The relationship between pressure and volume changes in living and dead rabbit eyes. John E. Eisenlohr and Maurice E. Langham

The relationship between pressure and volume changes in living and dead rabbit eyes John E. Eisenlohr and Maurice E. Langham A study has been made of the relationship between the volume of fluid infused into the living and dead eyes of individual rabbits and the intraocular pressure. To minimize loss of fluid from the eye during the experimental procedure the infusion was made either at 197 pi minr 1 or by a very rapid injection procedure. In individual rabbits the pressure/infusion volume relationship was found to differ significantly between the living ami dead eye. In the dead eye the coefficient of ocular rigidity was found to be approximately constant for pressures of 30 to 60 mm. Hg and to decrease below and above these pressures. In the living eye the coefficient of ocular rigidity was found to be at a maximum at a pressure of 25 to 30 mm. Hg and to decrease at pressures below and above this value provided the blood pressure tvas not exceeded. When the intraocular pressure exceeded the blood pressure the coefficient increased and approximated closely the value found in the dead eye at similar pressures. The marked influence of the blood supply to the eye on the pressure/infusion volume of the living eye has been demonstrated in experiments in which either the general blood pressure was varied or the local blood supply to the eye ivas decreased. The physiologic and practical significance of the results are discussed. It is emphasized that the volume of fluid injected into the living eye is not equal to the volume change of the eye. Thus, it is quite unjustifiable to utilize this type of relationship in tonographic measurements of the intraocular dynamics. T,. he measurements of intraocular pressure and dynamics by the conventional techniques of indentation tonometry and tonography are dependent on a knowledge of the exact relationship between pressure and volume changes of the living eye. However, at the present time the pressure volume relationship in general use is based on Friedenwald's 5 analysis of the experimental observations of earlier investigators on dead animal and human eyes and his own studies 0 on the volume of corneal indentation produced by a standard Schiotz From the Wilmer Institute, The Johns Hopkins University Medical School, Baltimore, Md. This work was supported in part by Special Fellowship BT627, National Institute of Neurological Diseases and Blindness, United States Public Health Service, and by United States Public Health Service Grant B2591. 63 tonometer. Friedenwald concluded that the volume change of the eye was directly proportional to the common logarithm of the ratio of the final and initial pressures and that the proportionality constant, which he termed the coefficient of ocular rigidity, had a mean value of 0.0215 for human eyes. The validity of this relationship was confirmed by Grant and Trotter 7 in manometric studies on enucleated human and rabbit eyes before and after death. On the other hand, Perkins and Gloster 13 " and Macri and coworkers 10 in similar studies on rabbit and cat eyes, respectively, could not confirm the relationship. None of these investigators reported detailed comparative studies on living and dead eyes of individual animals, but Grant and Trotter 7 observed no difference in the coefficient of ocular rigidity in two rabbits before and immediately after death, while Perkins and

64 Eisenlohr and Langham Investigative Ophthalmology February 1962 Gloster 13 stated that the pattern of results was similar in living and dead eyes but that values at any given pressure tended to be lower in life. Finally, Macri and coworkers 10 concluded that the reaction of living and dead eyes was essentially identical. It is the principal purpose of this investigation to evaluate the pressure response of the living and dead eyes of individual rabbits to injections of a known volume of fluid and to ascertain to what extent this response is influenced by the blood supply to the eye. In this connection the assumption made in previous studies that the volume of fluid injected and the volume change of the eye are equal has been examined experimentally. At the same time it was hoped to resolve the apparent discrepancies in the literature concerning the validity of the constancy of the coefficient of ocular rigidity by a reexamination of the method used in the earlier studies. This method comprised a measurement of pressure changes resulting from a very rapid injection of a known volume of fluid into the eye. In this study the results with this technique have been compared with those derived from observations of the pressure changes induced by a continuous infusion of physiologic saline into the eye. Our results indicate that living and dead eyes react differently to infused volumes of fluid and that the results on the living eye vary significantly with the level of the systemic blood pressure. The results agree closely with those found in a similar manometric study on the human eye before and after enucleation.' 1 Methods Adult rabbits of the New Zealand white strain which weighed between 2.0 and 3.0 kilograms were used throughout the experiments. Animals were anesthetized with urethane (1.0 to 1.75 Gm. per kilogram). Vascular and intraocular pressures were recorded by means of Sanborn transducers, Model No. 267B, in conjunction with Sanborn 1100 AS carrier preamplifiers and a multichannel 150 M rectilinear recorder. Rapid injections of fluid into the eye were made MICROMETER SYRINGE RECORDER CONTINUOUS I NFUSION APPARATUS Fig. 1. Diagram of the infusion and recording apparatus. By means of the saline reservoir the intraocular pressure could be established at any predetermined level. Then by excluding the reservoir from the system by closing the tap in the plastic block, saline could be injected into the eye from either the micrometer syringe or the continuous infusion machine. The pressure response was then recorded by the transducer through the same carmula used for infusion. using an Agla micrometer syringe. This was fitted with a mechanical stop so that predetermined volumes of physiologic saline (5 to 7 /tl) could be injected with one quick twist of the micrometer screw; the time taken for this volume of fluid to be expelled from the syringe was 100 to 300 msec. The accuracy of the micrometer screw in delivering the calculated volume was within 0.5 per cent. The plunger of the syringe and all taps were lubricated with DowCorning silicone stopcock grease. Continuous injections of physiologic saline were made with the apparatus described recently by Langham. 9 A diagram of the apparatus used to make the rapid and continuous infusion studies is shown in Fig. 1. Infusions into and pressure recordings from the eye were made through the same needle.

Volume 1 Number 1 Pressure and volume relationship 65 This was accomplished by the use of a fourway block which permitted interconnection of the injection apparatus, a saline pressure reservoir, the pressure transducer, and the anterior chamber either simultaneously or separately. A 23 gauge needle was inserted into the eye and connected to the fourway block by a short length (approximately 3 inches) of No. 50 polyethylene tubing. Prior to an experiment, the entire apparatus was cleaned, filled with deoxygenated physiologic saline, and examined carefully for trapped air bubbles and leaks. The recording system was balanced hydrostatically and the sensitivity set at 1 mm. per millimeter mercury for the continuous technique and 2 mm. per millimeter of mercury for the rapid technique. Since a small volume of fluid was required to raise the pressure of the transducer and the infusion system itself, this amount had to be subtracted from the total volume delivered by the syringe to obtain the net volume of saline entering the eye. Fig. 2 illustrates the method of measuring the system's volume capacity which was found to be linear with respect to pressure and to average 0.014 1*1 mm. Hg 1 (range 0.003 to 0.027 id. mm. Hg 1 ) for the rapid technique and 0.012 /il mm. Hg 1 (range 0.003 to 0.037 jul mm. Hg 1 ) for the continuous technique. In addition, a small correction for the amount of fluid escaping through the outflow channels of the eye had to be applied in the case of the continuous method. The value for this estimate of aqueous outflow at varying pressures was taken from the mean steadystate perfusion data of Langham. s Experimental procedure. The anesthetized animal was placed on an operating table equipped with a warming plate so that the body temperature could be maintained constant. The femoral artery was cannulated for the blood pressure record and in certain experiments a similar cannulation was made of the median ear artery. In general, it was found unnecessary to use a head holder and cannulation of the eye was made with the animal lying on its side. In the rapid injection experiments at least three infusions of 5 to 7 pi were performed at each pressure level. Determinations were made at the lowest pressure level first and then the pressure in the eye was increased in steps of 10 mm. Hg over the pressure range of 15 to 55 mm. Hg. Similar experiments were then made at descending steps of 10 mm. Hg starting at 50 mm. Hg. In this way die entire pressure range from 15 to 55 mm. Hg was covered in 5 mm. Hg intervals. No systematic difference was found between the first and third reading at any given pressure and the average pressure change was used in the subsequent calculations. In certain additional experiments similar studies were made for pressure exceeding 55 mm. Hg. The continuous infusion technique was carried out in the following manner: The volume capacity was measured by connection of the apparatus directly to the pressure transducer. Then all four taps on the Lucite block were opened and the pressure in the eye brought to 15 mm. Hg. The tap to the reservoir was then turned oft, and the record taken. This was repeated three or more times and the average readings vised in the subsequent calculations. Changes in the general blood pressure were induced by allowing blood from a femoral artery VOLUME CAPACITY CONTINUOUS TIME (SEO Fig. 2. Method for the determination of the volume capacity of the system, excluding the eye. On the left, 0.8 id. of fluid was injected rapidly into the closed system and the pressure rose by approximately 40 mm. Hg. On the right is shown a record of the pressure responses of the closed system when an infusion of 197 id min. 1 was made.

66 Eisenlohr and Lansham tigatioe Ophthalmology February 1962 to flow into a pressure reservoir set at the desired pressure level with respect to the heart. Results Preliminary experiments were carried out to confirm the validity of recording pressure from the anterior chamber through the same needle used for infusion. For this purpose a No. 23 gauge cannula was passed into the anterior chamber and arranged for infusion and pressure recording as outlined in Fig. 1. In addition, a second cannula attached to a separate transducer was inserted into the anterior chamber for an independent measurement of pressure. During rapid injections of saline the pressure changes recorded by the two cannulas were identical. However, during an infusion of 197 /xl min." 1 the pressure recorded directly from the anterior chamber was 2 mm. CONTINUOUS RAPID Fig. 3. Equilibration of pressure throughout the eye. The pressures in the anterior chamber and the vitreous humor were recorded simultaneously during the rapid and continuous infusion of fluid into the anterior chamber. The infusion of 197 la min. 1 into the anterior chamber caused the pressure in the anterior chamber to appear to be 2 mm. Hg above that in the vitreous. This constant pressure difference was due to the resistance to flow in the infusion system. After the rapid injection of saline into the anterior chamber, the pressures recorded in the two spaces were identical. In this case, however, it will be noticed that the pressure overshoot within the first short interval of time was greater in the anterior chamber. The paper speed was equivalent to one large square per second. Hg lower than that recorded from the infusion cannula. This pressure gradient was found to be a function of the rate of infusion and independent of the intraocular pressure. This discrepancy could be eliminated by employing a larger cannula, but this in turn increased the volume capacity of the system. Furthermore, in order to minimize disturbance to the eye it was desirable to use the smallest needle practicable. Since the 2 mm. Hg pressure gradient was found to be constant for all pressures studied up to 100 mm. Hg, the 23 gauge needle was used in all the infusion studies. The resistance to flow offered by the cannula and its polyethylene connection was responsible also for a pressure overshoot observed during rapid saline injections. This overshoot had a measured duration of approximately 0.25 second. Therefore, in agreement with the methods of previous investigators, the earliest portion of each rapid infusion record was ignored and the slope of the decay curve 1 second after injection was extrapolated back to zero time. Typical results from both the rapid and continuous infusion methods in which pressures in the anterior chamber and in the vitreous humor were recorded simultaneously are shown in Fig. 3. The pressure changes resulting from the rapid and continuous injection of saline were identical in the two compartments. Similar results were observed in dead eyes. A typical record of the continuous infusion of 197 ja min." 1 of normal saline into the anterior chamber of the rabbit eye before and after death is seen in Fig. 4. The slope of the infusion curve in the dead eye, which is a reflection of the eye's ocular rigidity, was consistently found to be higher than that of the living eye in each of the animals studied. Altogether 12 living eyes of 10 rabbits were examined by the continuous technique and of these 8 eyes were also studied in situ after death from an overdose of urethane. Mean values for the coefficient of ocular rigidity (where AV equaled the volume of saline infused)

Volume 1 Number 1 Pressure and volume relationship 67 LI VING DEAD 20 4 15 20 0 5 10 : TIME CSEO Fig. 4. A typical pressure response of the rabbit eye to the infusion of 197 MI min. 1 into the anterior chamber before and after death from an overdose of urethane. were calculated from the continuous tracings in steps of 10 mm. Hg and the results are summarized in Fig. 5 and Table I. It will be seen that the mean coefficients for both the living and the dead eye increased until the intraocular pressure reached 25 to 30 mm. Hg after which there was a progressive decrease in rigidity with a further increase in pressure. The values for the dead eye were higher than those for the living eye, the difference being significant at the 5 per cent level at 28 mm. Hg (0.02 < P < 0.05) and highly significant for pressure levels above this (P < 0.001). A plot of the logarithm of the intraocular pressure against the volume of saline infused into the 12 living eyes gave a curve of sigmoid shape with a point of inflexion at a pressure of 25 to 30 mm. Hg (Fig. 6). The effect on the intraocular pressure of a rapid injection of saline into the eye was studied on 16 eyes of 12 anesthetized rabbits, and one eye of each rabbit was also Table I. The coefficients of ocular rigidity of the living and dead rabbit eye calculated over successive intervals of 10 mm. Hg from the continuous infusion records. The figures shown are mean values (±S.E.) from 12 living and 8 dead eyes in situ Table II. The coefficients of ocular rigidity of the living and dead rabbit eye measured at varying intraocular pressures by the rapid technique. The figures shown are mean values (±S.E.) from 16 living and 12 dead eyes in situ Intraocular pressure (mm. Hg) Living Dead 1828 2333 2838 3343 3848 4353 4858 5363 6373.0216 ±.0026.0242 ±.0014.0246 ±.0014.0234 ±.0014.0217 ±.0013.0194 ±.0013.0174 ±.0014.0154 ±.0014.0134 ±.0019.0253 ±.0047.0289 ±.0026.0295 ±.0016.0307 ±.0020.0293 ±.0014.0280 i.0016.0270 ±.0014.0243 ±.0008.0241 ±.0013 Intraocular pressure (mm. Hg) 15 20 25 30 35 40 45 50 55 Living.0164 dt.0029.0237 dt.0013.0281 db.0014.0260 d:.0010.0256 dt.0013.0241 dt.0016.0209 dt.0016.0199 dt.0019.0156 : t.0013 Dead.0227 : t.0031.0291 dt.0022.0351 dt.0016.0360 d:.0014.0374 dt.0013.0356 dt.0012.0357 dt.0009.0353 dt.0012.0344 dt.0012

68 Eisenlohr and Langham Investigative Ophthalmology February 1962 o 030 020 * 0 0 * 20 30 40 50 60 70 80 10 P (MMHG) Fig. 5. The variation in the coefficient of ocular rigidity with pressure. The values were calculated from the results of the continuous infusion technique and were based upon 12 living ( ) and 8 dead (O) rabbit eyes. The results are expressed as the arithmetic mean ± the standard error of the mean (A.M. ± S.E.). saline over the same pressure range failed to reveal a tendency for each successive infusion curve to become steeper than the preceding one. The influence of blood pressure on the pressure response of the living eye to infusions of fluid was studied by reduction of the general blood pressure and by local restriction of the blood supply to the eye. Six rabbits were anestlietized and prepared for study in the usual manner, and, in addition, a femoral artery was attached to a blood reservoir by means of a polyethylene cannula. By opening the cannula and adjusting the level of the blood reservoir with respect to the heart, stable vascular hypotension could be readily produced. Under these conditions a marked dependence of ocular rigidity on blood pressure examined in situ after death by the same technique (Fig. 7). The results of these experiments are summarized in Fig. 8 and Table II. The mean coefficients of ocular rigidity derived from these results are similar to those of the continuous infusion method. Thus, the coefficient of ocular rigidity in the living eye increased until the initial intraocular pressure reached 25 mm. Hg after which it fell progressively. In dead eyes, the coefficient of ocular rigidity was virtually constant above a pressure of 25 mm. Hg. Statistically, the difference between the living and dead eye was significant at the 5 per cent level (0.02 < P < 0.05) at 20 mm. Hg, and highly significant for all pressure levels above this (P < 0.001). In the rapidinjection studies the volume of saline injected was maintained constant while observations were made first with increasing and then decreasing initial pressure steps of 10 mm. Hg. The values obtained for ocular rigidity during the descending phase tended to be slightly higher than those noted at ascending pressure steps, particularly in the dead eye. However, repeated continuous infusions of 70 60 50 40 CL O 30 O O 20 10 20 30 Fig. 6. The relationship between the logarithm of the intraocular pressure (plotted as ordinate) and the net volume of saline infused into the anterior chamber by the continuous technique. The solid and dotted lines represent, respectively, the A.M. ± S.E. of 12 living eyes. It will be noted that the curve has a point of inflection at approximately 25 mm. Hg. By definition the slope of this line at any pressure is equivalent to the coefficient of ocular rigidity and it will be seen that this increased in value up to the point of inflection and then decreased over the remaining range of pressures studied.

Pressure and volume relationship 69 LIVING DEAD o I 2 2 Fig. 7. A typical record of the intraocular pressure following three rapid injections of 5 ill each into the anterior chamber of the living and dead eye of an individual rabbit. The rjaper speed was equivalent to one large square per second. Note that the pressure response to equal volumes of saline injected was greater in the dead than in the living eye. The record of the femoral arterial pressure has been electronically damped to record the mean blood pressure. tsu > o.040 1.030 20.010 20 30 40 50 60 I0P(MM HG) Fig. 8. The effect of pressure on the coefficient of ocular rigidity calculated from experiments on 16 living ( ) and 12 dead (O) eyes, studied by the rapid injection technique. The results are expressed as the A.M. ± S.E. was observed. Fig. 9 illustrates the effect on intraocular pressure of the continuous infusion of 197 /A min." 1 into the eye of the same animal before and during vascular hypotension. It will be observed that, as the intraocular pressure approached the mean systemic blood pressure, the rate of increase in the intraocular pressure diminished significantly. Then, as the intraocular pressure exceeded the blood pressure, as evidenced by a cessation of the ocular pulse, there was a marked increase in the slope of the infusion curve. In all animals studied, this characteristic plateau in the continuous infusion records was found to be correlated with the level of systemic blood pressure. In the hypotensive animal, the plateau and the subsequent rapid pressure increase were noted at a correspondingly lower pressure than in the normotensive animal. Comparative tracings from one animal immediately after cannulation, during hypotension, and after death from an overdose of urethane are seen in

70 Eisenlohr and Langham Inoestigativ Ophthalmology February 1962 Fig. 10. The infusion curve from the hypotensive animal became indistinguishable from that obtained after death once the ophthalmic blood pressure had been exceeded. The effect of hypotension on the ocular rigidity measurements of 3 individual eyes studied by both infusion techniques is seen in Fig. 11 and is characteristic of all 6 animals in which the effect of hypotension was evaluated. A localized reduction in the vascular pressure of the eye was also accomplished by clamping one or both carotid arteries. A typical result is shown in Fig. 12. NORMOTENSIVE HYPOTENSIVE Fig. 9. The influence of vascular hypotension on the pressure response to a continuous infusion of 197 /d. min. 1 into the anterior chamber of the eye of an anesthetized rabbit. The first curve shows the results on the normal animal and it will be seen that, as the intraocular pressure approached the blood pressure, the slope of the pressure record decreased considerably, but then increased again as the blood pressure was exceeded. The second curve shows the same experiment when the animal was made hypotensive and exhibits the characteristic plateau at a correspondingly lower level of intraocular pressure. The blood pressure was electronically damped for part of the tracing. NORMOTENSIVE HYPOTENSIV E DEAD M^liUbiUdiliUilil imiufauil^yi 40 I. UI 7 20 20 Fig. 10. Comparative tracings of the pressure in the anterior chamber during an infusion of 197 la min." 1 into an animal made hypotensive and then killed. The characteristic marked difference in response of the hypotensive eye is seen, and, in addition, it will be noted that when the intraocular pressure exceeded the arterial pressure, the record in the living eye became very similar to that in the dead eye.

Volume 1 Number 1 Pressure and volume relationship 71 2 3 RAPID or Q_ O O 20 30 40 SO 60 20 30 40 50 60 CONTINUOUS 20 30 40 30 SO 20 30 40 SO 60 20 30 40 JO 60 20 30 40 SO I OP (MMHG) Fig. 11. The effect of vascular hypotension on the coefficient of ocular rigidity of 3 individual eyes studied by both the rapid and continuous injection techniques. The values for the rigidity of the living ( ) and dead (OO) animals are compared with those obtained from the same animal during hypotension ( ). The figures to the right of the records of the living and hypotensive rabbits indicate the mean femoral arterial blood pressure during the experiment. It will be seen that the ocular rigidity was dependent upon the systemic blood pressure and that only when the intraocular pressure approached or exceeded the blood pressure did the living and dead eyes react similarly to infusions of saline. A reduction in the median ear arterial pressure which is known to agree closely with that of the ophthalmic arterial pressure 1 was associated with a downward displacement of the pressure/infusion curve. Discussion The results of this study give strong support to the conclusion of recent investigators that the coefficient of ocular rigidity in both living and dead eyes varies with pressure, but they differ in certain qualitative and quantitative aspects which merit discussion. Furthermore, the results fail to support the contention of recent investigators that there is no significant difference in the pressure/infusion volume relationship before and after death. On the contrary, two of the most important observations in this study have been the difference in response of living and dead eyes and the marked influence of the vascular pressures on the pressure/infusion volume relationship of the living eye. The results derived from the rapid injection technique on the living and dead

72 Eisenlohr and Langham.stigatiue Ophtliahnology February 1962 TIME E0 Fig. 12. The effect of localized ocular hypotension on the pressure response, to a continuous infusion of 33 /id min.' of saline. The top record shows the pressure in the median ear artery which closely approximates that of the ophthalmic arterial pressure. The three records show the response in the normal animal (A) and following ipsilateral (B) and bilateral ligation (C) of the common carotid arteries. It will be noted that there was a downward shift in the plateau of the pressure record of the eye corresponding to each successive fall in the median ear arterial pressure. This response was completely reversible on release of the arterial ligatures. eyes of rabbits agree in some, but not in all, respects with those of Perkins and Gloster. 33 These investigators concluded that the coefficient of ocular rigidity increased with pressure in both living and dead eyes and that, in 5 out of 10 animals studied, the values were similar before and after death. In the remaining 5 animals, the values after death were approximately 20 per cent above those in life. No details were given of their experiments on dead eyes but analysis of their results on living animals showed that the mean coefficients were 0.007 ± 0.001, 0.015 ± 0.003, and 0.018 ± 0.002 for pressures of 11 to 18 mm. Hg, 22 to 29 mm. Hg, and 33 to 40 mm. Hg, respectively. In the present studies, however, an increase in the coefficient of ocular rigidity in both living and dead eyes was found only for pressures up to 25 to 30 mm. Hg. Furthermore, the absolute values for the coefficient of ocular rigidity reported by Perkins and Gloster were less than those of the present study at corresponding levels of intraocular pressure. The reason for these discrepancies could well lie in the relatively insensitive rubber membrane manometer used by these workers. It is, however, of interest that in Fig. 2 of a subsequent paper by Perkins and Gloster 14 the ocular rigidity of an eye before and after death is recorded over a pressure range of 40 to 107 mm. Hg and indicates a difference between the living and dead eye. Furthermore, the coefficient of ocular rigidity reached a minimum value at 73 mm. Hg in the living eye and then increased until it reached a similar value to that of the dead eye. This finding is in excellent agreement with the results of this study in which the ocular rigidity increased when the intraocular pressure exceeded the systemic blood pressure. The coefficient of ocular rigidity in living and dead eyes of cats was studied in a similar manner by Macri and coworkers. 10 They found the coefficient to vary with pressure in the living eye and their results agree closely with those found in the present study on the eyes of living rabbits.

Volume 1 Number 1 Pressure and volume relationship 73 However, their results on dead eyes differed from the present findings in that they could find no difference between living and dead eyes. This led these investigators to conclude that the coefficient of ocular rigidity was a function of the ocular coats and was not influenced by either nervous activity or circulatory changes. Although no difference was found between living and dead eyes, no specific studies were made to rule out the possibility of a vascular effect. Indeed the influence of vascular pressures on ocular rigidity in the living cat is strongly suggested in the results of a subsequent paper. 11 The marked difference observed in the pressure/infusion volume relationship between the living and dead eyes must be considered from two aspects: first, the validity of the assumption made by previous investigators that the volume of fluid infused into the eye was equal to the volume change of the eye; and second, the significance of this difference to tonometric and tonographic studies. The equation of volume infused to the volume change of the eye was made by Friedenwald 5 in his contribution to the theory and practice of tonometry and has never since been seriously questioned. His view, however, cannot be upheld by the findings in this study of a significant difference in the pressure response of eyes before and immediately after death and of a similarity in response of a living eye to that of the dead eye when the intraocular pressure exceeds that of the blood pressure. In the living eye it was evident that the inequality between the volume of fluid injected and the volume change of the eye became very great when the intraocular pressure approached that of the blood pressure. Under these conditions, the rate of change of intraocular pressure fell nearly to zero even when 197 fa min." 1 of fluid was infused into the eye. This means that intraocular fluid was leaving the eye at nearly the same rate. Further, this could be effected for any pressure level in the eye by a suitable adjustment of the blood pressure indicating the vascular nature of this phenomenon. In the case of the enucleated eye, it is not possible to determine from these studies whether the volume infused equaled the volume change of the eye. Although there cannot be any vascular displacement under these conditions, there still remains the possibility that an increased pressure caused a collapse of the intraocular vessels. Certainly a pressure of 30 mm. Hg should be more than sufficient to compress all the veins and smaller vessels of the eye and the present results are consistent with this hypothesis. The observation that the coefficient of ocular rigidity in the dead eye was approximately constant over the pressure range of 30 to 60 mm. Hg is in agreement with Friedenwald's contention. Thus, it may be that the relationship between actual volume and pressure change of the eye follows his original expression. Such a relationship would be of theoretical interest rather than of practical importance to the measurement of pressure and intraocular dynamics. The significance of the inequality of volume infused and volume change in the living eye lies in its application to tonometric and tonographic studies. The measurement of intraocular pressure by indentation tonometry is analogous to the infusion of a certain volume of fluid into the eye. The tonometer causes a volume of indentation and an increased pressure in the eye (P t ) and derivation of the pressure in the untouched eye (P o ) must be based on knowledge of the relationship between the volume of corneal indentation and the pressure change of the living eye. In tonography the object is to determine the rate of loss of aqueous humor per unit pressure gradient from the eye, and at least two assumptions are made which cannot be justified on the basis of the present studies, namely, that the coefficient of ocular rigidity is constant, and that the volume change calculated is due solely to loss of aqueous humor from the eye. Comparison of the results of the rapid

74 Eisenlohr and Lanzham Inoextigatioc Ophthalmology February 1962 and continuous infusion studies showed general agreement in the living and dead eyes although the coefficients of ocular rigidity derived from the continuous infusion studies were as much as 30 per cent below those of the rapid technique. The techniques differ principally in the rate of pressure change induced in the eye. It is known that the elastic properties of the dead eye include a viscoelastic component which would tend to cause a reduction in the coefficient of ocular rigidity with slower rates of infusion. The alternative explanation that the discrepancy could be due to an error in the correction for outflow of aqueous humor appears most unlikely as it represented only a very small fraction of the volume infused. REFERENCES 1. Barany, E. H.: The influence of local arterial blood pressure on aqueous humour and intraocular pressure. Experimental study of the mechanisms maintaining intraocular pressure. Intraocular pressure and local blood pressure from seconds to hours after unilateral carotid occlusion. A search for homeostatic reflexes in the undisturbed eye, Acta ophth. 24: 337, 1946. 2. Becker, B., and Friedenwald, J. S.: Clinical aqueous outflow, A. M. A. Arch. Ophth. 50: 557, 1953. 3. Bettman, J. W., Fellows, V., Chao, P., and PrattJohnson, E. J.: The effect of tonography and other pressures on the intraocular blood volume, A. M. A. Arch. Ophth. 60: 231, 1958. 4. Eisenlohr, J. E., Langham, M. E., and Maumenee, A. E.: Manometric studies of the pressure volume relationship in living and enucleated eyes of individual human subjects, Brit. J. Ophth. (in press). 5. Friedenwald, J. S.: Contribution to the theory and practice of tonometry, Am. J. Ophth. 20: 985, 1937. 6. Friedenwald, J. S.: Standardization of tonometers. Decennial report by the Committee on Standardization of Tonometers, Am. Acad. Ophth. 1954, pp. 93152. 7. Grant, W. M., and Trotter, R. R.: Tonographic measurements in enucleated eyes, A. M. A. Arch. Ophth. 53: 191, 1955. 8. Langham, M. E.: Influence of the intraocular pressure on the formation of the aqueous humour and the outflow resistance in the living eye, Brit. J. Ophth. 43: 705, 1959. 9. Langham, M. E.: Steady state pressure flow relationships in the living and dead eye of the cat, Am. J. Ophth. 50: (Pt. 2) 280, 1960. 10. Macri, F. J., Wanko, T., Grimes, P. A., and von Sallmann, L.: The elasticity of the eye, A. M. A. Arch. Ophth. 58: 513, 1957. 11. Macri, F. J.: Outflow patterns of the cat eye, Am. J. Ophth. 47: (Pt. 2) 547, 1959. 12. Monnik, A. J. W.: Ein neuer Tonometer und sein Gebrauch, v. Graefe's Arch. Ophth. 16: 49, 1870. 13. Perkins, E. S., and Gloster, J.: Distensibility of the eye, Brit. J. Ophth. 41: 93, 1957. 14. Perkins, E. S., and Gloster, J.: Further studies on the distensibility of the eye, Brit. J. Ophth. 41: 475, 1957. 15. Ytteborg, J.: The role of intraocular blood volume in rigidity measurements on human eyes, Acta ophth. 38: 410, 1960. Discussion Dr. Robert A. Moses, St. Louis, Mo. It is a privilege to be allowed to comment on a work which tackles the thorny problem of change in intraocular blood volume with changing intraocular pressure and changing blood pressure. There can be little doubt that intraocular blood volume is not fixed. The authors have demonstrated this and have quoted supporting documentation. That there are authors who did not find these phenomena is not surprising for the quantities are small. Since there is such a shift of volume, it is clear that what has been called "ocular rigidity" includes this alteration. It is astounding that the masterful simplifications of Dr. Friedenwald have been so useful in our work. I offered further evidence of the influence of blood pressure and blood volume changes at the Wilmer Resident's Association meeting this Spring when I showed that ocular rigidity not only changes but becomes anomalous with change in the patient's position (Fig. 13) (my figures numbered in sequence with the authors'). Their illustrations of relatively slow infusion into the anterior chamber are particularly provocative (Fig. 12). They interpret the plateaus as evidence of blood leaving the eye as aqueous is entering it. This is the interpretation Macri gave in experiments performed in the reverse order. That is, Macri distended the eye and studied the slope of the

Volume 1 Number 1 Pressure and volume relationship 75 >1bUo DC UI * 1UUcc iii fln =j 60 50 LLJ cr _^ / n (/) in UJ V 0 \\ \ 3 d _S 4 \ V 0 _\ 4 0; V. c _l v1 \ ^«FRIEDENWALD 1955 NOMOGRAM FOR SCHIOTZ TON 0 M ETER T DC OL 30 25 20 0 I I. >*6 \ _ ; i ] PI D 7 V rfo hs.06 TO MEASUII SCUHAl IICIDITV... in on,. non.0,,..,n. oppl.no.i.n onon,.,., 4 7 c < 6. 7 S.05 z k~9~ is p. j.04 > z > 12 ar 8 03b ^" JJ j H ~«.03.025 i 4 0 13 5.0»> di.plocm.nl ii oboul 0.3 cw. mm.) ond ioin 1h. Iwo point I "Po" 1 4 1? 15 14 1? 1J "~7.02 *» 15 8 16 IS 13". 17 M. 32 18" 7 i «> c 20 1!.015 61... i,c 19 18~ RIGIDITY 1') 0 zjz 18 5.5? y r c ṇ 1 i 1 i c > P1. 'Z.005 n* 5 \ r i 9 8 7 6 5 10 15 20 25 30 35 40 45 50 55 VOLUME OF INDENTATION CUBIC MILLIMETERS Fig. 13. Ocular rigidity measurements on supine subject with applanation and Schiotz tonometers (5.5 and 10 Cm. plunger loads). The lower curve was obtained with the patient horizontal, the upper solid curve with the table tilted head down 30 degrees, and the dotted curve after 15 minutes in the headdown position. pressure decay curve. The phenomenon of volume shift may be studied in a highly simplified model (Fig. 14). Here we have an "artery" which has a constant pressure at its origin. It traverses the "eye" and is exposed to "intraocular pressure" and discharges into an "extraocular vein" of fixed height. Since the "corneosclera" is glass it should have a virtually infinitely great rigidity, but as we inject volume and record pressure changes in this "eye" we find a pattern (Fig. 15) not unlike that found by the authors in a living rabbit. (Correction for volume loss to the manometer has been made.) I want to call attention to the similarity of the authors' normotension and hypotension curves (Fig. 9) to those of the model. In both cases, when the arterial blood pressure is lowered, apparent ocular rigidity is lowered also. I would like to ask the authors' interpretation of their Fig. 10 in which the total volume injected into the eye of the hypotensive animal is considerably greater at each pressure level than in the normotensive. This would seem to imply that, when hypotensive, the animal had more blood in its eye than when normotensive. The model and several experiments with it are to be reported elsewhere. Quantitation of the role of blood volume in what we record as ocular rigidity is difficult. This may be seen if one superimposes some of the authors' records, on the same eye. In Fig. 16, I have attempted to match some of the records

76 Eisenlohr and Langham Investigative Ophthalmology February 1962 Fig. 14. Model eye. A, glass jacket; B, thinwalled rubber tube; C, syringe for altering pressure in jacket; D, manometer for jacket pressure; E, water supply of constant pressure head; F, clamp for adjusting resistance of supply tube; G, manometer for pressure of water near entrance to thinwalled tube; H, flow exit. e E y/ C 3 5 7 AV (ml) 1 3 5 7 9 AV (ml) Fig. 15. "Pressurevolume" relationships of model eye. Q_ o /JF* from the authors' Fig. 4, and we see that the records from the living eye lie between those of the dead eye. There is a bonus in the paper. Dr. McEwen has been studying the rheology of the eye and he should be very interested in the peaks of pressure shown in the recordings from the vitreous when fluid is injected rapidly into the anterior chamber (Fig. 3). Such pressure peaks have been explained away by several authors on an instrumental basis. Here we have the pressure recording completely isolated from the injection system. The pressure peak is real and its decay must represent either a somewhat slow stretch of the ocular coats, or the time required for blood to be forced out of the eye, or both. I would like to ask the authors if they have noted the behavior of dead eyes similarly doubly cannulated? 60 40 20 1 1 0 5 10 Time (Sec.) Fig. 16. Superposition of curves of Eisenlohr and Langham's Fig. 4; a, b, living eye; c, d, dead eye. Dr. Eisenlohr (closing). Dr. Moses' experiments with volume shift in a simplified model eye are most intriguing and appear to be analogous to our findings in normotensive and hypotensive rabbit eyes. Regarding our interpretation of Fig. 10, we have no reason to think that the ocular blood volume is greater in the hypotensive than in the normotensive animal. The plateaus in the infusion records are indicative of that pressure range over which the greatest blood loss occurs from the eye during continuous saline infusions. Since the plateau occurs at a lower level of intraocular pressure in the hypotensive animal, the conclusion to be drawn is that the greatest blood loss from these eyes occurs at a correspondingly lower level of intraocular pressure.

Volume 1 Number 1 Pressure and volume relationship 77 The interpretation of the pressure peaks or overshoot associated with the rapid injection of saline is a complex problem. With independent pressure recordings from both the vitreous and the anterior chamber, the overshoot was not abolished in either compartment when the animals were killed. Furthermore, an artificial system consisting of a salinefilled piece of rubber tubing clamped at both ends could be shown to demonstrate the same type of overshoot when cannulated in the same manner with two separate needles. Thus, it may be that a mechanical or kinetic factor contributes to the overshoot and at the same time masks the very rapid changes occurring as a result of blood volume changes or the stretch of the ocular coats.