By S. CASSIN,* G. S. DAWES, JOAN C. MOTT, B. B. Rosst AND L. B. STRANG4 From the Nuffiekl Institute for Medical Research, University of Oxford

Transcription

1 J. Physiol. (1964), 171, pp With 10 text-figures Printed in Great Britain THE VASCULAR RESISTANCE OF THE FOETAL AND NEWLY VENTILATED LUNG OF THE LAMB By S. CASSIN,* G. S. DAWES, JOAN C. MOTT, B. B. Rosst AND L. B. STRANG4 From the Nuffiekl Institute for Medical Research, University of Oxford (Received 18 July 1963) Ventilation of the lungs of a foetal lamb causes an immediate large reduction in pulmonary vascular resistance, whether the lungs are ventilated with air, oxygen or nitrogen (Dawes, Mott, Widdicombe & Wyatt, 1953). Further experiments on isolated perfused lungs (Born, Dawes & Mott, 1955) and in vivo (Dawes & Mott, 1962; Cook, Drinker, Jacobson, Levison & Strang, 1963) showed that substitution of air for nitrogen as the ventilating gas often caused a further increase in pulmonary vasodilatation. Cook et al. (1963) also observed that addition of C02 to the ventilating gas mixture caused vasoconstriction, and that stepwise static inflations and deflations of the foetal lung with nitrogen did not cause a large reduction in pulmonary vascular resistance. The present experiments were undertaken to determine whether rhythmic inflation of the foetal lungs with a gas mixture, chosen so as to cause little change in arterial Pco, or Po2, would result in pulmonary vasodilatation; and to measure the relative magnitude of the vascular changes caused by ventilation with different gas mixtures. METHODS Thirty-one foetal lambs of days gestation age, weighing kg, were delivered by Caesarean section under light chloralose anaesthesia (30 mg/kg i.v. initially). Further doses of chloralose 10 mg/kg i.v. were given to the ewe at intervals of 1-2 hr; these caused no discernible change in pulmonary vascular resistance. The lambs were placed on a warmed table alongside the mother, still attached by the umbilical cord. A cannula filled with saline was introduced into the foetal trachea, and the chest was opened between the fourth and fifth ribs on the left side, to give access to the origin of the left pulmonary artery within the pericardium. In a few experiments the right lung alone was ventilated; it was necessary to open the chest more widely and to divide the left hemiazygos vein in * Special Fellowship Award, BT-1032, N.I.H., U.S.P.H.S.; on leave from the Department of Physiology, College of Medicine, University of Florida, Gainesville, Fla., U.S.A. t U.S.P.H.S. Career Development Award, GM K3-15, 216-C4-B; on leave from the Department of Physiology, University of Oregon Medical School, Portland, Oregon, U.S.A. t On leave from the Institute of Child Health, University of London. Present address: University College Hospital, London, W.C. 1.

2 62 S. CASSIN AND OTHERS order to gain access to and tie the left bronchus. The pulmonary end of the bronchus was opened to allow drainage of pulmonary fluid. Heparin (10 mg/kg) was injected i.v. and the left pulmonary artery was divided. The peripheral end was connected with the central end of the left carotid artery via an electromagnetic flowmeter (Wyatt, 1961) and pressure was measured with an Elema inductance manometer as described previously (Dawes & Mott, 1962). In addition, a vertical polyethylene tube, 100 cm long and 6 or 8 mm internal diameter was attached between the carotid artery and the point at which pressure was measured (Fig. 1). Pulmonary pressure-flow curves were produced by filling the vertical tube with blood, waiting for a minute for the consequent small changes in pressure and flow to subside, and then letting the blood from the tube flow into the left pulmonary artery while the carotid supply was temporarily interrupted (Fig. 4). Such records, made Vertical tube Left pulmonary Left carotid artery /_ artery Flow /, Pressure Fig. 1. Schematic diagram of arrangement for producing pulmonary pressure-flow curves by perfusion of blood from a vertical tube. over sec according to the rate of flow, showed a regular fall in flow as pressure declined. Changes in left atrial pressure were small (1-3 mm Hg), both during such manoeuvres and during the whole course of an experiment. Pressure-flow diagrams were recorded by displaying arterial pressure on the X axis and flow on the Y axis of an oscilloscope (which was photographed) or of an X- Y recorder. The surge of pressure produced on opening the vertical tube and the consequent increase of flow were damped by the elastic properties of the pulmonary vascular bed and the tubing in the external circuit, so that a rise time of about 0 5 sec was required before recording the decay of pressure and flow as the tube drained (Fig. 4). Neither the writing speed of the X- Y recorder (at the amplification used), nor the frequency responses of the flowmeter (20 c/s), manometer (6 c/s) and recording apparatus were such as to distort the slow pressure-flow decay curve. Normal pulsatile pressure and flow variations drew a loop on the X- Y recorder, asymmetrically placed about a pressure-flow curve taken before or afterwards. At the end of the experiment the cannula was removed from the left pulmonary artery and pressure-flow curves were generated through the external open-ended system. Exaamples of these, labelled 'open system', are to be seen in Figs. 8 and 10. In calculating

3 VASCULAR RESISTANCE OF LUNG 63 vascular conductance (A flow/a pressure), A pressure has been measured as the difference in pressure at two given flows corrected for the pressures in the external system. Conductance was always measured from the steep part of the curve (Fig. 5). The intercept of this part of the pressure-flow curve on the pressure axis was obtained by extrapolation (Fig. 5). The purpose of these measurements was solely to define the upper part of the pressure-flow curve in mathematical terms which could be used for calculations. The remainder of the apparatus has already been described (Dawes & Mott, 1962). The lungs were rhythmically ventilated with a constant-volume Starling Ideal pump, which had been modified to prevent any leakage. The pump was usually stopped while pressure-flow curves were produced; this brief period of arrest did not modify the result. The compositions of the inspired gas mixtures used were checked by analysis in the Lloyd- Haldane apparatus (Lloyd, 1958). Arterial blood samples (0-6-0*8 ml.) were taken from the carotid-pulmonary loop into 1 ml. syringes in which the dead space was filled with a solution of heparin 0-4 % and NaFl 8 g/100 ml. They were analysed at once for ph, with a Metrohm capillary glass electrode at C and an E.I.L. ph meter Model 23A, and for Pco, with an electrode of the type described by Severinghaus & Bradley (1958) and a Beckman 160 gas analyser. Arterial PO, was measured with a modified Clark oxygen electrode (Severinghaus & Bradley, 1958). The oxygen saturation was determined by a modification of the Barcroft-Haldane method with Warburg manometers (Born et at. 1955). In some experiments the arterial E0, was calculated from a HbO2 dissociation curve derived from in vivo measurements (see Results). Where possible the distinction between calculated Po2 and that measured directly has been preserved, but the mean figures quoted (e.g. in Table 1) contain measurements of both types. RESULTS The HbO2 di8sociation curve of foetal blood There is little information available by which the PO. of foetal lambs' blood can be deduced from measurements of ph and 02 saturation. earlier experiments (Barcroft, 1946; Barron, 1951; Barron & Meschia, 1954; Born, Dawes, Mott & Rennick, 1956) the dissociation curves were related to PCO0. In nine lambs of days gestation age, carotid arterial blood was analysed for ph, 02 saturation and Po2. Multiple regression analysis was used to obtain constants for the type of equation used by Meschia, Hellegers, Blechner, Wolkoff & Barron (1961): log Po, = K1-K2 ph + K3log S/(100-S), (1) where S is percentage 02 saturation. The constants derived from 39 samples were K1 = 3-63, K2 = (s.e.) and K3 = The standard error of the estimate of log PO, was 0-069, corresponding to + 1 mm Hg at a PO, of 10 mm Hg, and + 7 mm Hg at a Po2 of 40 mm Hg. The relatively large error of estimate at the higher Po2 may be due to variations in acid-base balance between lambs or during an experiment, as indicated by the large standard error of K2. These constants are not significantly different from those calculated by Meschia et al., using other analytical methods on blood from three lambs of days gestation age and of a different variety. However, the error in estimating arterial PO, from 02 In

4 64 S. CASSIN AND OTHERS saturation and ph was reduced for our purposes by using the constants derived above, and these have been used in the present paper where necessary. Figure 2 shows the dissociation curves calculated from our data at ph 7-2, 7-4 and 7-6 (solid lines). The experimental observations a 0._ (mm Hg) P02 Fig dissociation curves calculated for ph 7-2, 7-4 and 7-6 ( ) from equation (1), and individual observations also from nine mature foetal lambs at ph > 7*4 (-) or ph < 7-4 (0). The interrupted line (a) is a dissociation curve calculated by Meschia et al. (1961) for one lamb at 135 days gestation. are shown for ph < 7X4 (0) and ph > 7-4 (e); the range was ph Figure 2 (a, -- -) also shows the dissociation curve at ph 7-4 calculated by Meschia et al. for one lamb at 135 days gestation age. Unexplained changes in pulmonary blood flow When the circuit between the left carotid and the left pulmonary arteries was completed flow rose rapidly to reach a high value (Fig. 3)

5 VASCULAR RESISTANCE OF LUNG 65 and then subsided gradually as described previously (Dawes & Mott, 1962). In most lambs pulmonary flow then reached a steady value which was maintained over periods of observation from 10 to 55 min. But in thirteen out of thirty-one lambs, large seemingly spontaneous and quite rapid changes in pulmonary flow occurred soon after the preparation was completed. In eight of the thirteen the principal change was an increase in Pressure-flow curves (nos.) V E 40< Arterial 20- Left atrial E 200_ 100t Pco0 (mm Hg) Po. (mm Hg) Time (min) Fig. 3. Foetal lamb, 139 days gestation, 3 97 kg. The left pulmonary artery was connected to the left carotid through a flowmeter at time zero (see Fig. 1). There was an initial period of vasoconstriction, followed after 20 min by vasodilatation with no appreciable change in arterial Pco, or Po2. Examples of the pressure-flow measurements taken at intervals in the record are shown in Figs. 4 and 5. flow and in the remainder there was a decrease. Figure 3 illustrates an experiment in which flow decreased for the first 20 min, and then increased rapidly and considerably. It only reached a moderately steady intermediate state after min. As flow increased there was a small rise in left atrial pressure. Figure 4 shows records of left pulmonary arterial pressure and flow from a vertical tube previously filled with blood (see Methods), and Fig. 5 shows examples of simultaneously recorded pressure-flow curves, also from the experiment imustrated in Fig. 3 during the initial vasoconstriction (1 and 3), during the subsequent vasodilatation (7) and when a steady state was 5 Physiol. 171

6 66 S. CASSIN AND OTHERS 2 7 c E xe E A. OL I 30 sac Fig. 4. As Fig. 3. Records (retouched) of left pulmonary arterial flow (above) and pressure (below) during pressure-flow curves nos. 2 and F - _ 400- I_- E i a: / /./ I7. u~ - r Arterial pressure (mm Hg) 3 I 100 Fig. 5. As Fig. 3. Tracings of oscilloscope photographs of pressure-flow curves nos. 1, 3, 7 and 10. The interrupted lines indicate the prolongation of the conductance lines to cut the abscissa at the 'pressure intercept'.

7 VASCULAR RESISTANCE OF LUNG 67 reached (10). The conductance per kilogram (A flow/a pressure, see Methods for a full definition) increased from a minimum of 0-91 to 2d14 ml./min. mm Hg, and the intercept of the conductance line (Fig. 5, - - -) on the pressure axis fell from 57 to 29 mm Hg. In other experiments in which unexplained vasodilatation occurred the conductance per kilogram rose to a maximum of 2 5 ml./min. mm Hg and the pressure intercept fell below 20 mm Hg. In six of the eight lambs, in which large increases of flow were observed, blood samples were taken before, during, and sometimes afterwards. In no instance was there any appreciable change in arterial 50 0O O Conductance per kilogram=1.0 E o 8 0 / 4I 09 d C.. 30 _@0* / ~~* *- 30~~~~~~~~ 20 I I II Po, (mm Hg) Fig. 6. Simultaneous observations on twenty-two foetal lambs of arterial Pco, and P02 at conductances per kilogram < 1 (0) and conductances per kilogram > 1 (0). The isopleth was calculated from equation (2) at a conductance per kilogram of 10. PC02, ph, 02 saturation or in calculated PO, (Fig. 3). Though fewer measurements were taken in lambs in which flow decreased, there was also no evidence of a change in blood gas composition. Seemingly spontaneous increases or decreases in pulmonary flow were observed in lambs with a moderately high or low vascular conductance or pressure intercept, with an initial arterial ph of *73, a Pco, of mmhg, an 02 saturation of % and a calculated PO, of mm Hg. In none of these ways, nor in the length or magnitude of the dissection, did they differ from other lambs. The initial blood pressure was on the average higher in lambs which showed spontaneous increases

8 68 S. CASSIN AND OTHERS in flow (55 + 2*5 mm Hg) than in the remainder ( mm Hg); the difference between these means is small but significant. In addition to the seemingly spontaneous variation in conductance observed in individual lambs, there was considerable variation in conductance between lambs when a steady state was reached. In twenty-two lambs from which blood samples were obtained the mean conductance per kilogram in the foetal state was 1*11 ml./min.mm Hg, with a range of *94. For this variation, a definite relation was established between con- Regression analysis by the method ductance and arterial PCO, and Po2. of least squares gave the equation: conductance/kg = Po PC02, (2) where the residual error was , and standard errors for the regression constants for Po2 and Pco2 were (P < 0 05 that the constant is 0) and (P < 0.001) respectively. Figure 6 shows the simultaneous values of Pco, and Po0 for the arterial blood in thirty-two observations; the conductance per kilogram is indicated as < 1 (0) or > 1 (e). An isopleth is shown for a conductance per kilogram of 1, derived from the equation given. Although this division of conductance is arbitrary, the results show that lambs in a more asphyxiated state tend to have lower pulmonary conductances. Ventilation with N2 and C02 Four lambs were ventilated with a pump whose inlet and outlet were attached to a bag, so that they rebreathed from it. The bag was filled with a mixture of 3 % 02 and 7 % CO2 in N2, which was thought to be approximately in equilibrium with the pulmonary blood gases. Samples were withdrawn for gas analysis at 5-10 min intervals. In three lambs the 02 content of the gas mixture rose (to a maximum of 3-95 %) and the CO2 content fell (to a minimum of 5-4 %) over 5-20 min; in the fourth lamb there was no immediate change in the gas composition. It was concluded that if the foetal lungs were ventilated with a gas mixture containing 3 % 02 or less and 6-8 % CO2 in N2, this should cause only small changes in arterial blood gas tensions. Measurements of pulmonary arterial pressure-flow curves, accompanied by blood gas analyses, were made on fifteen lambs which were judged to be in a steady state, before and after either ventilation with 7 % CO2 in N2 or (in two lambs) rebreathing 3 % 02 with 7 % CO2 in N2. In every lamb there was an immediate increase of pulmonary blood flow and a small fall of pulmonary arterial pressure when ventilation was begun. Not too much weight can be put on this immediate change, because, although the tracheal cannula and connecting tubes were flushed

9 VASCULAR RESISTANCE OF LUNG 69 with the gas mixture to be used for ventilation (during the removal by suction of pulmonary fluid), we cannot be entirely certain that some air may not yet have entered (into the bronchi, for instance) and ventilation with air causes a further fall in pulmonary vascular resistance (see TABLE 1. Effect of ventilation with 7 % C02 in N2, or of rebreathing 3% 02 and 7 % CO2 in N2, on pulmonary vascular resistance and arterial gas tensions in fifteen foetal lambs Ventilated with N2 and C02 (13 lambs) Lungs Unexpanded or rebreathing (2 lambs) Vascular conductance/kg (ml./min.mm Hg) Pressure intercept (mm Hg) Arterial Pco * * Arterial Po2t 21 +l1 17 +Lt The values given are means + S.E. * Difference not significant. t In thirteen lambs. I Difference highly significant (P < 0-001). 30 Ventilated with C02 in N2 Foetal I/ / e r 4~// ~/ Arterial pressure (mm Hg) Fig. 7. The mean left pulmonary vascular conductances per kilogram and pressure intercepts were measured in fifteen foetal lambs before and after ventilation of the lungs with 7 % 002 and N2 (see text) in order to construct the mean pressure-flow diagrams shown above ( ). In eight lambs in which the initial conductance per kilogram was < 1 0 (.---) the decrease in vascular resistance on ventilation was greater than in the other seven in which conductance per kilogram had been below). When flow and pressure became steady, 5-10 min after ventilation was begun, 2 or 3 more pressure-flow curves were taken for comparison with those previously obtained in the foetal state. They all indicated that pulmonary vasodilatation had taken place, often of very considerable

10 70 S. CASSIN AND OTHERS size. Table 1 shows that, on the average, vascular conductance had increased 78 %, while the pressure intercept had fallen 34 %. The effect of these changes on the mean pressure-flow curves is illustrated in Fig. 7 ). The increase in vascular conductance on ventilation was relatively greater in eight lambs with a low conductance per kilogram initially (< 1-0 ml./min. mm Hg ) than in seven lambs with a higher conductance (-- -). After ventilation the mean conductances of the two groups were not significantly different. Table 1 shows that ventilation with the gas mixture was accompanied by a small fall in arterial Po2 and an insignificant rise in Pco,. The relative changes in blood gas tensions were similar in the two groups of lambs. After ventilation with 7 % CO2 in N2, in nine lambs an attempt was made to return the lung to an airless condition similar to that in the foetal state, by ventilation with mixtures of gases which are readily absorbed (02, CO2 and/or N20). Ventilation was then stopped and the lungs were allowed to collapse at atmospheric pressure. Their external appearance after ventilation was not identical with that before ventilation; some gas appeared to be retained. In only one experiment did the vascular conductance fall below that observed during ventilation with 7 % CO2 in N2, though it always decreased to this level. It was concluded that this procedure was unlikely to cause the lung to return to the true foetal condition under these experimental circumstances. Ventilation with N2, air and CO2 mixtures Some of the experiments were designed to determine to what extent the composition of the gas used to ventilate the lungs would alter pulmonary vascular conductance. Observations with one gas mixture were bracketed between two observations with another gas mixture in order to detect otherwise spontaneous changes in vascular conductances, and all measurements were recorded in a steady state, at least 5-6 min after changing the gas mixture. Figure 8 shows the pressure-flow curves obtained during part of an experiment. Ventilation with N2 caused an increase in vascular conductance greater than that on ventilation with 7 % CO2 in N2, but less than that observed with air. In each of seven lambs either a decrease in the CO2 content of the ventilating gas (from % to zero), or an increase in its 02 content (from less than 0-8 % to *7 %) both caused an increase in vascular conductance and a fall in the pressure intercept. In order to give a quantitative picture of these effects, the mean conductances per kilogram and the mean pressure intercepts have been calculated for the unventilated foetal lung and for each gas mixture, and these are shown in Fig. 9. Removal of CO2 from the

11 VASCULAR RESISTANCE OF LUNG 71 ventilating gas mixture caused as large a fall in pulmonary vascular resistance in the presence of 02 as it did in its absence (Table 2). During these experiments the lamb was still connected to its mother by an intact umbilical cord, and umbilical blood flow must have been large relative to pulmonary flow, since when the ventilating gas mixtures Arterial pressure (mm Hg) Fig. 8. Foetal lamb, 138 days gestation, 3-44 kg. Left pulmonary pressure-flow diagrams traced from an X- Y recorder in the foetal condition (nos. 4 and 5) on ventilation with 7 % CO2 and N2 (nos. 8 and 9), with N2 only (nos. 6, 7, 10 and 11), with air (nos. 12 and 13) and with the cannula removed from the left pulmonary artery (open system). were replaced with 7 % CO2 in N2 the arterial PO, only fell to values a little below those observed in the foetal state before ventilation was begun. The effects of ventilation with different gas mixtures upon the arterial Po2 and PCOS are shown in Table 2. Ventilation with N2 or with air caused a fall in arterial Pco, by more than 10 mm Hg, but ventilation with air (whether with or without C02) caused only a small rise in Po2. This is probably accounted for by admixture in the left atrium with large

12 72 S. CASSIN AND OTHERS bo X t ^ ~+l +l +l +l +l e _ g o0 t- " t Q oq tom c0 9 r% X6 > Q Q C 0 Q n +1 +l+l +l+l CD "- e= = 00 ox = - X X> t> i~~~~~a C) R 3.., > X o oo oo~~~ b + l l +l+d CD so X >oxb + t m14 e: en o ~ ~~~~C) Cae oo~ ~ D- P z

13 VASCULAR RESISTANCE OF LUNG 73 quantities of less well oxygenated blood, derived in part from the umbilical circulation through the placenta. As in the unventilated foetal lung, so in the ventilated lung there was a definite relation between pulmonary vascular conductance and arterial 40t- Ventilated with air 301F ho E Z-A2 E 0 10 F Arterial pressure (mm Hg) Fig. 9. The mean left pulmonary vascular conductances per kilogram and pressure intercepts were used to construct mean pressure-flow diagrams for seven lambs in the foetal condition, and after ventilation with different gas mixtures. Pco, and Po,. a I I Regression analysis of thirty-five measurements on the seven lambs gave the equation: conductance/kg = Po Pco0, (3) where the residual error was , and standard errors for the regression constants for Po, and Pco, were and respectively. Both regression constants were significantly different from zero (P < 0 01). Comparing equations (2) and (3), the regression constants for PO, are identical and those for Pco, are not significantly different. However, the remaining constant is significantly different (P < 0 001). 60

14 74 S. CASSIN AND OTHERS Ventilation of the right lung only In four lambs the right lung (in one the right upper lobe only) was ventilated with gas mixtures of different compositions, while pressureflow curves were generated from the left unventilated lung. In each lamb changes in vascular conductance in the unventilated lung were observed, 400 Open system Ventilated Arterial 9 with air P234 2 (mm Hg) Ventilated with 300 // 7% CO2 In N2 12 E~~~~~~~~~~~~~~ E Arterial ioop0 (mm Hg) Pco. (mm Hg) Arterial pressure (mm Hg) Fig. 10. Foetal lamb, 140 days gestation, 4-01 kg. Left pulmonary pressure-flow diagrams traced from an X- Y recorder on ventilation of the right lung only with 7% CO2 in N2 (nos. 7, 8, 12 and 13) or air (nos. 9, 10 and 11), and also with the cannula removed from the left pulmonary artery (open system). The changes in arterial P0, and Pco2 are indicated. which varied with the composition of the gas mixture used. The changes in conductance were related to small variations in arterial blood gas tensions due to admixture, in the left atrium, of blood from the ventilated right lung and from the unventilated left lung, with blood returning from the placenta through the foramen ovale. A small increase in PO, or fall

15 VASCULAR RESISTANCE OF LUNG75 in Pco,, or both, in the arterial blood (which also supplied the left pulmonary artery) caused a rise in conductance and a fall in pressure intercept. Figure 10 shows an experiment in which a simultaneous rise in arterial PO, and a fall in PCO2 caused a relatively large decrease in the vascular resistance of the left unventilated lung. Regression analysis of twenty-five measurements on these four lambs gave the equation: conductance/kg = Po Pc2, (4) where the residual error was 0-14, and standard errors for the regression constants for Po, and Pco, were and respectively. Both regression constants were significantly different from zero (P < 0 01). DISCUSSION Changes in vascuktr resistance on ventilating the foetal lungs The experiments show that rhythmic ventilation of the foetal lungs with a gas mixture containing 7 % CO2 in N2 causes pulmonary vasodilatation. Ventilation with this gas mixture led to a small fall in arterial PO, but no change in Pco2. In other experiments in unventilated lungs (Dawes & Mott, 1962), and when the right lung only was ventilated, a reduction in arterial Po2 caused pulmonary vasoconstriction. Hence the small changes in blood gas tensions in these experiments could not account for the pulmonary vasodilatation which was observed on ventilation. Comparison of equations (2) and (3) leads to the same conclusion. The regression constants for Po2 and Pco, in these equations are not significantly different, but the residual constants are different. This difference provides a quantitative measure of the decrease in pulmonary vascular conductance on ventilation of the lungs, independent of changes in Po2 and Pco,, which amounts to *82 = 0-60 ml./min.mm Hg. kg. There was also the possibility that local changes in gas tensions might have taken place within the lung. Yet vasodilatation was also observed in the rebreathing experiments in which the ventilating gas mixture came into equilibrium with the lungs. And ventilation hypoxia would be expected to cause vasoconstriction, as in the isolated lung (Born et al. 1955). It is interesting that the effect of ventilation with 7 % CO2 in N2, or of rebreathing, was relatively greater in lambs whose pulmonary vascular conductance was low to start with (Fig. 7). After ventilation vascular conductance increased to the same value in all lambs, whether it had been low or high initially. The mechanism by which pulmonary ventilation causes a fall in vascular resistance is still obscure. It could be a direct mechanical effect upon the

16 76 S. CASSIN AND OTHERS lung, or a nervous reflex. If it is a direct mechanical action (for example, distension or uncoiling of the smaller blood vessels as the lung is expanded) we should need to explain how it is that equal or greater vasodilatation is seen on injection of acetylcholine or histamine (Dawes & Mott, 1962) or in response to changes in blood gas tensions in the unventilated lung. It could be argued either that these agents cause vasodilatation in channels which are not contorted, or that expansion of the foetal lungs relaxes the vascular spasm by mechanical means. We also have to explain how it is that stepwise inflation and deflation of the lung with nitrogen did not cause any considerable vasodilatation in the experiments of Cook et at. (1963), although the lung on deflation contained gas at atmospheric pressure. Figure 9 and equation (3) show the consequences of ventilating the foetal lungs with different gas mixtures, and Table 2 shows that ventilation with a gas mixture containing no CO2 will cause a fall in arterial Pco,, and hence pulmonary vasodilatation. The vasodilatations caused either by lowering the CO2 content (by 7 %) or by raising the 02 content of the gas mixture (by 21 %) are of about the same size, and are additive. But when expressed in terms of arterial gas tensions, equations (2) and (3) both show that a 10 mm Hg fall in PCo2 or rise in Po, causes about the same increase in pulmonary vascular conductance. After birth the arterial PO, rises from, say 25, to 80 mm Hg or more; changes in PCo2 are less certain. In the present experiments few measurements were made at a Po2 > 40 mm Hg and we cannot predict what will be the effect of a further increase on pulmonary vascular conductance. The additional vasodilatation on ventilating with air in place of N2 is small when compared with that seen on first ventilating the foetal lungs with N2. This fact is consistent with previous observations. The mechanisms by which changes in the 02 and CO2 content of the ventilating gas mixtures cause alterations in pulmonary vascular resistance are uncertain. Previous experiments (Dawes & Mott, 1962) had shown that small changes in the arterial 02 saturation of the foetus caused large changes in the vascular resistance of the unexpanded foetal lung. In the present paper, also, small changes in arterial Po2 and Pco2, arising naturally (equation (2)), or caused by varying the composition of the gas mixture ventilating the right lung (equation (4)), caused alterations in the vascular resistance of the unexpanded left lung qualitatively similar to those observed when both lungs were ventilated (equation (3)). Changes in PO, and Pco2 might affect pulmonary blood vessels directly (either from the alveoli or arterioles) or indirectly by actions elsewhere in the body, e.g. through the chemoreceptors. The fact that the regression constants for both 02 and CO2 were not significantly different in measure-

17 VASCULAR RESISTANCE OF LUNG 77 ments on the unventilated foetal lungs (equation (2)) and on ventilated lungs (equation (3)) suggests that local changes in alveolar gas tensions are considerably less important than changes in arterial gas tensions in determining pulmonary vascular resistance. When air was substituted for N2 as the ventilating gas the Po, of the inspired gas was raised by 150 mm Hg, but that of the arterial blood rose only 17 mm Hg (Table 2); equations (2) and (3) show that the responses of the pulmonary vessels were quantitatively similar, whether this increase in arterial PO, was effected by pulmonary ventilation or otherwise. Nevertheless, it should be borne in mind that these experiments were not primarily designed to study the mechanisms by which changes in Po2 and PCO2 alter pulmonary vascular resistance. Substitution of air for N2 as the ventilating gas in the isolated perfused lungs of newly delivered lambs caused vasodilatation (Born et al. 1955) as in adult mammals. Changing the Po2 and PCO2 of the blood supply to isolated perfused unventilated foetal lungs (in unpublished experiments) has sometimes caused small alterations in vascular resistance, but the results have not been consistent. There are, therefore, a number of possibilities to be investigated. The experiments in which the right lung only was ventilated gave an equation (4) with constants significantly different from those of equations (2) and (3). This was not wholly surprising, since the dissection required to isolate and divide the left bronchus was extensive, and involved the destruction of nerves and small blood vessels in the vicinity. These could have included efferent nerves to the unventilated left lung in which pressure-flow curves were measured. Nevertheless, equation (4) shows that there was a highly significant relation between pulmonary vascular conductance in the unventilated lung and arterial Po2 and Pco,, under conditions in which the Po2 was varied from 11 to 64 mm Hg and the PcoS from 24 to 53 mm Hg. This was a much wider range of PO, than that observed in the twenty-two unventilated lambs from which the data used to derive equation (2) were obtained. Spontaneous changes in pulmonary flow in the foetal lung The observations on seemingly spontaneous changes in pulmonary blood flow are particularly interesting, because they demonstrate that vascular resistance in the foetal lungs is not solely determined by the arterial Po2 or Pco.. Similar changes, usually smaller in size, were seen during a previous series of experiments, in some of which the left pulmonary artery was divided and the cut ends rejoined through a flowmeter (Dawes & Mott, 1962). The phenomenon is therefore not dependent on the unusual, but convenient, circulatory arrangement used in the present experiments, in which the left pulmonary artery was supplied from the

18 78 S. CASSIN AND OTHERS left carotid. Such changes were not observed in earlier experiments under Dial-urethane anaesthesia (Dawes et al. 1953). Not only was the anaesthesia deeper, but the exposure was greater and the preparation slower than in the present experiments, partly because the density flowmeter was a clumsier instrument than the electromagnetic flowmeter which is now used. We do not know to what extent delivering the foetus, handling it and opening the chest, have altered pulmonary blood flow and vascular resistance. It has always been recognized that the functional model of the foetal circulation which has been built up from previous experiments is tentative, and dependent on more information about the mechanisms which alter the distribution of cardiac output in the foetus. The present experiments serve to emphasize the caution needed in drawing general conclusions, for it is now evident that pulmonary blood flow is not always low in the foetus. In four of the present experiments it rose, for a short while, to ml./kg. min through the left lung alone. We may suppose that, if flow through the right lung had risen to the same extent, total pulmonary flow would be ml./kg. min. For comparison, mean umbilical blood flow in a series of mature foetal lambs, heparinized and under light chloralose anaesthesia, was 177 ml./kg.min (Dawes, 1962) and right heart output is commonly ml./kg.min in the first 10 days from birth (Cross, Dawes & Mott, 1959). SUMMARY 1. The mean 02 dissociation curve was calculated on blood from mature foetal lambs, delivered by Caesarean section. The umbilical cord was intact in all experiments. 2. Large, unexplained changes in pulmonary blood flow and vascular resistance were often seen in the unexpanded lungs of individual foetal lambs, with no significant change in arterial PO, or Pco,. 3. Multiple regression analysis of data from twenty-two lambs indicated that lambs with a higher Pco2 and lower P0X tended to have a higher pulmonary vascular resistance. 4. First ventilation of the lungs with 7 % CO2 in N2 caused a fall in pulmonary vascular resistance with a small decrease in arterial Po2 and no significant change in Pco,. 5. Ventilation with N2 caused a further fall in vascular resistance and a decrease in arterial Pco,; ventilation with air caused a still further fall in vascular resistance and a rise in Po2. Addition of 7 % C02 to the ventilating gas caused about the same degree of vasoconstriction in the presence of air or of N2. 6. Ventilation of the right lung only (with gas mixtures which caused changes in the arterial Po2 or Pco2 similar to those observed when both

19 VASCULAR RESISTANCE OF LUNG 79 lungs were ventilated) led to similar changes in vascular resistance in the unventilated left lung. 7. It was concluded that the vascular tone of the foetal lung is variable, and that it is controlled by other factors as well as the arterial PCO, and PO. We are grateful to Dr E. Stoneman, E. Bernard, A. Ryder and A. Stevens for help with the experiments. REFERENCES BARCROFT, J. (1946). Researches on Pre-natal Life. Oxford: Blackwell. BARRON, D. H. (1951). Some aspects of the transfer of oxygen across the syndesmochorial placenta of the sheep. Yale J. Biol. Med. 24, BARRON, D. H. & MESCHIA, G. (1954). A comparative study of the exchange of the respiratory gases across the placenta. Cold. Spr. Harb. Symp. quant. Biol. 19, BORN, G. V. R., DAWES, G. S. & MoTT, J. C. (1955). The viability of premature lambs. J. Physiol. 130, BORN, G. V. R., DAwEs, G. S., MoTT, J. C. & RENNICK, B. (1956). The constriction of the ductus arteriosus caused by oxygen and by asphyxia in newborn lambs. J. Physiol. 132, COOK, C. D., DRINKER, P. A., JACOBSON, H. N., LEVISON, H. & STRANG, L. B. (1963). Control of pulmonary blood flow in the foetal and newly born lamb. J. Phy8iol. 169, CROSS, K. W., DAWES, G. S. & MoTT, J. C. (1959). Anoxia, oxygen consumption and cardiac output in new-bom and adult sheep. J. Phy8iol. 146, DAWES, G. S. (1962). The umbilical circulation. Amer. J. Obstet. G(ynec. 84, DAWES, G. S. & MorT, J. C. (1962). The vascular tone of the foetal lung. J. Physiol. 164, DAWES, G. S., MoTT, J. C., WIDDICOMBE, J. G. & WYATT, D. G. (1953). Changes in the lungs of the newborn lamb. J. Phy8iol. 121, LLOYD, B. B. (1958). A development of Haldane's gas-analysis apparatus. J. Physiol. 143, 5 P. MESCHIA, G., HELLEGERS, A., BLECHNER, J. N., WOLKOFF, A. S. & BARRON, D. H. (1961). A comparison of the oxygen dissociation curves of the bloods of maternal foetal and newborn sheep at various phs. Quart. J. exp. Physiol. 46, SEVERINGHAUS, J. W. & BRADLEY, A. F. (1958). Electrodes for blood P02 and pco2 determination. J. appl. Phy8iol. 13, WYATT, D. G. (1961). A 50 c/s cannulated electromagnetic flowmeter. Electron. Engng 33,