COMPARISON OF PORTABLE EMERGENCY VENTILATORS USING A LUNG MODEL

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British Journal of Anaesthesia 1993; 70: 2-7 APPARATUS COMPARISON OF PORTABLE EMERGENCY VENTILATORS USING A LUNG MODEL L. ATTEBO, M. BENGTSSON AND A.

1 British Journal of Anaesthesia 1993; 70: 2-7 APPARATUS COMPARISON OF PORTABLE EMERGENCY VENTILATORS USING A LUNG MODEL L. ATTEBO, M. BENGTSSON AND A. JOHNSON SUMMARY A lung model was used to test the performance of five emergency ventilators (MEDUMA T Elektronik, Variabel, OSIRIS, and rescupac 2DM). The model comprised two glass jars filled with water to suitable compliances and connecting tubes. A resistance (obstruction) was added to one of the "bronchial" tubes in order to simulate a patient with obstructive lung disease. Preset minute volume was compared with measured minute volume and the gas distribution produced by the different ventilators. Acceptable performance was found with the Elektronik, Variabel, OSIRIS and ventilators. (Br. J. Anaesth. 1993; 70: 2-7) KEY WORDS Equipment, emergency ventilators. Ventilation: lung mode/. There is a need for simple, easy to handle, portable emergency ventilators capable of ventilating the lungs of intensive care patients. The performance of earlier ventilators has been described previously [1, 2]. The purpose of the present study was to evaluate the performance of five newer portable ventilators in a lung model in which resistance and compliance could be changed in order to simulate both healthy and diseased lungs. METHODS Five gas-powered portable ventilators were studied ( Elektronik, Variabel, OSIRIS, and rescupac 2DM) (table I). Correct function of the ventilators was assured by either the manufacturers or the technical department of our hospital. The fresh gasflowsused were within the manufacturers' recommendations. Ventilatory frequencies ( and b.p.m.) and minute volume (F) settings (6, 9 and litre min" 1 ) typical of clinical practice were chosen, with the exception of two ventilators ( and rescupac 2DM) which had features that made such settings more difficult. The ventilator was therefore set to 10, 15, 25 and 35 b.p.m. and 3, 7, 10, 15 and 20 litre min- 1. The rescupac 2DM ventilator could only be set by tidal volume (FT) with consequent automatic adjustment of ventilatory frequencies. Thus FT was set at 300, 400, 500, 800, 900, 1100 and 1450 ml with ventilatory frequencies 22, 20,, 14, 13, 11 and 9 b.p.m. and calculated F 6.6, 8, 9, 11.2, 11.7,.1 and 13.1 litre min"'. One ventilator (rescupac 2DM) had varying I:E ratios which depended on the FT setting. Three different settings of i:e could be chosen with the Elektronik; the other ventilators had fixed I:E ratios (table I). The ventilator to be tested was connected to a twocompartment lung model as has been described and used previously in other studies [1, 3-5]. The lung model consisted of two 60-litre glass bottles ("lungs") each connected to a ("bronchial") tube and with a Y-piece attached to a common ("tracheal") tube. The bottles werefilledpartly with water to obtain a compliance of 35 ml/cm H 2 O for FT 1 litre. One bronchial tube could be occluded partially with a resistance [6] of 200 cm H 2 O litre" 1 s" 1 ("bronchial obstruction"). Tracheal pressure and the pressures in the two lung compartments were measured with pressure transducers. Tracheal gas flow was measured with a pneumotachograph and a differential pressure transducer. Signals were fed through an amplifier and a computer to a recorder to obtain readings of pressure, flow and FT. A calibrator [7] was used to calibrate the signals. The final gas distribution between the lungs was calculated as a ratio from the pressure curve in each lung (the ratio between the pressure difference in the obstructed lung and the pressure difference in the non-obstructed lung during ventilation). RESULTS Minute volume The relationship between preset and recorded F was almost linear in all ventilators, the exception being the rescupac 2DM (figs 1-4). The Elektronik ventilator at the low pressure setting did not deliver the preset F. When tested without and with the bronchial obstruction and an I: E ratio of 1:2, the mean differences between LENA ATTEBO, M.B., B.S.; MATS BENGTSSON, M.D., PH.D.; ANDERS JOHNSON, M.D., PH.D. ; Department of Anaesthesiology, University Hospital, S Linkoping, Sweden. Accepted for Publication: August 4, Correspondence to A. J.

2 PORTABLE VENTILATORS 3 TABLE I. General characteristics of the ventilators (data obtained from the manufacturers' manuals) General Information Manufacturer Dimensions (mm) (ht x width x depth) Power supply Weight (kg) Functions Minute volume (litre man" 1 ) Ventilatory frequency (b.p.m.) I: E ratio PEEP (cm H,O) Alarm "2 o Elektronik Weinmann Germany 150 x 220 x :1-1:3 As additional equipment Yes: audiovisual for no O,, empty batteries 14 n Variabel Weinmann Germany 85x322x :1.7 OSIRIS L'air Liquide medical France 150 x 245 x (100% O,) 4-30(60% O,) : Yes: audiovisual for minimum pressure 8 10 Preset volume (litre min" 1 ) Dragerwerk Germany 80 x 200 x :1.5 E. High V. High OSIRIS rescupac 2DM E. Low V. Low y= 0.90x rescupac 2DM Pneumatics for Medicine England 92x220x Varying Yes: audible for maximum pressure FIG. 1. Relationship between preset and recorded volume for the ventilators tested without obstruction at b.p.m. The two regression lines (_y = 0.90.x and.y = 1 lox) are drawn to illustrate the relationship between preset volume and recorded volume and represent 90% and 110% of preset volume, respectively. E = Elektronic; V = Variabel. High = High pressure setting; Low = low pressure setting. preset and recorded V were 15% and %, respectively. This difference increased further with obstruction at slower ventilatory frequencies and with a smaller I: E ratio. With this ventilator, different manual settings of I: E ratio could be chosen, which in turn correlated well with the measured i: E ratio at different values of V. When the high pressure setting was used, the mean differences between preset and recorded minute volume were +2% and 10% without and with obstruction, respectively. Again, V was smaller than the preset value when the ventilatory frequency was decreased, bronchial obstruction was present and I: E ratio decreased. Similar results were demonstrated with the Variabel ventilator. On the low pressure setting, the discrepancy between preset and recorded values of V was greater than with Elektronik. The mean difference between preset and recorded V was 27 % without obstruction and 51 % with obstruction (figs 1^4). Again, ventilatory frequency affected the recorded value of V especially with bronchial obstruction. At reduced frequencies the recorded V was less than at increased frequencies. There was a fixed i:e ratio of 1:1.7, which correlated well with the measured I:E ratio. At high pressure settings, recorded V was within % of dialled volume both without and with obstruction. With the OSIRIS ventilator, the recorded V differed from the preset value by a mean of 2 % without obstruction and 8% with obstruction. The fixed I:E ratio of 1:2 correlated well with the measured I:E ratio. With the ventilator, the difference between preset and recorded V was noted to be less

3 4 BRITISH JOURNAL OF ANAESTHESIA 14 i E 10 y= 1.1 Ox ^ K" E. High /OSIRIS i V. High o s 8 6- rescupac 2DM " E. Low ^ V. Low 8 10 Preset volume (litre min" 1 ) FIG. 2. Relationship between dialled and recorded volume for the ventilators tested with obstruction at b.p.m. The two regression lines, abbreviations as shown in figure T I 10 o r 8 - T3 "2 oo Preset volume (litre min" 1 ) y 1.10x y~ 0.90x /TE. High ' OSIRIS >" DUMAT V. High r rescupac 2DM "I E. Low V. Low FIG. 3. Relationship between preset and recorded volume for the ventilators tested without obstruction at b.p.m. The two regression lines, abbreviations as in figure 1. at a greater ventilatory frequency that is, with a smaller tracheal pressure. At a ventilatory frequency of 10 b.p.m. the mean difference between set and recorded V was 8% without obstruction and 25% with obstruction. Increasing the ventilatory frequency to 25 b.p.m. gave a smaller mean difference between preset and recorded V (+ 4 % without obstruction; 10% with obstruction). The discrepancy in Fwas noted to be least when K7-15 litre min" 1 and ventilatory frequency 25 b.p.m. were chosen, i:e ratio was fixed at 1:1.5, according to the manufacturer, and was verified from recordings. With rescupac 2DM, there was a discrepancy between the preset and recorded V which increased as the preset V was increased. This discrepancy increased further when bronchial obstruction was added to the lung model. Recorded V differed from preset V with a mean of 11 % without obstruction and a mean of 26% with obstruction. Furthermore, with the rescupac 2DM the I:E ratio increased from 1:5 to 1:2 as preset V was increased from 6.6 to 13.1 litre min" 1, respectively. Alternation of the I: E ratio occurred automatically and could not be changed manually. Tracheal pressure During inspiration, the tracheal pressures increased linearly with time in all ventilators except when the low pressure setting was used with the Elektronik and Variabel. In the latter cases, the deceleration of tracheal flow was accentuated by tracheal obstruction with a consequent reduction in pressure increase (fig. 5). The greatest tracheal pressures were recorded with obstruction, ventilatory frequency b.p.m. and V litre min" 1 in the following ventilators: Elektronik 42 cm H 2 O (high pressure setting), Variabel 35 cm HjO (high

4 PORTABLE VENTILATORS i I.IOX,y=0.90x E o EC OSIRTS E. High V. High E. Low rescupac 2DM V. Low 8 10 Preset volume (litre mirr 1 ) FIG. 4. Relationship between preset and recorded volume for the ventilators tested with obstruction at b.p.m. The two regression lines, abbreviations as in figure 1. V(litre min" 1 ) fib.p.m.) Elektronik Variabel High Low High Low 10 OSIRIS rescupac 2DM FIG. 5. Tracheal pressure (Press.) andflowcurves for the ventilators tested without and with obstruction. V = Minute volume ;/= ventilatory frequency. For comparison, distances along the abscissa (time) are identical for each ventilator shown. Gain on the ordinate differs between ventilators. High, Low = High and low pressure settings. 1 = n-linear increase in tracheal pressure as a result of 2; 2 = deceleration of trachealflowwas accentuated by tracheal obstruction; 3 = activation of high pressure alarm with resultant plateau of tracheal pressure; 4 = inspiratory flow becomes decelerating because of activation of the high pressure alarm pressure setting) and OSIRIS 48 cm H 2 O. The greatest tracheal pressure recorded with the rescupac 2DM ventilator was 34 cm H 2 O at a ventilatory frequency of 9 b.p.m. and Vi 13.1 litre min" 1 (measured I:E ratio 1.9:4.3) with obstruction. With the ventilator, the greatest recorded tracheal pressure was 74 cm H 2 O at a ventilatory frequency of 10 b.p.m. and Vi of 20 litre min" 1 with obstruction. Flow All ventilators gave a constantflow during inspiration, with the exception of: the Elektronik on the low pressure setting, the Variabel on the low pressure setting and the rescupac 2DM. The flow patterns of these three ventilators warrant further comment. The inspiratory flow of the Elektronik and Variabel became decelerating when low pressure was chosen (fig. 5). This pattern of inspiratory flow was more pronounced with a small I:E ratio and the addition of bronchial obstruction. The reduced driving pressure in the ventilator was the cause for the decelerating flow, as constant flow was observed when the ventilator was set to a greater working pressure. The inspiratory flow of the rescupac 2DM ventilator was constant with and without bronchial obstruction at preset V ^ 9 litre min" 1. However, at V > 9 litre min" 1 with bronchial obstruction, the

5 6 BRITISH JOURNAL OF ANAESTHESIA TABLE II. Final gas distribution in the lung vnth bronchial obstruction expressed as a percentage of the gas distribution to the lung with no obstruction, f = Ventilatory frequency (Jb.p.m.); V = minute volume {litre min~'); Vr = tidal volume (ml). Each value represents a graphical mean of four to six breaths distribution (%) Elektronik Variabel Settings Low High Low High OSIRIS Settings Settings rescupac 2DM /=, V = 6 /=, V = ~ 9 /=, V = i:e ratio 1:3 I:E ratio 1:1 /=, V = 6 /=, V = 9 /=, V = Mean SD Range high pressure alarm was activated and the resultant inspiratory flow became decelerating. At F 13.1 litre min~ l, the high pressure alarm was activated without obstruction. Tracheal pressure of 30 cm H 2 O activated the alarm. distribution The final gas distribution in the lung with bronchial obstruction expressed as a percentage of the gas distribution to the lung with no obstruction is shown in table II. The obstructed lung was never emptied during expiration with any ventilator tested. All ventilators produced a better gas distribution at the slower ventilatory frequencies than at the higher frequencies. The I:E ratio could be varied with the Elektronik and an I: E ratio of 1:1 produced a better distribution (33%) than an I:E ratio of 1:3, with which the distribution was (23 %). The rescupac 2DM ventilator increased the gas distribution to the lung with bronchial obstruction with increasing preset F. distribution increased from 11 % at FT 300 ml to % at FT 1450 ml. This is a result of the ventilator not being able to manage the large tracheal pressures generated at greater FT and the inspiratory flow consequently becoming decelerating. Positive end-expiratory pressure (PEEP 10 cm HjO) did not improve gas distribution in the OSIRIS ventilator. DISCUSSION The lung model used in this study has been previously tested in other studies and shown to be acceptable for studying the ventilatory performance of anaesthesia ventilators [1,3-5]. Adiabatic effects have been disregarded because no discernible temperature difference was observed during the experiment. In addition, final gas distribution was calculated at the end of inspiration when any redistribution (pendelluft) had taken place /=10,K=7 /= 10, V = 10 /= 10, V= 15 /=15, V = l f= 15, V= 10 /= 15, K= y = 11, 13,, 20, 22, Vi = 1450 (K=13.1) VT = 1100 KT^-WO 0 (K=11.7) Vr = 500 (K = 9) KT = 400 \r O) KT = The requirement to ventilate the lungs of intensive care patients during transport demands that these ventilators are capable of delivering predetermined ventilatory parameters in varying conditions of pulmonary pathophysiology. Schapera and coworkers showed in a clinical study on patients with varying pulmonary compliance that a ventilator with a large inspiratory flow capability was preferable to a ventilator without such capability [8]. In the present study, a "bronchial" tube was occluded partially with a resistance in order to simulate a clinical situation. The resistance used has been described previously [6] and found to increase linearly with gas flow. Each ventilator was tested under conditions simulating healthy lungs (i.e. without resistance) and conditions simulating pathological lungs with increased airway resistance and decreased compliance (i.e. with resistance), and with a range of ventilatory frequencies and minute volumes in order to gain an appreciation of their capabilities. We found that these portable devices are far from being equal when challenged with varying ventilatory demands. Ventilation corresponded to preset V with the Elektronik (high pressure), Variabel (high pressure), OSIRIS and ventilators. More reliable minute ventilation was attained at greater ventilatory frequencies. When bronchial obstruction was applied, V decreased by about 10 % in these four ventilators. Bronchial obstruction at decreased ventilatory frequency generally produced a more favourable final gas distribution. On the other hand, both the Elektronik and the Variabel on the low pressure setting could not produce ventilation within % of the preset V. Their performance improved marginally at a greater frequency, but worsened when a bronchial obstruction was applied. The low pressure setting on these ventilators seemed

6 PORTABLE VENTILATORS 7 unreliable and, on the basis of our finding, we would not recommend this setting. The ventilatory frequency on the rescupac 2DM was unpredictable and varied with changing variables, despite a "fixed" setting on the console. It produced poor final distribution at preset V less than litre min" 1. Minute volume settings less than 9 litre min" 1 produced ventilation within % of the preset values in the absence of obstruction. Greater preset minute volumes caused activation of the high pressure alarm, resulting in a decelerating inspiratory flow and reduced ventilation. This was accentuated with bronchial obstruction. We conclude that the Elektronik and Variabel on high pressure settings, and the OSIRIS and ventilators, performed well during the varying conditions in the lung model and should be suitable for artificial ventilation in critically ill patients with lung pathology. The performance of the rescupac 2DM ventilator was not acceptable under the conditions studied. ACKNOWLEDGEMENT The authors are grateful to Paabo Consulting Group AB, Linkoping, Sweden for valuable assistance with the graphics. REFERENCES 1. Dahlgren B-E, Johnson A, Lofstrom JB. Portable emergency ventilators. A lung model study. Acta Anaesthesiologica Scandinavica 1983; 27: Park GR, Manara AR, Bodenham AR, Moss CJ. The pneupac ventilator with new patient valve and air compressors. Anaesthesia 1989; 44: Hedenstierna G, Johansson H. Different flow patterns and their effect on gas distribution in a lung model study. Acta Anaesthesiologica Scandinavica 1973; 17: Johnson A, Bengtsson M. Comparison of anaesthesia ventilators using a lung model. Acta Anaesthesiologica Scandinavica 1990; 34: Hesselvik JF, Bengtsson M, Johnson A. A new ventilator converter with the Siemens Servo Ventilator evaluation in a lung model. Acta Anaesthesiologica Scandinavica 1992; 36: Saklad M, Weyerhaeuser R. The construction of linear resistances for the testing of ventilators. Anesthesiology 1980; 52: Herzog P, rlander OP. A precision method for the dynamic volume flow calibration during pneumotachography. Ada Anaesthesiologica Scandinavica 1966; (Suppl.) 24: Schapera A, Marks JD, Minagi H, Goodman P, Katz JA. Perioperative pulmonary function in acute respiratory failure: Effect of ventilator type and gas mixture. Anesthesiology 1989; 71:

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