Summary. This study compared the continuous positive airways pressure mode of the demand valve system of the Engstrom Erica ventilator with a custom-made ...
Anaesthesia, 1994, Volume 49, pages 391-393
Effect of weaning on oxygen consumption and cardiovascular function A comparison of continuous flow and demand valve systems P. C. IP YAM, I. R. APPADURAI
AND
W. J. KOX
Summary This study compared the continuous positive airways pressure mode of the demand valve system of the Engstrom Erica ventilator with a custom-made continuousflow continuous positive airways pressure system in terms of the oxygen cost of breathing during weaning from mechanical ventilation. Ten consecutive patients in our intensive care unit, with thermodilution pulmonary artery Jotation catheters in situ, were studied. Measurements were carried out under steady-state conditions, initially when breathing spontaneously with continuous positive airways pressure via the Erica and then when transition to the continuousflow system was achieved. There were no significant diferences between the two methods of providing continuous positive airways pressure in terms of the measured and derived physiological variables studied, with the exception of oxygen consumption. Oxygen consumption with the continuous flow system was significantly less than with the Erica (142.8 ( S E M 31.4) ml.min-’.m-’ compared with 165.8 (SEM 30.5) ml.min-’.m-’, p < 0.05). This diference reflects the reduced oxygen cost of breathing when the custom-made continuous flow system was used during weaning.
Key words Ventilation;continuous positive-pressure breathing. Oxygen; consumption.
In critically ill patients with septicaemia, the increased oxygen requirements demand increased oxygen delivery and a reduction in the metabolic demands of tissues. Respiratory muscles in particular can utilise large amounts of oxygen when the work of breathing is increased. A reduction in the work of breathing leads to a reduced oxygen cost of breathing, thus avoiding ‘oxygen steal’ from nonrespiratory tissues and consequent impairment of vital organ function. Continuous positive airways pressure (CPAP) is a widely used technique for respiratory support in patients with respiratory failure. With CPAP, a significant decrease (25%) in the work of breathing has been demonstrated in babies [l], and a similar decrease suggested in adults [2]. To minimise the work of breathing during CPAP, the airway pressure must be maintained at a near constant value throughout the respiratory cycle [3]. It is also important
that the time delay between the instigation of the patient’s inspiratory effort and the onset of inspiratory gas flow is minimal, thus avoiding an intrapulmonary pressure drop which will greatly increase the work of breathing. A survey of 55 major intensive care units in the United Kingdom in 1984 found that this physiological goal was achieved in 82% of units by custom-made CPAP circuits, while 78% had ventilators equipped with a CPAP facility [4]. Of the CPAP systems investigated, the most acceptable adult CPAP delivery system was the traditional custom-made type with a high flow (greater than 301.min-’), a large inspiratory reservoir, and a water bottle. These considerations led us to compare the CPAP mode of the Engstrom Erica ventilator with a custom-made continuous flow CPAP system in terms of measured and derived haemodynamic variables and oxygen consumption and delivery during weaning from mechanical ventilation.
P.C. Ip Yam, FRCA, Lecturer, Department of Anaesthesia, University of Liverpool, I.R. Appadurai, FRCA, Lecturer, Department of Anaesthetics, University of Wales College of Medicine, W.J. Kox, MD, PhD, Professor of Anaesthesiology, University of Essen, Germany. An abstract of this work was presented in poster form at the Annual Scientific Meeting of the Association of Anaesthetists of Great Britain and Ireland in Bournemouth, September 1992. Accepted 2 March 1993. 0003-2409/94/050391+ 03 $08.00/0
@ 1994 The Association of Anaesthetists of Gt Britain and Ireland
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.P.C.Ip Yam, I.R. Appadurai and W.J. Kox nised intermittent mandatory ventilation (SIMV) to spontaneous breathing with CPAP through a tracheal tube. Measurements were carried out under steady-state conditions, initially when the patients started to breathe spontaneously with CPAP via the Erica (Engstrom, Sweden) ventilator, and then when transition to the Bird continuous flow CPAP system was achieved. The same levels of CPAP and inspired oxygen were used and no alterations were made to therapy. Inspiratory assist on the Erica was set at 5 cmH,O to offset the internal trigger resistance. Heart rate, mean arterial pressure, central venous pressure, mean pulmonary artery pressure and pulmonary artery occlusion pressure were recorded. Arterial and mixed venous blood were sampled and oxygen saturations determined by co-oximetry (482 co-oximeter, Instrumentation Laboratories Ltd., UK). Cardiac and left ventricular stroke work indices, pulmonary and systemic vascular resistances and oxygen delivery and consumption were derived. Variables were submitted to analysis by Student's paired 2-test and differences were considered significant if p < 0.05.
Oxygenblender
Bird mark VIII high
Fig. 1. Custom-made CPAP system.
Patients and methods In the custom-made CPAP system [5], a Bird ventilator generated a continuous flow of approximately 40 1.min-I with a 6 1 reservoir bag positioned between hinged, springloaded clapper boards to provide inspiratory support. Gas was supplied through a T-piece and CPAP was provided by an underwater pipe (Fig.1). Local ethics committee approval was obtained for this study. Ten consecutive, haemodynamically stable patients on our intensive care unit were studied (Table 1). In all patients, a pulmonary artery flotation catheter had been inserted and all satisfied the generally accepted criteria for weaning [6]. They all progressed successfully from synchro-
Results
There were no significant differences in physiological measurements between the two methods of providing CPAP, with the exception of oxygen consumption (Table 2). When the custom-made continuous flow system was used, oxygen consumption (142.8 (SEM 3 1.4) ml.min-'.m-*) was significantly less than that associated with the use of the Erica (165.8 (SEM 30.5) ml.min-'.m-2
Table 1. Characteristics of patients studied. Patient No. 1
2 3 4 5
6 7 8 9 10
Sex
Age (years)
Weight (kg)
Height (cm)
M F F M M M F F F M
80 65 68 68 75 65 48 52 45 67
74 60 69 75
175 164 162 170 170 159
85
70 59 84 65 70
CPAP (cmH20)
Admission diagnosis Pulmonary contusion Pneumococcal pneumonia Post-op. aortic aneurysm Post-op. aortic aneurysm Laparotomy for peritonitis Pen-operative MI Post-op. cerebral aneurysm Laparotomy for peritonitis Pelvic clearance Post-TURP haemorrhage
164
160 165 169
5
7.5 5
5 5 7.5 5 7.5 5 5
MI, myocardial infarction; TURP, transurethral resection of the prostate. Table 2. Comparison of physiological measurements. Demand flow system Measurements
Mean
Cardiac index; l.min-'.m-2 Mean arterial pressure; mmHg Left ventricular stroke work index; g.m.m-2 Systemic vascular resistance index; dyn.s.cn-s.m-2 Pulmonary vascular resistance index; dyn.~.m-~.m-~ Pulmonary artery occlusion pressure; mmHg Mixed venous oxygen saturation; % Oxygen consumption; ml.min-1.m-2 Oxygen delivery; ml.min-l.m-2 Mixed venous lactate; mmol.1-a
4.65 100.0 70.4
*p < 0.05 @aired Student's 1-test).
1588 183 12 76 165.8 748 0.66
Continuous flow system Mean
(0.65) (16.4) (21.7) (477) (66.4) (3.5) (5.17) (30.5) (86) (0.42)
4.52 92.9 67.6 1494 177.5 12 78 142.8 712 0.69
(0.72) (15.8) (27.9) (384) (38) (3.2) (5.26) (31.4)* (81)
(0.39)
Efect of weaning on oxygen Consumption and cardiovascular function p < 0.05). Under steady-state conditions, this difference in oxygen consumption between the two methods of providing CPAP reflects the oxygen cost of breathing [A.
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tages of a continuous flow CPAP system in terms of the oxygen cost of breathing during weaning. Acknowledgment
Discussion
These results demonstrate the reduced oxygen cost of breathing during weaning with CPAP using the custommade system when compared with the Erica ventilator. In healthy subjects, the oxygen cost of breathing, defined as the percentage of total oxygen consumption used by the muscles of ventilation, is 5% or less [7]. In patients with impaired pulmonary function, the oxygen cost of breathing may increase fivefold [8]. The mode of ventilation also affects the oxygen cost of breathing [9]. Our results show that the oxygen cost of breathing depends also upon the type of circuit used, even when the same level of CPAP is employed. The design of the Erica ventilator is representative of the efforts made by manufacturers to overcome the technical disadvantages of demand valve systems by providing pressure support modes. Although this reduces the inspiratory work of breathing, and thus oxygen consumption, [9, 101 the design of the valve remains critical if no time delay is to occur before the onset of gas flow. The superiority of continuous flow systems has been demonstrated in an investigation of different ventilators including the Erica ventilator [l 11. This is confirmed in the present study. It may be postulated that when oxygen consumption of the respiratory muscles assumes an inordinately high fraction of total oxygen consumption, nonrespiratory tissues may obtain insufficient oxygen to maintain normal metabolic function. This leads to tissue hypoxia, impaired vital organ function and, ultimately, failure of the weaning process. A significant exponential correlation exists between the oxygen cost of breathing as a percentage of total oxygen consumption during spontaneous ventilation, and the duration of weaning [12]. Our results show that the oxygen consumption is lower using a custom-made continuous flow CPAP system when compared with the demand valve system incorporated in the Engstrom Erica ventilator and emphasise the advan-
We thank the nursing staff of the intensive care unit, Charing Cross Hospital for their understanding and support during this study. References [I] COGSWELLJJ, HATCHDJ, KERR AA, TAYLOR B. Effects of continuous positive airway pressure on lung mechanics of babies after operation for congenital heart disease. Archives of Diseases in Childhood 1975;5 0 799-804. [2] VENUSB, JACOBSHK, LIM L. Treatment of the adult respiratory distress syndrome with continuous positive airway pressure. Chest 1979;7 6 257-61. S, PETERSRM, VIRGILIORW. Mechanical work on [3] GHERINI the lungs and work of breathing with positive end expiratory pressure and continuous positive airway pressure. Chest 1979; 7 6 251-6. [4] Cox D, NIBLETTDJ. Studies on continuous positive airway pressure breathing systems. Brirish Journal of Anaesrhesia 1984; 56: 905. [5] Kox WF. The physiological basis of ventilatory and respiratory support. In: Kox WJ, BIHARID, eds. Shock and the adult respiratory distress syndrome. London: Springer-Verlag, 1988: 139-52. (61 BROWNEDRG. Weaning patients from ventilators. Hospital Update 1988; 14 1809-18. [7] CAMPBELL EJM, WFSTLAKEEK, CHERNIACK RM. The oxygen consumption and efficiency of the respiratory muscles of young male subjects. Clinical Science 1959; 18: 55-64. [8] FIELD S, KELLY SM, MACKLEMPT. The oxygen cost of breathing in patients with cardiorespiratory disease. American Review of Respiratory Diseases 1982; 126 9-13. [9] KANAKR, FAHEY PJ, VANDERWARF C. Oxygen cost of breathing. Changes dependent upon mode of mechanical ventilation. Chest 1985;87: 1267. [lo]BROCHARD L, PLUSKWA F, LEMAIRE F. Improved efficacy of spontaneous breathing with inspiratory pressure support. American Review of Respirarory Diseases 1987; 136: 41 1-15. [Ill Cox D, TINLOISF, FARRIMOND JG. Investigation of the spontaneous modes of breathing of different ventilators. Intensive Care Medicine 1988; 1 4 532-7. [12] MCDONALDNJ, LAVELLEP, GALLACHER WN, HARPINRP. Use of the oxygen cost of breathing as an index of weaning ability from mechanical ventilation. Intensive Care Medicine 1988; 14: 50-4.
