Respiratory system compliance decreases after cardiopulmonary resuscitation and stomach inflation: impact of large and small tidal volumes on calculated peak airway pressure¹

Abstract

The purpose of the present study was to evaluate respiratory system compliance after cardiopulmonary resuscitation (CPR) and subsequent stomach inflation. Further, we calculated peak airway pressure according to the different tidal volume recommendations of the European Resuscitation Council (7.5 ml/kg) and the American Heart Association (15 ml/kg) for ventilation of an unintubated cardiac arrest victim. After 4 min of ventricular fibrillation, and 6 min of CPR, return of spontaneous circulation (ROSC) after defibrillation occurred in seven pigs. Respiratory system compliance was measured at prearrest, after ROSC, and after 2 and 4 l of stomach inflation in the postresuscitation phase; peak airway pressure was subsequently calculated. Before cardiac arrest the mean (±S.D.) respiratory system compliance was 30±3 ml/cm H₂O, and decreased significantly (P<0.05) after ROSC to 24±5 ml/cm H₂O, and further declined significantly to 18±4 ml/cm H₂O after 2 l, and to 13±3 ml/cm H₂O after 4 l of stomach inflation. At prearrest, the mean±S.D. calculated peak airway pressure according to European versus American guidelines was 9±1 versus 18±3 cm H₂O, after ROSC 12±2 versus 23±4 cm H₂O, and 15±2 versus 30±5 cm H₂O after 2 l, and 22±6 versus 44±12 cm H₂O after 4 l of stomach inflation. In conclusion, respiratory system compliance decreased significantly after CPR and subsequent induction of stomach inflation in an animal model with a wide open airway. This may have a significant impact on peak airway pressure and distribution of gas during ventilation of an unintubated patient with cardiac arrest.

Introduction

The distribution of ventilation volume between lungs and stomach in an unintubated patient with cardiac arrest depends on variables such as upper airway resistance, respiratory system compliance, and lower oesophageal sphincter pressure. Of equal importance are the differences in technique applied by the healthcare professional while performing basic or advanced airway support, such as head position, inspiratory flow rate and time, and especially, peak airway pressure. The combination of these variables determines gas distribution between the lungs and the oesophagus, and subsequently, the stomach 1, 2.

The respiratory system compliance deteriorates in an awake adult from ∼100 ml/cm H₂O [3]to ∼50–65 ml/cm H₂O 4, 5in anesthetized, paralyzed supine patients. In patients being resuscitated after prolonged periods of cardiac arrest, the respiratory system compliance further declined to 20 ml/cm H₂O [6]. Lower oesophageal sphincter pressure in healthy humans is ∼20 cm H₂O 7, 8, but recent observations in an animal model showed that this variable may decrease rapidly to 5 cm H₂O within the first 5 min of an untreated cardiac arrest [9]. Thus, the respiratory mechanics of a cardiac arrest patient may render gastric inflation during basic life support very likely.

The purpose of the present study was to evaluate the effect of cardiopulmonary resuscitation (CPR) and subsequent gastric inflation on the respiratory system compliance in a laboratory investigation. Recently, the European Resuscitation Council recommended a decrease in the tidal volume during ventilation of an unintubated cardiac arrest victim from 15 ml/kg as recommended by the American Heart Association [10]to 7.5 ml/kg [11], partly in an effort to decrease peak airway pressure. Therefore, we further calculated peak airway pressure with tidal volumes recommended by both organizations in this model.