Influence of tidal volume on the distribution of gas between the lungs and stomach in the nonintubated patient receiving positive-pressure ventilation


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Abstract

Objectives: When ventilating a nonintubated patient in cardiac arrest, the European Resuscitation Council has recently recommended a decrease in the tidal volume from 0.8 to 1.2 L to 0.5 L, partly in an effort to decrease peak flow rate, and therefore, to minimize stomach inflation. The purpose of the present study was to examine the validity of the European Resuscitation Council's recommendation in terms of gas distribution between lungs and stomach in a bench model that simulates ventilation of a nonintubated patient with a self-inflatable bag representing tidal volumes of 0.5 and 0.75 L.Design: A bench model of a patient with a nonintubated airway was used consisting of face mask, manikin head, training lung (lung compliance, 50 mL/cm H2 O; airway resistance, 5 cm H2 O/L/sec), adjustable lower esophageal sphincter pressure (LESP) and simulated stomach.Setting: University hospital laboratory.Subjects: Thirty healthcare professionals.Interventions: Healthcare professionals performed 1-min bag-mask ventilation at each LESP level of 5, 10, and 15 cm H2 O at a rate of 12 breaths/min, using an adult and pediatric self-inflating bag, respectively. Volunteers were blinded to the LESP, which was randomly varied.Measurements and Main Results: Both types of self-inflating bags induced stomach inflation, with higher stomach and lower lung tidal volumes when the LESP was decreased. Lung tidal volume with the pediatric bag was significantly (p < .05) lower at all LESP levels when compared with the adult bag, and ranged between 240 mL at an LESP of 15 cm H2 O and 120 mL at an LESP of 5 cm H2 O. Stomach tidal volume with the adult bag ranged between 250 mL at an LESP of 15 cm H2 O and increased to 550 mL at an LESP of 5 cm H2 O. Stomach tidal volume with the pediatric bag was significantly lower (p < .05) at all LESP levels when compared with the adult bag and ranged between 70 mL at an LESP of 15 cm H2 O and 300 mL at an LESP of 5 cm H2 O.Conclusions: Our data support the recommendation of the European Resuscitation Council to decrease tidal volumes to 0.5 L when ventilating a cardiac arrest victim with an unprotected airway. A small tidal volume may be a better trade-off in the basic life support phase, as this may provide reasonable ventilation while avoiding massive stomach inflation. (Crit Care Med 1998; 26:364-368)When ventilating a nonintubated cardiac arrest patient, the European Resuscitation Council has recently recommended a decrease in the tidal volume from 0.8 to 1.2 L [1] to 0.5 L [2], partly in an effort to decrease peak flow rate, and therefore, to minimize stomach inflation. Ventilation volume has an effect on pH, CO2 elimination, and oxygenation when pulmonary blood flow is extremely low, such as during cardiopulmonary resuscitation (CPR) or shock [3]. The Airway and Ventilation Management Working Group of the European Resuscitation Council [2] stated that ventilating a nonintubated cardiac arrest patient with a smaller tidal volume may be a better trade-off in order to provide reasonable ventilation, while avoiding massive stomach inflation that may result in life-threatening pulmonary complications.The gas distribution between lungs and stomach during positive-pressure ventilation in a nonintubated airway depends on patient characteristics (lower esophageal sphincter pressure [LESP], airway resistance, and respiratory system compliance) and performance variables of the rescuer applying positive-pressure ventilation (tidal volume, peak flow rate, and upper airway pressure). Thus, assessing the above-mentioned recommendation in a clinical study is difficult to perform due to many confounding variables that are difficult to control and to evaluate when emergently managing therapy during CPR.As an example of the usefulness of laboratory models of ventilation, the American Heart Association [1] recommended increasing inspiratory time when ventilating a nonintubated cardiac arrest patient [1,4]. For example, such a model has the advantage that each variable of respiratory mechanics can be controlled and adjusted to investigate a certain hypothesis. We [4,5] previously studied the effect of tidal volume and peak flow rate on gas distribution between lungs and stomach during mechanical positive-pressure ventilation, using a modification of the earlier described bench model of an unprotected airway. Although a mechanical ventilator is a valuable tool to study gas distribution in an unprotected airway, a self-inflatable bag is the device usually used by the emergency medical service and in the hospital during the initial care of a cardiac arrest victim.Thus, the purpose of the present study was to examine the validity of the European Resuscitation Council recommendation [2] in terms of gas distribution between lungs and stomach in a bench model that simulates positive-pressure ventilation of a nonintubated patient with self-inflatable bags representing tidal volumes of 0.5 L and 0.75 L. Since observations in an animal model [6] showed that the LESP decreases rapidly after an untreated cardiac arrest, we further evaluated the effect of a decreased LESP on gas distribution between lungs and stomach in this model.

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