Mouth-to-Mouth Ventilation During Cardiopulmonary Resuscitation: Word of Mouth in the Street Versus Science
When the airway is unprotected, the distribution of ventilation volume between lungs and stomach depends mainly on variables such as lower esophageal sphincter pressure (2), airway resistance, and respiratory system compliance (3). Of equal importance are differences in technique applied while performing basic or advanced airway support, such as head position, tidal volume, inflation flow rate, and duration, all of which determine upper airway pressure (4). The combination of these variables determines gas distribution between the lungs and the esophagus and, subsequently, the stomach. There are several fundamental differences between these components of respiratory mechanics in a healthy, awake adult, an anesthetized supine patient, and a victim of cardiac arrest (Table 1). On the basis of these measurements, we evaluated simulated basic life support ventilation in both bench models and apneic patients and found that even experienced paramedics and anesthesiologists can cause stomach inflation when performing routine bag-valve-mask ventilation (5,6). Moreover, we found that mouth-to-mouth ventilation skills in first-year medical students (who may be somewhat comparable to lay bystanders) deteriorated within 6 mo to levels that did not ensure proper basic life support ventilation performance at all (7).
Extrapolating these observations, patients who received mouth-to-mouth ventilation by lay bystanders in the city of Vienna, Austria (1), would be expected to have much more frequent rates of stomach inflation and pulmonary aspiration, especially when considering that mouth-to-mouth ventilation was performed for approximately 7 min. In fact, our bench model investigations showed severe levels of stomach inflation when basic life support ventilation was simulated in an unintubated cardiac arrest patient for this duration (8). There could be a number of explanations for this discrepancy. First, stomach inflation and/or pulmonary aspiration may have occurred without being detected on chest radiograph; second, mouth-to-mouth ventilation skills were better than we thought; and third, stomach inflation may have been self-limiting (9). None of these possibilities can be ascertained or discounted by the investigators of the present article. Moreover, the authors correctly identified the limitation that only surviving patients were examined, indicating the possibility that if lay bystanders had performed nonsurvivable basic life support maneuvers, these catastrophic complications would not have been detected as a result of the study design. Although speculative, an additional possibility may be the most likely underlying reason for the observations of Oschatz et al. (1) in Vienna. When paramedics performed bag-valve-mask ventilation, inspiratory times were ∼0.5 s instead of 2 s (10), indicating that rescuers may squeeze a self-inflating bag in such a rapid and powerful manner that extremely high flow rates are produced, and subsequently, high peak airway pressures with stomach inflation are very likely. However, it seems obvious that when bystanders perform mouth-to-mouth ventilation, they have less muscle power in their cheeks than paramedics can exert with their hands during bag-valve-mask ventilation, which may result in lower inspiratory flow rates, lower peak airway pressure, and lower rates of stomach inflation.