Are There Adverse Effects of Mouth-to-Mouth Ventilation?

Respiratory Effects
When no endotracheal tube is in place, assisted ventilation maneuvers often are associated with gastric insufflation and pulmonary aspiration of gastric contents, which in turn may cause adult respiratory distress syndrome, pneumonitis, and possible death.⁶³ Some studies suggest a 10% to 35% incidence of pulmonary aspiration of gastric contents associated with CPR. During mouth-to-mouth ventilation, positive pressure in the oropharynx forces air into the lungs. Gas, however, flows down the path of least resistance, which may be either the trachea and lungs or the esophagus and stomach. Gastric insufflation is more apt to occur when (1) pulmonary compliance decreases (eg, with CPR, pulmonary edema, atelectasis, obesity, and the supine position), (2) airway resistance increases (eg, obstructive pulmonary disease), or (3) lower esophageal sphincter tone decreases.

Lower esophageal sphincter tone usually prevents regurgitation of gastric contents and provides resistance to gastric air flow during positive pressure ventilation. The normal esophageal sphincter opening pressure is about 20 to 25 cm H₂O.^{90 91} However, this energy-dependent muscular tone decreases rather quickly after circulatory arrest. In a swine model of CPR, mean esophageal sphincter opening pressure decreased from 20.6 cm H₂O before cardiac arrest to 5.6 cm H₂O after 5 minutes of cardiac arrest.⁹² In anesthetized patients with adequate circulation, gastric insufflation commonly occurs with bag-valve mask ventilation unless cricoid pressure is provided.⁹³ During cardiac arrest and CPR, pulmonary compliance decreases, increased inspiratory pressures may be needed to inflate the lung, and lower esophageal sphincter tone may decrease, all factors that increase gastric insufflation. Regurgitation was noted to occur primarily after the stomach was insufflated with air.¹⁶ Not surprisingly, in one series gastric insufflation and pulmonary aspiration were documented in nearly half of cardiac arrest victims after CPR with mouth-to-mouth ventilation.¹⁷

Another consideration is whether there are important differences between exhaled gas and ambient air. While room air has 21% O₂ and 0.03% CO₂, exhaled gas was observed to contain a mean O₂ concentration of 16.6% to 17.8% and a mean CO₂ concentration of 3.5% to 4.1% during one- and two-rescuer CPR.¹⁹ Thus, as provided during mouth-to-mouth ventilation, expired gas is slightly hypoxic and contains considerably more CO₂ than ambient air ventilation achieved with gasping and compression-induced ventilation alone.⁹⁴

Although expired gas rescue breathing is safe and may be life-saving for patients with respiratory arrests, the hypercarbia may have adverse cardiovascular effects when compared with ambient air ventilation during circulatory arrest. In one study of isolated hypercarbia, animals ventilated (12 mL/kg) with 95% O₂ and 5% CO₂ did as poorly as animals receiving no assisted ventilation at all during CPR.⁸⁴ Another investigation showed that swine ventilated with room air during 6 minutes of CPR had a rate of successful resuscitation (83%) twice that of animals ventilated with simulated exhaled gas ventilation when both groups received similar tidal volumes (38%, P<.01).⁹⁵ Consistent with animal studies are cellular studies that demonstrate modest increases in concentration of CO₂ can inhibit the rate and force of cardiac contraction, suggesting that elevated CO₂ has a direct cardiodepressive effect.^{96 97}

Circulatory Effects
Obviously a single rescuer performing CPR on an adult or child cannot provide chest compression and mouth-to-mouth ventilation simultaneously. Thus, it follows that with more time spent attempting ventilation, less time will likely be allocated to chest compression and vice versa. As noted, successful resuscitation has been highly correlated with the timing and degree of restored myocardial blood flow and coronary perfusion pressure, which are in turn dependent on the effective provision of chest compression of a sufficient rate and depth.^{98 99 100 101} Therefore, time spent attempting ventilation may take away valuable coronary perfusion. In support of this concept are recent studies that suggest that when rescuers attempt ventilation using current AHA recommendations, the compression rate and depth become inadequate.¹⁰² Current AHA guidelines for adult CPR recommend a chest compression rate of 80 to 100 per minute and a respiratory rate of approximately 12 breaths per minute, with compression/ventilation ratios of 15:2 with one rescuer and 5:1 with two rescuers. However, achieving these recommended guidelines in the real world has been demonstrated as problematic for both one- and two-rescuer adult CPR with mouth-to-mouth ventilation.^{18 19 20 101 102 103} One study of in-hospital two-rescuer CPR found that only 2 of 12 rescuers gave 80 compressions or more per minute,¹⁰³ whereas another investigation of simulated two-rescuer CPR on a manikin found that compression rates averaged 75 per minute and that the depth of compression was inadequate in 14% to 22%.¹⁰¹ These inadequacies of chest compression observed in two-rescuer CPR appear to be even worse in studies of one-rescuer CPR.^{18 19 20} In a study of healthcare professionals performing one-rescuer CPR on a manikin, only 15% achieved a rate of 80 compressions per minute despite continuous coaching.¹⁹ Further investigation confirmed that even immediately after successful completion of a basic CPR course, compression rates were particularly inadequate; on average, only 56 compressions per minute were provided by the 129 medical students studied.²⁰

These data suggest that the competition between time for ventilation and time for compression during one-rescuer CPR is a "zero-sum" game; time spent on ventilation takes precious time away from chest compression and support of myocardial blood flow. It is surprising that this aspect of CPR as practiced in the real world (ie, cycle time spent on ventilation versus cycle time spent on compression) has not been studied. It is worrisome that multiple studies have demonstrated survival rates to be consistently correlated with coronary perfusion pressure whereas no studies of fibrillatory arrest have shown improved rates of survival with early ventilation. Future studies need to take these real-world factors into account and not assume that performance of CPR is in complete adherence with AHA guidelines (which appears to be infrequently achieved).

Does Mouth-to-Mouth Ventilation Inhibit Performance of Bystander Cardiopulmonary Resuscitation?
Despite widespread acknowledgment of its value and efficacy, CPR is not performed by bystanders in the majority of cases for which it is indicated. Furthermore, several recent studies have documented a lower overall frequency of bystander CPR performance compared with earlier investigations.³ Although this latter finding may have several possible explanations (including the failure of the medical community and public health officials to effectively teach the public the skills of CPR), the perceived risk of disease transmission during CPR, even by healthcare workers, has become increasingly suspect as a major factor.

The actual risks of disease transmission during mouth-to-mouth ventilation are quite small. There are isolated reports of possible transmission of Helibacter pylori,¹⁰⁴ Mycobacterium tuberculosis,¹⁰⁵ meningococcus,¹⁰⁶ herpes simplex,^{107 108 109} shigella,¹¹⁰ streptococcus,¹¹¹ and salmonella.¹¹² No reports on transmission of HIV can be found. Nevertheless, despite the remote chances of its occurring, fears regarding disease transmission are common in the current era of universal precautions. Indeed, not only laypersons but physicians, nurses, and even BLS instructors are extremely reluctant to perform mouth-to-mouth ventilation.^{7 8 9 12 113} The most commonly stated reason for not performing mouth-to-mouth ventilation is fear of contracting AIDS.¹³ In one survey, only 15% of 975 respondents reported a willingness to perform chest compression with mouth-to-mouth ventilation on a stranger, whereas 68% would "definitely" perform chest compression alone if it was offered as an effective alternative CPR technique.¹²

Fear of disease transmission may not be the only reason that mouth-to-mouth ventilation inhibits bystanders from initiating CPR. When mouth-to-mouth ventilation is combined with chest compression, the CPR technique becomes a complex psychomotor task that can be difficult to teach, learn, remember, and perform.^{82 114} Educational principles suggest that a simpler technique, such as chest compression without mouth-to-mouth ventilation, would be far easier to teach the public. If it were known that the use of chest compression alone was nearly as efficacious as when combined with mouth-to-mouth ventilation, potential rescuers might start chest-compression CPR faster and more frequently because it is easier to perform in an actual emergency. In addition, the greater ease of learning, retaining, and performing such a simple procedure could lead to more widespread performance of bystander CPR, thereby improving survival rates for victims of cardiac arrest.

A Reappraisal of Mouth-to-Mouth Ventilation during Bystander-Initiated Cardiopulmonary Resuscitation

Circulation Magazine 1997