Gas concentrations in expired air during basic life support using different ratios of compression to ventilation

In cardiopulmonary resuscitation, different ratios of compression to ventilation with regard to optimal oxygen transport are considered. We hypothesised that the end tidal fraction of oxygen might increase from levels found in the conventional compression-ventilation ratio of 15:2 if more consecutive ventilations are given because the rescuer would hyperventilate. The second hypothesis was that the air blown into an infant with mouth to mouth ventilation consists of rescuer's dead space air only, meaning that the fraction of oxygen should increase.

In a basic life support simulation, we measured the expired air of rescuers using a VmaxST (Sensormedics, USA) respiratory gas analyser connected to an adult and to an infant resuscitation manikin. Fourteen participants performed five different compression-ventilation ratios (30:2, 30:5, 50:5, 100:10 and 5:1). These were compared to a ratio of 15:2 (control group).

We found a significant increase in end tidal oxygen in 30:2 (16.3%), 30:5 (16.8%), 50:5 (16.8%), 100:10 (17.0%) compared to 15:2 (15.9%), p< or =0.004 for all groups versus control; p for trend: 0.014. In the infant CPR observation (ratio 5:1), the difference with the adult control group (15:2) also reached statistical significance (17.9% versus 15.9%, p=0.0005).

Increasing consecutive compressions and ventilations above 15:2 leads to a statistically significant increase in expired fraction of oxygen. In infant ventilation, the air exhaled into a victim contains some dead space air with a higher end tidal oxygen fraction than in adults.