Abstract 188: Arterial Oxygen Saturation May Be the Limiting Factor for O2 Delivery During Compression-Only CPR
Background Previous studies show that there is no difference in outcomes between bystander conventional and chest-compression-only (CCO) CPR. However, several points need to be addressed: 1) the equivalent, but low, survival rate indicates that there is much room for improvement. The question is whether effective ventilation can be achieved during CPR regardless of approach. 2) the subgroup of patients whose cardiac arrest was not of cardiac origin did not sustain any benefit from ventilation. This finding would imply that adequate ventilation was not achieved. Because cardiac output (CO) generated by chest-compression is low, O2 delivery (DO2) is mainly dependent on arterial O2 content. We hypothesize that DO2 is the rate limiting factor and improvement in DO2 is mainly determined by O2 content of arterial blood, since CO is at best only 1/3 of the normal value.
Methods We modeled DO2 in a range of 0 to critical DO2 (8.2 ml/kg/min) at variable CO and O2 contents of arterial blood. We assumed a 70kg patient with a hemoglobin level (Hb) of 15g/dl. Arterial O2 Content (ml/L) = Hb × Arterial O2 Saturation × 1.34 ×10. DO2 (ml/min) = Arterial O2 Content (ml/L) × CO (L/min).
Result The half-critical DO2 at arterial blood O2 saturation of 80% can be achieved with CO of 1.8L/min. While to achieve the same level of DO2 at O2 saturation 40%, require CO of 3.6 L/min. Critical O2 delivery and half-critical DO2 are represented with horizontal yellow and red lines respectively. The vertical mahogany and blue lines represent the CO where half-critical delivery at O2 saturations of 80% and 40%, respectively.
Conclusion Since the CO obtained with chest compression is at best 1/3 of the normal value; to reach half-critical O2 delivery requires an O2 saturation of 80%. CCO seems unlikely to produce O2 saturation greater than 80%. Therefore, effective ventilation during CPR is essential. Further study is needed to demonstrate whether confirmed adequate ventilation improves outcome during CPR.
- © 2011 by American Heart Association, Inc.