Reducing Ventilation Frequency During Cardiopulmonary Resuscitation in a Porcine Model of Cardiac Arrest

INTRODUCTION: American Heart Association/American College of Cardiology guidelines recommend a compression-to-ventilation ratio (C/V ratio) of 15:2 during cardiopulmonary resuscitation (CPR) for out-of-the-hospital cardiac arrest. Recent data have shown that frequent ventilations are unnecessary and may be harmful during CPR, since each positive-pressure ventilation increases intrathoracic pressure and may increase intracranial pressure and decrease venous blood return to the right heart and thereby decrease both the cerebral and coronary perfusion pressures.

HYPOTHESIS: We hypothesized that reducing the ventilation rate by increasing the C/V ratio from 15:2 to 15:1 will increase vital-organ perfusion pressures without compromising oxygenation and acid-base balance.

METHODS: Direct-current ventricular fibrillation was induced in 8 pigs. After 4 min of untreated ventricular fibrillation without ventilation, all animals received 4 min of standard CPR with a C/V ratio of 15:2. Animals were then randomized to either (A) a C/V ratio of 15:1 and then 15:2, or (B) a C/V ratio of 15:2 and then 15:1, for 3 min each. During CPR, ventilations were delivered with an automatic transport ventilator, with 100% oxygen. Right atrial pressure, intratracheal pressure (a surrogate for intrathoracic pressure), aortic pressure, and intracranial pressure were measured. Coronary perfusion pressure was calculated as diastolic aortic pressure minus right atrial pressure. Cerebral perfusion pressure was calculated as mean aortic pressure minus mean intracranial pressure. Arterial blood gas values were obtained at the end of each intervention. A paired t test was used for statistical analysis, and a p value < 0.05 was considered significant.

RESULTS: The mean ± SEM values over 1 min with either 15:2 or 15:1 C/V ratios were as follows: intratracheal pressure 0.93 ± 0.3 mm Hg versus 0.3 ± 0.28 mm Hg, p = 0.006; coronary perfusion pressure 10.1 ± 4.5 mm Hg versus 19.3 ± 3.2 mm Hg, p = 0.007; intracranial pressure 25.4 ± 2.7 mm Hg versus 25.7 ± 2.7 mm Hg, p = NS; mean arterial pressure 33.1 ± 3.7 mm Hg versus 40.2 ± 3.6 mm Hg, p = 0.007; cerebral perfusion pressure 7.7 ± 6.2 mm Hg versus 14.5 ± 5.5 mm Hg, p = 0.008. Minute area intratracheal pressure was 55 ± 17 mm Hg · s versus 22.3 ± 10 mm Hg · s, p < 0.001. End-tidal CO₂ with 15:2 versus 15:1 was 24 ± 3.6 mm Hg versus 29 ± 2.5 mm Hg, respectively, p = 0.001. Arterial blood gas values were not significantly changed with 15:2 versus 15:1 C/V ratios: pH 7.28 ± 0.03 versus 7.3 ± 0.03; P_aCO2 37.7 ± 2.9 mm Hg versus 37.6 ± 3.5 mm Hg; and P_aO2 274 ± 36 mm Hg versus 303 ± 51 mm Hg.

CONCLUSIONS: In a porcine model of ventricular fibrillation cardiac arrest, reducing the ventilation frequency during CPR by increasing the C/V ratio from 15:2 to 15:1 resulted in improved vital-organ perfusion pressures, higher end-tidal CO₂ levels, and no change in arterial oxygen content or acid-base balance.