Intraosseous and central venous blood acid-base relationship during cardiopulmonary resuscitation

The objectives of this study were: 1) to determine whether obtaining intraosseous (IO) blood samples was practical during cardiopulmonary resuscitation (CPR), and 2) to compare the acid-base status (pH and partial pressure of CO2 (PCO2) of venous and IO blood during CPR.

A prospective repeated measure study.

An animal laboratory at a university medical center.

Nine mixed breed piglets (mean weight 43 kg) were anesthetized, tracheotomized, and placed on a ventilator (Siemens 900C Elema, Sweden). Placement of a pulmonary artery catheter was done via a surgical incision in the neck. An IO cannula was then placed in the tibial marrow cavity. The animals were positioned under a mechanical thumper (Thumper, Michigan Instruments, Grand Rapids, MI) for chest compressions. Blood gases were analyzed during steady state (baseline) after five minutes of ventricular fibrillation and during CPR at seven, nine, 11, 13, 15 and 18 minutes.

Blood samples for acid-base analysis were easily obtained from the IO sites during all sampling times. Mixed venous blood was slightly more acidic than IO blood, especially at 13, 15, and 18 minutes. However, there were no significant differences in pH and Pco2 values between IO and central venous (CV) gases at all time intervals except the PCO2. At nine minutes, a significant difference (P < 0.006) was found in PCO2 (59 +/- 4 vs 47 +/- 5 torr) for the CV versus IO sample, respectively. As the duration of CPR progressed, the differences in PCO2 between IO and CV sites were clinically relevant (though not statistically significant).

Obtaining blood from the IO site is practical during CPR. The divergence in values as CPR progresses suggests that, during longer periods of CPR, IO blood may reflect local acidosis and yield lower PCO2 and higher pH values that CV blood. This finding may limit the usefulness of IO blood to judge acid base status as CPR progresses.