Ischemic threshold during cold blood cardioplegic arrest: monitoring with tissue pH and pO2.

BACKGROUND

Several studies have suggested that measuring interstitial pH and pO2, may be useful to monitor ischemia throughout cardioplegic arrest during cardiac surgery.

METHODS

To evaluate the levels of myocardial tissue pH and pO2 that correlate with significant ischemia, 7 dogs underwent cold blood cardioplegic arrest and subsequent incremental episodes of 5, 10, 20 and 40 min of ischemia interrupted by cardioplegic infusion over 10-min periods.

RESULTS

Myocardial tissue pH and pO2 were monitored with probes implanted in the anterior and lateral walls of the left ventricle. The release of CK, troponine T and lactate was measured before and after each episode of ischemic arrest. Tissue pH decreased from 7.08+/-0.15 to 7.03+/-0.15 (p>0.05), 7.21+/-0.15 to 7.07+/-0.11 (p>0.05), 7.17+/-0.15 to 6.82+/-0.14 (p<0.05) and 7.0+/-0.18 to 6.63+/-0.08 (p<0.05) after 5, 10, 20 and 40 min of ischemic arrest. Tissue pO2 decreased from 74+/-10 to 38+/-11 mmHg (p<0.05), 83+/-16 to 18+/-4 mmHg (p<0.05), 9+/-22 to 14+/-5 mmHg (p<0.05) and 64+/-24 to 16+/-10 mmHg (p<0.05) after 5, 10, 20 and 40 min of ischemic arrest. CK, troponine T and lactate serum levels increased significantly only following 40 min of ischemic arrest. Myocardial temperature decreased to an average minimum of 14+/-1 degrees C during cardioplegic infusion.

CONCLUSIONS

A myocardial tissue pH lower than 7.04 (90% CI, upper limit of 6.82+/-0.14) and a tissue pO2 lower than 22 mmHg (90% CI, upper limit of 14+/-5 mmHg) correlate with anaerobic metabolism and myocardial ischemia during cold cardioplegic arrest.