Measurement of myocardial oxygen consumption

Monitoring of pacemaker induced changes in cardiac output with inspired to endtidal oxygen difference in paediatric cardiac surgery patients

METHODS

Fourteen children aged 4-15 months were studied after corrective cardiac surgery. Heart rate was increased by 20% with an external pacemaker. Cardiac output (CO) was measured with thermodilution. Oxygen saturation was measured in systemic artery (SaO2), central vein (ScvcO2) and pulmonary artery (SvO2). Inspiratory to endtidal oxygen difference (FI-ETO2) was measured using a paramagnetic technique. SvO2 was measured continuously using a spectrophotometric technique.

RESULTS

CO increased in three patients and decreased in 11 patients during pacing. Regression between DeltaCO and Delta(1/Sa-vO2), Delta(FI-ETO2/Sa-vO2), Delta(FI-ETO2/Sa-cvcO2) showed r=0.70, r=0.76 and r=0.75, respectively. DeltaCO exceeded 10% in 17 of 26 interventions. Changes in FI-ETO2 of equal direction as changes in CO occurred in 12 of these 17 interventions.

CONCLUSION

Estimations of CO changes, based on SvO2, can be enhanced if changes in FI-ETO2 are also measured. ScvcO2 instead of SvO2 gives equivalent results. Sudden changes in FI-ETO2 after pacemaker initiation or termination can predict the direction of CO changes.

Ischemic preconditioning reduces infarct size in swine myocardium

We evaluated the hypothesis that stunning swine myocardium with brief ischemia reduces oxygen demand in the stunned region and increases tolerance of myocardium to longer periods of ischemia. Wall function was quantified with ultrasonic crystals aligned to measure wall thickening, and stunning was achieved with two cycles of left anterior descending coronary artery (LAD) occlusion (10 minutes) and reperfusion (30 minutes), after which the LAD was occluded for 60 minutes and reperfused for 90 minutes. Infarct size (as a percent of risk region) was then determined by incubating myocardium with para-nitro blue tetrazolium. Regional oxygen demand was measured as myocardial oxygen consumption before the 60-minute LAD occlusion in the stunned region; tracer microspheres were used to determine blood flow, and blood from the anterior interventricular vein and left atrium was used to calculate oxygen saturations. After the second reperfusion period, wall thickening in the stunned region was reduced to 1.4 +/- 2.4% compared with 36.7 +/- 2.5% (mean +/- SEM) before ischemia (p less than 0.001). Regional myocardial oxygen consumption after stunning (3.1 +/- 0.7 ml O2/min/100 g) was no different from regional myocardial oxygen consumption before stunning (3.7 +/- 0.6 ml O2/min/100 g). In the nine pigs "preconditioned" by stunning, infarct size was 10.4 +/- 6.3% of the risk region compared with 48.0 +/- 12.7% in the six control pigs subjected to 60 minutes of ischemia without prior stunning (p less than 0.005). The risk regions were similar (14.4 +/- 1.5% vs. 14.6 +/- 1.9% of the left ventricle, preconditioned vs. control pigs, respectively). We conclude that stunning swine myocardium with two cycles of a 10-minute LAD occlusion followed by reperfusion increases ischemic tolerance but that changes in regional demand in stunned myocardium do not predict the marked reduction in infarct size that follows a subsequent 60-minute period of ischemia.

Myocardial blood flow and oxygen consumption after aortic cross-clamping

The effect of four repetitive periods of 10 min global ischemia, followed by 15 min of reperfusion on myocardial blood flow and oxygen consumption at 34 degrees C (group N) and 25 degrees C (group H) were studied in comparison with that of 60 min continuous ischemia combined with multidose St. Thomas Cardioplegia (group C) in dogs on cardiopulmonary bypass (CPB). Two groups, in which a protocol comparable to the groups N and H, respectively, but without repetitive periods of ischemia was followed, served as control. In all groups a hyperemic response was observed after release of aortic cross-clamping (AC). Myocardial blood flow was not diminished at the end of CPB as compared to the values at the start of CPB. We conclude that the no-reflow phenomenon did not occur after these procedures of intermittent or continuous AC. Immediately following release of AC the arterial-coronary sinus difference of oxygen content reached a peak value in groups N and H indicating rapid replenishment of the low tissue oxygen content. These peak values appeared to be much smaller after cardioplegia. After 10 min of reperfusion a significant lower oxygen consumption was observed at 25 degrees C (0.2 mumole.g-1.min-1 O2) as compared to 34 degrees C (1.5 mumole.g-1.min-1 O2). This difference cannot be explained by temperature alone because oxygen consumption did not decrease below 0.5 mumole.g-1.min-1 in the control group of dogs put on CPB at 25 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Myocardial oxygen consumption, oxygen supply/demand heterogeneity, and microvascular patency in regionally stunned myocardium

Although oxygen consumption closely parallels mechanical work in the normal heart, previous studies have found that stunned myocardium may have normal or even increased oxygen consumption despite depressed function. In this study we used microspectrophotometry to measure the oxygen saturations within arteries and veins of less than 100 micron diameter in quick-frozen biopsy samples from normal and regionally stunned myocardium of 10 open-chest anesthetized dogs. Regional myocardial blood flow, measured by radioactive microspheres, was similar in stunned and normal regions, as was mean arteriolar oxygen saturation. However, mean venous oxygen saturation was lower in the stunned region (epicardium 38.0% vs 43.8%, p less than .02; endocardium 36.2% vs 39.5%, p = .12), indicating increased oxygen extraction and consumption, despite a marked reduction in mean systolic segmental shortening from 14.4% to 0.5%. In addition, there was greater vein-to-vein heterogeneity of oxygen saturation in the stunned region, with an excess of veins having low saturations (statistically significant in epicardium, nonsignificant trend in endocardium). Microvascular injections studies with Microfil or drafting ink revealed filling of over 95% of arterioles and 85% of capillaries in the stunned region, similar to the findings in the normal region. Our results are consistent with an inefficient transfer of energy into myocyte contraction or an increased use of energy for noncontractile activities in stunned myocardium. In addition, the finding of increased heterogeneity of oxygen extraction suggests that the injury to stunned myocardium may not be uniform to all contractile elements, but instead may be focally and irregularly distributed.