Dynamic changes of myocardial oxygen consumption at pacing increased heart rate – the first observation by the continuous measurement of systemic oxygen consumption

Abstract

Objectives. To assess dynamic changes in myocardial oxygen consumption (myoVO2) during atrial pacing increased heart rate by continuous measurement of systemic oxygen consumption (sysVO2). Methods. Six mechanically ventilated pigs were atrially paced to increase heart rate from baseline 98 ± 9 to 120-140-160-180 bpm for 10 minutes at each stage, with 10 minute intervals without pacing between stages. sysVO2 was continuously measured with a respiratory mass spectrometer. Left anterior descending coronary arterial flow, aorta and coronary sinus blood gases were measured to calculate index of whole heart myoVO2. Results. sysVO2 peaked at the initiation of pacing in the first two to three minutes, followed by a decrease and subsequent stabilization. As heart rate increased, sysVO2 increased by 0.08 ± 0.06 ml/kg/min, 0.14 ± 0.05 ml/kg/min and 0.17 ± 0.10 ml/kg/min, representing a 1.2 ± 0.9%, 2.1 ± 0.7% and 3.0 ± 1.8% increase of sysVO2 respectively; myoVO2 increased by 0.16 ± 0.12 to 0.31 ± 0.14 to 0.36 ± 0.24 ml/100 g/min, representing a 11 ± 9%, 21 ± 9% and 26 ± 12% increase of myoVO2, respectively. The absolute and relative increases in sysVO2 were significantly correlated with the increases in myoVO2. Conclusions. On-line continuous sysVO2 monitoring by respiratory mass spectrometry allows non-invasive assessments of dynamic changes in myoVO2 in vivo. The mechanism for the peaked increase in sysVO2 at the initiation of pacing remains to be explored.