Coupling between regional myocardial oxygen consumption and contraction under altered preload and afterload.

OBJECTIVES

This study was designed to assess the relation between left ventricular regional myocardial oxygen consumption (VO2) and variables of regional myocardial contractile function under various loading conditions.

BACKGROUND

Although the relation between global VO2 and global ventricular function has been extensively studied, the relation between regional VO2 and regional myocardial contraction is not fully understood.

METHODS

Myocardial shortening (regional area shrinkage), regional work, regional total mechanical energy index and regional VO2 were measured under variously altered loading conditions in the isolated, blood-perfused dog left ventricle. Regional total mechanical energy per beat was quantified by wall tension-regional area area (TAA) by the analogy of left ventricular pressure-volume area. Left ventricular loading conditions were altered by changing end-diastolic volume and stroke volume with a servo pump as follows: 1) increased preload (increased end-diastolic volume and stroke volume at a constant ejection fraction), 2) decreased afterload (increased stroke volume at a constant end-diastolic volume), 3) increased preload and afterload (increased end-diastolic volume at a constant stroke volume), and 4) altered mode of contraction (ejecting vs. isovolumetric contractions).

RESULTS

During increased preload, all three variables correlated positively with regional VO2 (r = 0.78 to 1.00). During decreased afterload, the correlation was negative for area shrinkage (r = -0.65 to -0.91) and variable for regional work (r = -0.55 to 0.98) but positive and highly linear for TAA (r = 0.80 to 0.99). During increased preload and afterload, the correlation was again negative for area shrinkage (r = -0.77 to -0.97) but positive for regional work (r = 0.83 to 0.93) and TAA (r = 0.95 to 0.99). During altered mode of contraction, the correlation was insignificant for area shrinkage (r = 0.24 to 0.57) and moderate for regional work (r = 0.50 to 0.79), whereas again highly linear for TAA (r = 0.95 to 0.98). Thus, only TAA correlated closely with regional VO2 under any loading conditions. Furthermore, the slope and regional VO2 intercept of the regional VO2-TAA relation was remarkably consistent among the different hearts and loading conditions.

CONCLUSIONS

We conclude that there is a tight coupling between regional VO2 and regional total mechanical energy represented by TAA regardless of left ventricular afterload and preload conditions.