Myocardial oxygen consumption after reversible ischemia

Increased cross-bridge cycling rate in stunned myocardium

Decreased Ca2+ responsiveness of the myofilaments underlies myocardial stunning. Given that cross-bridge cycling is a major determinant of myofilament behavior, we quantified cross-bridge cycling rate in stunned myocardium. After stabilization, rat hearts were subjected to 20 min of no-flow global ischemia and 30 min of reperfusion at 37 degrees C. Control hearts were perfused continuously at 37 degrees C for 60 min. Trabeculae were dissected and chemically skinned with 1% Triton X-100. The muscles were then activated with solutions of varied Ca2+ concentration ([Ca2+]). Force-[Ca2+] relations, rate of force redevelopment after release (k(tr)), muscle stiffness (k(m)), and myofilament ATP consumption were determined. Maximal Ca2+-activated force (Fmax) was depressed in stunned myocardium (49 +/- 5 vs. 82 +/- 5 mN/mm2, P < 0.01). Western immunoblotting showed degradation of troponin I in stunned myocardium. The k(tr) at Fmax was significantly increased in stunned muscles (19.82 +/- 2.74 vs. 13.19 +/- 0.96 s(-1), 22 degrees C, P < 0.01; 7.49 +/- 0.52 vs. 5.81 +/- 0.54 s(-1), 10 degrees C, P < 0.05). The ratio of k(m) measured at 100 Hz over that at 1 Hz, during Fmax, is lower in stunned muscles (8.22 +/- 1.56 vs. 12.94 +/- 0.71, P < 0.05). In comparison with k(m) at rigor, k(m) at Fmax is significantly lower in the stunned group (78.82 +/- 6.11 vs. 93.27 +/- 3.03%, P < 0.05). Myofilament ATP consumption at Fmax did not change in stunned muscles (5,901 +/- 952 vs. 5,596 +/- 972 pmol x microl(-1) x min(-1), P = 0.49). These results show that cross-bridge cycling is increased in stunned myocardium. Such increases are likely the result of increased transition rate from force-generating states to non-force-generating states. Thus stunned myocardium still maintains ATP consumption in spite of lower force development, rationalizing the long-standing paradox of decreased force but unchanged oxygen consumption in the postischemic heart.

Recovery after cardioplegia in the hypertrophic rat heart

BACKGROUND

Enhanced recovery after cardioplegic arrest has been observed in rat hearts with hypertrophy induced by hemodynamic overload. We hypothesize that this is related to altered characteristics of hypertrophied myocardium-reflected by increased V(3) isomyosin and glycolytic potential-other than increased left ventricular mass.

MATERIALS AND METHODS

Isolated hearts from age-matched nonoperated and sham-operated control rats and from aortic-banded, hyperthyroid, and hypothyroid rats-groups in which hypertrophy and V(3) as a percentage of left ventricular myosin vary independently-underwent 2 h of multidose cardioplegic arrest at 8 degrees C followed by reperfusion at 37 degrees C. Left ventricular V(3) isomyosin was evaluated after separation by gel electrophoresis.

RESULTS

Moderate left ventricular hypertrophy was produced by aortic banding or hyperthyroidism and atrophy by hypothyroidism. V(3) isomyosin was increased in banded (28%) and hypothyroid (75%) rats compared to control (12%) and hyperthyroid rats (7%). Myocardial glycogen content closely paralleled %V(3). At 30 min of working reperfusion, functional recovery (assessed as percentage prearrest cardiac output) was 66 +/- 4 and 68 +/- 5% in control and hyperthyroid hearts and 81 +/- 2 and 80 +/- 5% in hearts from banded and hypothyroid rats (each P < 0.05 vs controls), respectively. At 30 min, hearts from banded and hypothyroid rats were also more efficient (as indexed by cardiac output at constant mean aortic pressure/myocardial oxygen consumption) than control and hyperthyroid hearts.

CONCLUSIONS

The data suggest that recovery is related not to increased mass but to other changes in overload hypertrophy. Increased percentage V(3) isomyosin and glycogen reflect these changes and may themselves contribute to improved functional recovery after cardioplegic arrest, as may increased postischemic efficiency.

The relationship of myocardial stroke work to coronary flow velocity immediately after aortic valve replacement

BACKGROUND

The interrelations between myocardial stroke work and coronary flow velocity have not been fully defined during aortic valve replacement or with different cardioplegias.

METHODS

Twenty-six patients (15 men age 63+/-13 years) who had elective isolated aortic valve replacement were studied by transesophageal Doppler echocardiography with simultaneous high fidelity left ventricular pressure. Fifteen patients received cold blood cardioplegia and 11 had warm blood cardioplegia. Myocardial stroke work and flow velocities in proximal left anterior descending coronary artery were quantified simultaneously before cardiopulmonary bypass and at 1, 6, 12, and 20 hours afterwards.

RESULTS

Myocardial stroke work decreased postoperatively in both groups (160+/-19 versus 228+/-19 mJ/cm3 per minute, with cold blood cardioplegia; 135+/-22 versus 227+/-22 mJ/cm3 per minute with warm blood cardioplegia; both p<0.001 versus time, but p>0.05 versus cardioplegia, by two-way analysis of variance). Left anterior descending artery flow velocity-time integral per minute increased significantly in both groups (26.1+/-2.1 versus 15.0+/-2.1 m/min with cold blood cardioplegia; 32.8+/-2.5 versus 14.4+/-2.5 m/min with warm blood cardioplegia; both p<0.001 versus time, but p>0.05 versus cardioplegia). Thus, at 1 hour postoperatively the mJ x cm(-3) x m(-1) x min ratio of myocardial stroke work to left anterior descending artery flow velocity-time integral decreased significantly in both groups (4.3+/-1.6 versus 16.3+/-1.7 mJ x cm(-3) x m(-1) x min with warm blood cardioplegia, and 7.4+/-1.4 versus 17.9+/-1.4 J x cm(-3) x m(-1) x min with cold blood cardioplegia; both p<0.001 versus time). Warm blood cardioplegia was also associated with a lower mean ratio perioperatively than that with cold blood cardioplegia (7.8+/-0.9 versus 10.9+/-0.7 mJ x cm(-3) x m(-1) x min, p = 0.014).

CONCLUSIONS

Coronary hyperemia occurs for at least 20 hours postoperatively when myocardial stoke work has decreased. The ratio of myocardial stroke work to coronary flow velocity appears to be more sensitive than either alone in differentiating the effect of warm versus cold blood cardioplegia.