Effects of chronic heart failure on microvascular oxygen exchange dynamics in muscles of contrasting fiber type
In rat spinotrapezius muscle, chronic heart failure (CHF) speeds microvascular O 2 pressure ( pO 2 ; index of O 2 delivery-to-O 2 uptake) dynamics across the rest -contractions transition [Cardiovasc. Res. 56 (2002) 479]. Due to the mosaic nature of this muscle, the effect of CHF on microvascular pO 2 dynamics in different fiber types remains unclear. Objective: Based upon derangements of endothelial function and blood flow responses, we hypothesized that CHF would speed microvascular pO 2
... ovascular pO 2 dynamics (reduced O 2 delivery-to-O 2 uptake ratio) in type I muscle (soleus, f 84% type I), but not in type II muscle (peroneal, f 86% type II [J. Appl. Physiol. 80 (1996) 261]). Methods: Using phosphorescence quenching, microvascular pO 2 was measured at rest and across the rest -contractions transition (1 Hz) in soleus and peroneal of non-infarcted control (control; n = 7), and Sprague -Dawley rats with moderate (moderate; elevated left ventricular end-diastolic pressure (LVEDP) 10 F 2 mm Hg; n = 10) and severe (severe; LVEDP 28 F 4 mm Hg; n = 5) CHF. Results: The microvascular pO 2 mean response time (time delay + time constant) was progressively speeded with increasing severity of CHF in soleus (control, 38.7 F 2.0; moderate, 29.1 F 1.5; severe, 22.5 F 3.9 s; P V 0.05), but not in peroneal (control = moderate = severe). Conclusion: As type I fibers are recruited predominately for moderate intensity exercise, the more rapid lowering of soleus microvascular pO 2 in CHF would reduce the blood-muscle O 2 driving gradient, exacerbate phosphocreatine and glycogen breakdown, and provide a mechanism for slowed O 2 uptake kinetics and premature fatigue in CHF. Abbreviations: CHF, chronic heart failure; pO 2 , partial pressure of oxygen; Control, non-infarcted rats; Moderate, rats with moderate levels of CHF; Severe, rats with severe levels of CHF; LVEDP, left ventricle enddiastolic pressure; RV, right ventricle; MRT, mean response time (time delay+time constant); T 63 , time to 63% of final response; TD, time delay; s, time constant; k r pO 2 , rate constant of pO 2 change (DpO 2 /s); DpO 2 , change in microvascular pO 2 from non-contracting values to nadir.