Myocardial relaxation is an important energy-dependent process. Hypoxia, unlike ischemia, has not been shown to impair myocardial relaxation. This difference may be because (a) the traditional index to assess isometric muscle relaxation (half time to relaxation or RT½) reflects both changes in developed tension as well as relaxation and (b) the relaxation process is highly sensitive to temperature and previous papillary muscle studies have been conducted under hypothermic conditions. The present study examines the effect of hypoxia on the relaxation process of 31 isometrically contracting kitten papillary muscles at hypothermic (29°C) and euthermic (38°C) conditions using RT½, the peak rate of tension fall (−dT/dt) and −dT/dt normalized for tension ([peak −dT/dt]/T and max [−dT/dt per T]). Hypoxia at 29°C resulted in a fall in RT½ from 278±11 (SEM) to 230±17 ms (P < 0.01) and no change in (peak −dT/dt)/T and max (−dT/dt per T). However, at 38°C, hypoxia impaired relaxation as reflected in a prolongation of RT½ from 101±6 to 126±8 ms (P < 0.01) in spite of a substantial fall in peak tension. Moreover, (peak −dT/dt)/T decreased from −15.4±0.7 to −11.0±0.8/s (P < 0.01) and max (−dT/dt per T) decreased from −25.1±1.8 to −13.8±0.9/s (P < 0.01). In conclusion, the present study demonstrates that hypoxia impairs the relaxation process of cardiac muscle.
William H. Frist, Igor Palacios, Wm. John Powell Jr.