Signs of malignant hyperthermia, including progressive increases in PaCO2, skin temperature and heart rate, and elevated serum levels of potassium, inorganic phosphate, and creatine kinase, were identified in a halothane-anesthetized horse. Treatment was discontinuing halothane administration, applying ice and cold fluids, and hyperventilating with 100% oxygen. After an initial recovery, bilateral hindlimb myopathy and pigmenturia developed. The myopathy resolved after treatment with oral dantrolene, IV fluids, and hydrocortisone. Results of caffeine-halothane challenge, using semimembranosus muscle collected 2 weeks after the episode, were considered within normal limits for horses. The intraoperative abnormalities were evidently predictive of postanesthetic myopathy but the cause in this horse remained unclear.
The effects of 5- and 10-wk treadmill exercise training on cardiorespiratory function were evaluated in the horse. Cardiac output (Q), heart rate (HR), and stroke volume (SV), as well as left ventricular (LVP), aortic (AoP), and mean right atrial (RAP) pressures and the peak first time derivative of LVP (LV dP/dtmax), were measured at rest and at five different levels of exercise up to 90% of initial predicted maximal HR (HRmax). Oxygen uptake (VO2) and respiratory exchange ratio (R) were also obtained under the same conditions. At rest, although HR was no different after training, LV dP/dtmax was lower at 10 wk (2,369 +/- 502 vs. 1,615 +/- 302 mmHg/s). At the different measured work loads during exercise, Q and VO2 remained unchanged with training, whereas there were consistent trends toward lower LVP, AoP, and LV dP/dtmax and significant reductions in HR and R (both P less than 0.05). In contrast, SV and mean RAP were elevated (P less than 0.05) during exercise after the 10-wk training program. Although the observed changes in cardiorespiratory function in response to a training program in the horse are generally similar to those reported for other species, our data also suggest a training-induced increase in venous pressures during exercise as measured by elevated mean RAP values.
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