BLOOD LACTATE THRESHOLD AND TYPE-II FIBER PREDOMINANCE IN PATIENTS WITH EXERTIONAL HEATSTROKE

Objectives-Severe damage to skeletal muscle is usually seen in patient s with exertional heatstroke. Thirty seven young military recruits wit h exertional heatstroke in Taiwan from 1992 to 1995 were studied to ev aluate changes in muscle pathology and blood lactate with exercise. Me thods-A biopsy sample of the vastus lateralis was taken from recruits within 10 days of the initial presentation. Results were compared with those from 15 controls matched for age and sex. During the recovery p eriod, 90-150 days after exertional heatstroke, 29 patients participat ed in a constant work load test on the treadmill to assess their blood lactate threshold, and a second biopsy sample was taken. Each biopsy was examined histologically for pathology, distribution of fibre types , and fibre diameter. Results-Twenty four of the 37 patients with exer tional heatstroke developed rhabdomyolysis and 18 of these had type II fibre predominance in their muscle biopsy. The patients with type II fibre predominance had a higher tendency to develop rhabdomyolysis (ch i(2) = 6.84, P < 0.01). The time required to reach a blood lactate thr eshold during a constant treadmill work load after recovery was signif icantly shorter in the patients with exertional heatstroke who had typ e II fibre predominance (P < 0.01). There was a positive correlation b etween the highest value of blood lactate and the percentage of type I I fibres in all tested subjects (r = 0.82, P < 0.01). Conclusion-Patie nts with type II fibre predominance are more susceptible to exertional heatstroke and tend to have a higher blood lactate concentration and a shorter time to reach blood lactate threshold under a treadmill load test.