Human circulatory and thermoregulatory adaptations with heat acclimation and exercise in a hot, dry environment.

J Physiol. 1993 Jan;460:467-85

Nielsen B, Hales JR, Strange S, Christensen NJ, Warberg J, Saltin B.

August Krogh Institute, University of Copenhagen, Denmark.

1. Heat acclimation was induced in eight subjects by asking them to exercise until exhaustion at 60% of maximum oxygen consumption rate (VO2) for 9-12 consecutive days at an ambient temperature of 40 degrees C, with 10% relative humidity (RH).
Five control subjects exercised similarly in a cool environment, 20 degrees C, for 90 min for 9-12 days; of these, three were exposed to exercise at 40 degrees C on the first and last day.

2. Acclimation had occurred as seen by the increased average endurance from 48 min to 80 min, the lower rate of rise in the heart rate (HR) and core temperature and the increased sweating.

3. Cardiac output increased significantly from the first to the final heat exposure from 19.6 to 21.4 l min-1; this was possibly due to an increased plasma volume and stroke volume.

4. The mechanism for the increased plasma volume may be an isosmotic volume expansion caused by influx of protein to the vascular compartment, and a sodium retention induced by a significant increase in aldosterone.

5. The exhaustion coincided with, or was elicited when, core temperature reached 39.7 +/- 0.15 degrees C; with progressing acclimation processes it took progressively longer to reach this level. However, at this point we found no reduction in cardiac output, muscle (leg) blood flow, no changes in substrate utilization or availability, and no recognized accumulated ‘fatigue’ substances.

6. It is concluded that the high core temperature per se, and not circulatory failure, is the critical factor for the exhaustion during exercise in heat stress.

PMID: 8487204 [PubMed – indexed for MEDLINE]

Muscle blood flow and muscle metabolism during exercise and heat stress

J Appl Physiol. 1990 Sep;69(3):1040-6

Nielsen B, Savard G, Richter EA, Hargreaves M, Saltin B.

August Krogh Institute, University of Copenhagen, Denmark.

The effect of heat stress on blood flow and metabolism in an exercising leg was studied in seven subjects walking uphill (12-17%) at 5 km/h on a treadmill for 90 min or until exhaustion.

The first 30 min of exercise were performed in a cool environment (18-21 degrees C); then subjects moved to an adjacent room at 40 degrees C and continued to exercise at the same speed and inclination for a further 60 min or to exhaustion, whichever occurred first.

The rate of O2 consumption, 2.6 l/min (1.8-3.3) (average from cool and hot conditions), corresponded to 55-77% of their individual maximums. In the cool environment a steady state was reached at 30 min. When the subjects were shifted to the hot room, the core temperature and heart rate started to rise and reached values greater than 39 degrees C and near-maximal values, respectively, at the termination of the exercise.

The leg blood flow (thermodilution method), femoral arteriovenous O2 difference, and consequently leg O2 consumption were unchanged in the hot compared with the cool condition. There was no increase in release of lactate and no reduction in glucose and free net fatty acid uptake in the exercising leg in the heat. Furthermore, the rate of glycogen utilization in the gastrocnemius muscle was not elevated in the hot environment.

There was a tendency for cardiac output to increase in the heat (mean 15.2 to 18.4 l/min), which may have contributed to the increase in skin circulation, together with a possible further reduction in flow to other vascular beds, because muscle blood flow was not reduced.(ABSTRACT TRUNCATED AT 250 WORDS)

PMID: 2246151 [PubMed – indexed for MEDLINE]

Muscle blood flow is not reduced in humans during moderate exercise and heat stress

J Appl Physiol. 1988 Feb;64(2):649-57.

Savard GK, Nielsen B, Laszczynska J, Larsen BE, Saltin B.

August Krogh Institute, University of Copenhagen, Denmark.

The effect of heat stress on circulation in an exercising leg was determined using one-legged knee extension and two-legged bicycle exercise, both seated and upright.

Subjects exercised for three successive 25-min periods wearing a water-perfused suit: control [CT, mean skin temperature (Tsk) = 35 degrees C], hot (H, Tsk = 38 degrees C), and cold (C, Tsk = 31 degrees C). During the heating period, esophageal temperature increased to a maximum of 37.91, 39.35, and 39.05 degrees C in the three types of exercise, respectively. There were no significant changes in pulmonary O2 uptake (VO2) throughout the entire exercise period with either one or two legs.

Leg blood flow (LBF), measured in the femoral vein of one leg by thermodilution, remained unchanged between CT, H, and C periods. Venous plasma lactate concentration gradually declined over time, and no trend for an increased lactate release during the heating period was found.

Similarly, femoral arteriovenous O2 difference and leg VO2 remained unchanged between the three exercise periods. Although cardiac output (acetylene rebreathing) was not significantly higher during H, there was a tendency for an increase of 1 and 2 l/min in one- and two-legged exercise, respectively, which could account for part of the increase in total skin blood flow during heating (gauged by changes in forearm blood flow).

Because LBF was not reduced during exercise and heat stress in these experiments, the additional increase in skin blood flow must have been met by redistribution of blood away from vascular beds other than active skeletal muscle.

PMID: 3372423 [PubMed – indexed for MEDLINE]