No abstract

Using radioactive microsphere and electromagnetic techniques, hindleg vascular responses were studied in 38 conscious, chronically prepared sheep subjected to either exposure to a warm environment, and/or local warming of the hypothalamus, spinal cord, forelegs of hindlegs. The total proportion of cardiac output passing through AVA's was increased by all treatments. AVA flow in hindleg skin was increased but capillary flow was unchanged by warming the hypothalamus, spinal cord or forelegs. AVA flow was unchanged but capillary flow was increased by warming the ambient air or the hindlegs alone. Equivalent cooling treatments resulted in AVA and capillary flow changes converse to warming. It is concluded that, in sheep, blood flow through cutaneous AVA's is controlled by specific thermoregulatory reflexes, whereas capillary flow is the target of local temperature effects. A significant role for the direction of the thermal gradient across the skin is implicated.

1. A 'tissue perfusion monitor' (TPM) to non-invasively provide an index of skin blood flow (SkBF) has been developed; it employs photoelectric plethysmographic principles to measure changes in the nett flux of red blood cells in superficial microvasculature. 2. The 'tissue perfusion index' (TPI) varies in proportion to SkBF, provided local haemoglobin concentration does not change significantly. TPI of humans and experimental animals has been shown to indicate reliably, well established phenomena such as decreased SkBF in response to mechanical restriction, cold or Valsalva's manoeuvre, or increased SkBF in response to heat, acetylcholine, sodium nitrite or local nerve blockade. 3. SkBF in sheep was varied between 1 and 156 mL/100g per min as measured with radioactive microspheres. Simultaneous measurements were made using the TPM and four laser-Doppler instruments. The TPI yielded a correlation coefficient of 0.938, and when data were expressed as percentage change, the regression line did not differ significantly from the line of identity and the root-mean-square-error was 6.2%. Data for the laser-Doppler indices of SkBF were, respectively, 0.549-0.786, highly significant deviations in slopes, and 13.6-16.7%. 4. Thus, the TPI is a reliable index of changes in SkBF. Compared with some other available instruments, the TPM is more precise; it is also less sensitive to movement artefact, can be completely portable by battery operation, probes can be multiplexed to a single meter and it is likely to be much less expensive than current lasers. 5. Applications include, for example, experimental investigations of SkBF in man and animals, clinical uses such as evaluation of the efficacy of regional nerve blockade or of circulatory restitution after reconstructive surgery, and clinical tests of neurovascular function.

Radioactive microspheres were used to measure cardiac output and blood flow to most major tissues in sheep at rest and during treadmill exercise (3- to 6-fold increase in metabolic rate for 30 min) in thermoneutral (TN) [dry bulb temperature (Tdb) = 16 degrees C, wet bulb temperature (Twb) = 12 degrees C] and mildly hot (MH) (Tdb = 40 degrees C, Twb = 23 degrees C) environments. During exercise, rectal temperature increased more under MH than under TN conditions; exercise-induced changes in the major central cardiovascular parameters were unaffected by MH. Exercise in TN caused mild hypocapnia, and in MH, severe respiratory alkalosis. Skin blood flow in the torso decreased during exercise in TN and MH. Extremity skin blood flow was increased by heat but not exercise. Exercise-induced increases in flows to respiratory muscles and upper respiratory tract tissues were greatly enhanced in MH. Exercise caused large increases in blood flow to fore- and hindlimb muscles, which were less in MH than in TN. Effects of MH on exercise-induced changes in flow to these and other tissues (e.g., abdominal viscera and adipose tissue) are discussed in terms of the conflicting requirements of energy expenditure and body temperature regulation during exercise in sheep and other species, particularly humans.