Seasonal difference in responses of body fluids to heat stress

Seasonal Variation in Preeclampsia Based on Timing of Conception

OBJECTIVE

Studies have suggested that the incidence of preeclampsia may be partially dependent on the month or season of delivery. We sought to evaluate whether preeclampsia occurs seasonally in our population and whether the timing of conception or delivery is more strongly associated with risk.

METHODS

Between January 1995 and August 2003, we identified 142 primiparous women with singleton pregnancies who met the American College of Obstetricians and Gynecologists' definition for preeclampsia and compared them with 7,762 primiparous control deliveries. We analyzed rates of preeclampsia by individual month and 3-month seasonal blocks based on conception and delivery. Data were analyzed with the chi2 test, and logistic regression and odds ratios were calculated where appropriate.

RESULTS

Preeclampsia occurred in 1.8% of singleton primiparous gestations (142/7,904). Cases were younger than controls (26.5 +/- 5.6 versus 28.0 +/- 0 6.0 years, P < .003), of similar race (97% white versus 96% white, P = .69), and equally likely to have a female child (45% versus 48%, P = .41). We found no significant association of month (logistic regression P = .20) of delivery with the risk of preeclampsia. There was a significant association of month (P = .003) of conception with risk of preeclampsia. Conception during the summer months had the highest risk (incidence 2.3%; odds ratio 1.7; 95% confidence limits 1.06, 2.75) compared with spring (incidence 1.4%). Fall (1.7%) and winter (1.6%) conceptions were associated with intermediate rates of preeclampsia.

CONCLUSION

We identified a seasonal variation in preeclampsia that appears to be more strongly related to timing of conception than to the timing of delivery. The highest incidence of preeclampsia was associated with conception in the summer months.

LEVEL OF EVIDENCE

II-2.

Haematocrit: within-subject and seasonal variation

This review was undertaken, concerning within-subject biological variation and seasonal variation of haematocrit in normal healthy adults and athletes, to find the limits for natural, intra-individual variation in haematocrit values. The terminology and calculations followed well defined theories, from the field of laboratory medicine, about biological variation. Based on results from 12 studies of 638 normal healthy adults, and which used a sampling interval of 1 day to 1-2 months, the coefficient of within-subject biological variation of haematocrit is 3%. The normal within-subject biological variation (3%) and analytical variation (3%) may explain a relative change of approximately 12% (95% level) [e.g. a change from 0.42-0.47] between two successive haematocrit values, measured with a time interval between 1 day and 1-2 months, in a normal healthy adult. Partly due to haemodilution in warm weather, haematocrit often has a seasonal variation in normal healthy adults; based on results from 18 studies of 24 793 participants, the population mean is approximately 3% lower in summer than winter. Population mean values that are 7% lower in summer than winter have been found in some studies, although no seasonal effect may also be seen, especially in temperate climates. If haematocrit values are sampled at yearly peak and trough time points, with intervals of up to 6 months, a 15% relative change (95% level) can be seen in a normal healthy adult; e.g. a change from 0.42-0.48. Published values for haematocrit in athletes are scarce. It is known that the haematocrit value is influenced by training, especially in the first weeks before a new steady-state is reached. Theoretically, the biological variation in athletes could therefore be greater than in normal individuals. Based on two references addressing the biological variation of haematocrit in athletes, however, the above results are also valid for athletes. Further studies, both in the short term and throughout the seasons, are recommended about the natural variation of haematocrit in athletes.

Effect of an exercise-heat acclimation program on body fluid regulatory responses to dehydration in older men

We examined if an exercise-heat acclimation program improves body fluid regulatory function in older subjects, as has been reported in younger subjects. Nine older (Old; 70 +/- 3 yr) and six younger (Young; 25 +/- 3 yr) male subjects participated in the study. Body fluid regulatory responses to an acute thermal dehydration challenge were examined before and after the 6-day acclimation session. Acute dehydration was produced by intermittent light exercise [4 bouts of 20-min exercise at 40% peak rate of oxygen consumption (VO(2 peak)) separated by 10 min rest] in the heat (36 degrees C; 40% relative humidity) followed by 30 min of recovery without fluid intake at 25 degrees C. During the 2-h rehydration period the subjects drank a carbohydrate-electrolyte solution ad libitum. In the preacclimation test, the Old lost approximately 0.8 kg during dehydration and recovered 31 +/- 4% of that loss during rehydration, whereas the Young lost approximately 1.2 kg and recovered 56 +/- 8% (P < 0.05, Young vs. Old). During the 6-day heat acclimation period all subjects performed the same exercise-heat exposure as in the dehydration period. Exercise-heat acclimation increased plasma volume by approximately 5% (P < 0.05) in Young subjects but not in Old. The body fluid loss during dehydration in the postacclimation test was similar to that in the preacclimation in Young and Old. The fractional recovery of lost fluid volume during rehydration increased in Young (by 80 +/- 9%; P < 0.05) but not in Old (by only 34 +/- 5%; NS). The improved recovery from dehydration in Young was mainly due to increased fluid intake with a small increase in the fluid retention fraction. The greater involuntary dehydration (greater fluid deficit) in Old was accompanied by reduced plasma vasopressin and aldosterone concentrations, renin activity, and subjective thirst rating (P < 0.05, Young vs. Old). Thus older people have reduced ability to facilitate body fluid regulatory function by exercise-heat acclimation, which might be involved in attenuation of the acclimation-induced increase in body fluid volume.

Thermoregulation and body fluid osmolality

Thermoregulatory responses induce dehydration, and dehydration itself raises body temperature, causing an increase in the threshold temperature for cutaneous vasodilatation and sweating, the sensitivity of cutaneous vasodilatation in response to a unit rise in body temperature, and the maximum attainable level of cutaneous circulation, and sweat rate. The reduction of these thermoregulatory responses has been related to hypovolemia and hyperosmolality. Evidence showing the involvement of cardiopulmonary baroreceptors is discussed along with an introduction on the effect of hyperosmolality on skin blood flow and sweating and the involvement of central nervous mechanisms. Heat induced hyperosmolality triggers regulatory responses maintaining blood volume and circulatory function, including a fluid shift between body fluid compartments and the control of fluid intake. Evidence showing the importance of the osmotic regulation of body fluid by drinking is also presented. Finally, the effect of hypovolemia and hyperosmolality under thermal stress due to hot environment or physical activity is discussed from the viewpoint of the interaction between circulation, thermoregulation and body fluid homeostasis.

Changes in blood volume and vascular compliance during body heating in rats

The effects of hyperthermia on blood volume and effective vascular compliance were studied in control and heat acclimated rats (three weeks at 32 degrees C and 50% R.H.). Experiments were performed on conscious rats whose abdominal aorta and both jugular veins were cannulated. Continuous changes in blood volume (BV) were monitored by measuring 51Cr tagged erythrocyte dilution, using an arterio-venous extracorporeal shunt passing through a gamma counter. Total vascular compliance was calculated from the relation between changes in BV and central venous pressure during 10 min of infusion of saline at a rate of 1.6% body wt/10 min. Hyperthermia induced a significant blood volume expansion. This expansion was more pronounced in non-acclimated rats. Effective vascular compliance was similar in the normothermic, both non-acclimated and acclimated rats. However, while hyperthermia did not affect the vascular compliance of non-acclimated rats, it was decreased significantly in the acclimated hyperthermic rats. The data suggest that changes in vascular compliance play a role when rapid blood volume changes take place, especially in acclimated hyperthermic animals. The relations between changes in vascular compliance and heat induced redistribution of cardiac output are discussed.