As long ago as the nineteenth century, it was noted that uremia is associated with hypothermia in human patients, although to date the pathophysiology of uremic hypothermia (UH) in human and veterinary patients has not been completely elucidated. Metabolism in the kidneys is highly aerobic, so these organs contribute over 10% of total body heat. Almost one-quarter of pre-dialysis human patients with chronic kidney disease (CKD) are hypothermic.
In this retrospective review of medical records of uremic cats and dogs treated with intermittent hemodialysis (IHD) at New York City's Animal Medical Center from 1997 to 2013, UH was identified as a clinical phenomenon that occurs in both species, although cats are more severely impacted. These investigators hypothesized that azotemic dogs and cats are hypothermic upon presentation and that UH would resolve as the azotemia is corrected by IHD. Animals with end-stage CKD and those requiring IHD for nonuremic disease were not included in the study.
The temperature of the dialysate used was 99.7⁰F (37.6⁰C), and all patients were provided with a circulating water heating pad; additional external heat support was provided at the discretion of the attending clinician. The control group included nonazotemic intensive care unit patients matched to the azotemic group in terms of sex and age group. One control cat was identified for each azotemic feline patient. Mean rectal temperatures of the dog and cat cases were obtained before and after each IHD treatment.
The group of 79 uremic cats included 36 spayed females, 41 castrated males, and 2 intact males. The median age of the case cats was 7.8 years (range 0.6-15.9 years), while the median age of the controls was 9.0 years (range 0.5-15.7 years). The median weight of the uremic cats was 5.0 kg (range, 2.3-12.0 kg), while that of the control group was 4.2 kg. Of the uremic cats, 38% were hypothermic, while only 12.7% of the control cats were hypothermic. The mean body temperature of the uremic cats was significantly lower at 99.3⁰F (range, 92.3-103.4⁰F), while that of the control cats was 100.6⁰F (range 94.0-103.8⁰F).
At the end of IHD treatment, 54% of the dogs and 42% of the cats had an increase in temperature. For cats < 5 kg, mean body temperature decreased after dialysis, but this change was not found to be significant. The investigators concluded that the small body size of the cats < 5 kg may mask the effects of correcting azotemia on the UH, despite efforts to conserve body heat with external heat support. Cats in this group have a larger proportion of their blood in the extracorporeal portion of the dialysis circuit, where it is exposed to ambient room temperature. The authors suggest that for felines, increasing the dialysate temperature may be particularly beneficial in preserving body heat. Similar to the findings in dogs, in cats > 5 kg, the mean body temperature increased significantly, from a pre-dialysis temperature of 100.7⁰F to a post-dialysis temperature of 101.0⁰F.
In contrast to the uremic dogs, in which the UH was more significantly correlated with body weight than with blood urea nitrogen (BUN) or serum creatinine concentrations, hypothermia in the uremic cats appeared to be a consequence of uremia alone and was particularly associated with the magnitude of the BUN concentration. The cat's small body size and limited variability in body size in comparison to dogs, permits a significant amount of heat to be lost at the body surface. One of the few previous veterinary studies of UH demonstrated that male cats with urethral obstruction and hypothermia had significantly higher BUN and serum creatinine concentrations than those who were normothermic. Lower body temperature was found in another study to be a negative prognostic indicator for cats with acute kidney injury (AKI).
Cellular hypometabolism secondary to the effects of uremic toxins is considered to be one of the major etiologic reasons for UH. As some of the dialyzers used in the study only remove small molecules from the blood circulation, leaving medium-sized and large molecules behind, substances with small molecular size may be involved in the pathogenesis of UH. Hypothermia in a patient upon presentation, especially a feline patient, should raise the clinician's suspicion of uremia before renal function is evaluated clinicopathologically, and the degree of hypothermia may also correlate with the severity of uremia. Although IHD is not at the disposal of the primary care clinician, heat support of uremic patients via warmed parenteral fluids, blankets, and safe heating devices, for those with AKI and/or urethral obstruction in the hospital setting, and those with CKD in both the clinic and home environments, is crucial to the cat's survival and well-being.