TAKE HOME MESSAGES

• Measurement of fetlock temperature provides a practical, noninvasive, inexpensive, but only moderately useful method for predicting cardiac output and need for intravenous fluids in dehydrated calves housed at thermoneutral ambient temperatures (50-76 F).
• Fetlock temperature is of minimal to no value for predicting cardiac output and need for intravenous fluids in calves housed at <50 F.

INTRODUCTION

Diarrhea in neonatal calves is a major source of economic loss for the cattle industry, being the leading cause of death in dairy and beef calves in the United States. Financial losses occur not only from calf mortality, but also from the cost of medication and labor needed to treat sick calves. One of the major goals of treatment is increasing cardiac output and correcting dehydration, through administration of oral electrolyte solutions and/or intravenous fluids. Because of the expense associated with conventional methods for administering intravenous fluids, such treatment is usually reserved for severely dehydrated calves (> 8% dehydration). An accurate, practical, and inexpensive method for predicting cardiac output and the need for intravenous fluids in calves with diarrhea would therefore be of value in guiding treatment.

Peripheral temperature is correlated with cardiac output and survival in critically ill human beings, as they indicate the degree of peripheral blood flow and tissue perfusion. In humans, the extremities exhibit the greatest variation in blood flow, the change being greatest in the distal rather than the proximal portion of the extremities, and larger in the toes than in the fingers. Measurement of peripheral temperature may therefore provide a useful, noninvasive, and inexpensive method for evaluating cardiac output and the need for intravenous fluid administration in dehydrated calves. Although a number of studies in dehydrated calves have used fetlock (peripheral) temperature to monitor the response to treatment, the nature of the relationshipbetween these 2 factors and cardiac output is presently unknown. Accordingly, the main purpose of this study was to determine the relationship between peripheral (fetlock) temperature and cardiac output, in dehydrated calves with diarrhea.

MATERIALS AND METHODS

Twenty-eight male dairy calves, 3 to 10 days old, were examined in a prospective, experimental study. Severe dehydration and watery diarrhea were induced by administering diuretic agents (furosemide, hydrochlorothiazide, spironolactone) and sucrose solution. Cardiac output was measured by thermodilution, and fetlock temperature was measured by taping a thermistor to the fetlock of the left hind leg and insulating the thermistor from ambient air. This study was performed at an ambient temperature within the thermoneutral zone for neonatal calves (50-76 F), in order to minimize the effects of cold-induced vasoconstriction on peripheral blood flow. The relationshipbetween fetlock temperature and cardiac output was examined using linear regression. Eleven healthy and normally hydrated dairy calves, 6 to 21 days old and housed outside in calf hutches, were also examined at an ambient temperature of 26 F, in order to examine the effect of ambient temperature on the measurement of fetlock temperature.

RESULTS & DISCUSSION

Within thermoneutral ambient temperatures, fetlock temperature was constant and independent of cardiac output at normal or high values for cardiac output, but linearly dependent upon cardiac output below a critical value (78 % of normal cardiac output). Regression equations were developed that predicted cardiac output from a measured fetlock temperature. At ambient temperatures below the lower critical temperature for neonatal calves (46-50 F), normal values for fetlock temperature in healthy calves were significantly lower than those at thermoneutral ambient temperatures.

The thermoneutral zone represents an ambient temperature range (effective dry still-air temperature) within which an animal's heat production is constant and independent of the thermal environment. The lower and upper borders of the zone of thermoneutrality are called the lower and upper critical temperature, respectively. These critical temperatures are dependent on age, breed (body weight and thermal insulation), nutrition, time after feeding, behavior, housing, wetness of hair coat, and extent of solar radiation. For neonatal dairy and beef calves, the lower critical temperature is generally accepted to be 8-10^oC (50 F). The lower critical temperature decreases with age, being 8-10^oC in neonatal calves, 8^oC in 3 week old calves, 0^oC in 1 month old calves, and -14^oC in 3 month old veal calves. Calves are less cold tolerant than adult cattle because of lower values for heat production, tissue insulation, and external insulation when indexed to surface area. The upper critical temperature for neonatal calves is 24-27^oC.

Although peripheral temperature in critically ill human beings is measured either on the big toe or thumb, the fetlock provides the most convenient site for measuring peripheral temperature in calves, and appears to be more sensitive than hock temperature to changes in hydration status (and therefore cardiac output) and peripheral blood flow. Moreover, because there are no muscles present in the fetlock, the contribution of tissue metabolic heat from contracting muscle is negligible. The hind limb is preferred over the forelimb, because calves fold their front limbs when in sternal recumbency, allowing heat conduction from the thorax to potentially increase forelimb fetlock temperature. In yearling steers housed at cold ambient temperatures, the skin temperature of the distal hind limb is lower than that of the front limb and tail. Ear temperature is less accurate than fetlock temperature for evaluating peripheral blood flow, as periodic increases in blood flow through arterio-venous anastomoses occur in the ear veins of calves.

Peripheral temperature and core-peripheral temperature difference are practical, noninvasive, inexpensive, but only moderately useful methods for predicting cardiac output in hemodynamically stable calves housed in a thermoneutral environment. These parameters are therefore of some value in daily monitoring of the response to treatment and in determining the need for intravenous fluids in dehydrated calves housed at a dry still-air temperature of 10-24^oC, but are of minimal to no value in calves housed at <50 F.