Temperature-Monitoring Sites

The core thermal compartment is composed of highly perfused tissues whose temperature is uniform and high in comparison to the rest of the body. Temperature in this compartment can be evaluated in the pulmonary artery, distal part of the esophagus, tympanic membrane, or nasopharynx.^{[221] [222]} Temperature probes incorporated into esophageal stethoscopes must be positioned at the point of maximal heart sounds, or even more distally, to provide accurate readings.^[165] Even during rapid thermal perturbations (e.g., cardiopulmonary bypass), these temperature-monitoring sites remain reliable. Core temperature can be estimated with reasonable accuracy by using oral, axillary, rectal, and bladder temperatures, except during extreme thermal perturbations.^{[221] [222]}

Skin surface temperatures are considerably lower than core temperature. ^[223] Skin surface temperatures—when adjusted with an appropriate offset—nonetheless reflect core temperature reasonably well.^[224] However, skin temperatures fail to reliably confirm the clinical signs of malignant hyperthermia (tachycardia and hypercarbia) in swine^[225] and have not been evaluated for this purpose in humans ( Fig. 40-25 ). ^[225] Rectal temperature also normally correlates well with core temperature,^{[221] [222]} but it fails to increase appropriately during malignant hyperthermia crises^[225] and in other documented situations.^{[226] [227]} Consequently, rectal and skin surface temperatures must be used with some caution.

Even during cardiopulmonary bypass, the core temperature-monitoring sites (e.g., tympanic membrane, nasopharynx, pulmonary artery, and esophagus) remain useful. In contrast, rectal temperatures lag behind those measured in core sites. Consequently, rectal temperature is considered an "intermediate" temperature in deliberately cooled patients. During cardiac surgery, bladder temperature is equal to rectal temperature (and therefore intermediate temperature) when urine flow is low, but equal to pulmonary artery temperature (and thus core temperature) when flow is high.^[228] Because bladder temperature is strongly influenced by urine flow, it may be difficult to interpret in these patients. The adequacy of

Figure 40-25 Axillary and esophageal temperatures correlated well during acute malignant hyperthermia in swine, but forehead and neck skin temperatures did not. Rectal temperature also failed to promptly identify the onset of malignant hyperthermia. Elapsed time zero indicates an end-tidal PCO₂ of 70 mm Hg. These data indicate that forehead and neck skin surface temperatures will not adequately confirm other clinical signs of malignant hyperthermia. Valid core temperature monitoring sites include the distal part of the esophagus, pulmonary artery, nasopharynx, and tympanic membrane. Except during cardio-pulmonary bypass, body temperature can also be measured in the mouth, axilla, and bladder. Data are presented as means ± SD. (Redrawn with modification from laizzo PA, Zink RS, Kehler CH, et al: Skin and central temperature during malignant hyperthermia in swine [abstract]. Anesthesiology 77:A569, 1992.)