Temperature Monitors

TEMPERATURE

Indications

The temperature of patients undergoing anesthe-sia must be monitored. Postoperative temperature is increasingly used as a quality anesthesia indica-tor. Hypothermia is associated with delayed drug metabolism, increased blood glucose, vasoconstric-tion, impaired coagulation, and impaired resistance to surgical infections. Hyperthermia can likewise have deleterious effects perioperatively, leading to tachycardia, vasodilation, and neurological injury. Consequently, temperature must be measured and recorded perioperatively.

Contraindications

Th ere are no contraindications, although a particu-lar monitoring site may be unsuitable in certain patients.

Techniques & Complications

Intraoperatively, temperature is usually measured using a thermistor or thermocouple. Thermistors are semiconductors whose resistance decreases pre-dictably with warming. A thermocouple is a circuit of two dissimilar metals joined so that a potential difference is generated when the metals are at dif-ferent temperatures. Disposable thermocouple and thermistor probes are available for monitoring the temperature of the tympanic membrane, nasophar-ynx, esophagus, bladder, rectum, and skin. Infrared sensors estimate temperature from the infrared energy that is produced. Tympanic membrane tem-peratures reflect core body temperature; however, the devices used may not reliably measure the tem-perature at the tympanic membrane. Complications of temperature monitoring are usually related to trauma caused by the probe (eg, rectal or tympanic membrane perforation).

Each monitoring site has advantages and dis-advantages. The tympanic membrane theoretically reflects brain temperature because the auditory canal’s blood supply is the external carotid artery. Trauma during insertion and cerumen insulation detract from the routine use of tympanic probes. Rectal temperatures have a slow response to changes in core temperature. Nasopharyngeal probes are prone to cause epistaxis, but accurately measure core temperature if placed adjacent to the nasopharyn-geal mucosa. The thermistor on a pulmonary artery catheter also measures core temperature. There is a variable correlation between axillary temperature and core temperature, depending on skin perfusion. Liquid crystal adhesive strips placed on the skin are inadequate indicators of core body temperature dur-ing surgery. Esophageal temperature sensors, often incorporated into esophageal stethoscopes, provide the best combination of economy, performance, and safety. To avoid measuring the temperature of tracheal gases, the temperature sensor should be positioned behind the heart in the lower third of the esophagus. Conveniently, heart sounds are most prominent at this location. For more on the clinical considerations of temperature control.