Methodological investigation of measuring nasopharyngeal temperature as noninvasive brain temperature analogue in the neonate

Abstract

Objectives: (a) To investigate in a newborn animal model whether nasopharyngeal temperature is more closely related to epidural brain temperature than rectal temperature and (b) to investigate in human neonates whether measurement of nasopharyngeal temperature is dependent on the measurement site and other conditions. Design and setting: (a) Animal experiment in newborn piglets, at an institute for surgical research. (b) Prospective study in human neonates, at a neonatal intensive care unit of a tertiary care university hospital. Animals and patients: (a) Nineteen tracheostomized ventilated newborn piglets. (b) Twenty-two spontaneously breathing human newborns nursed either in an incubator or a cot. Measurements and results: (a) In the piglets nasopharyngeal temperature (T_nasoph) measured at the nose-ear distance, defined as distance from the inner brim of the nostril to the tragus and inner rim of the meatus accusticus, most closely reflected epidural temperature (T_epidur) at the epidural surface (r ²=0.89), followed by skin temperature at the temple, rectal temperature (T_rectum) at 2 cm depth, and esophageal temperature (T_esoph) in the middle esophagus. T_nasoph did not significantly differ before and after tracheostomy. (b) In the newborns T_nasoph was significantly lower than T_rectum. Measurements of T_nasoph at nose-ear distance within a feeding tube had a high precision and were unaffected by breathing or head turning. A nasopharyngeal probe was imaged by magnetic resonance imaging in four newborns of various body weight; its tip when inserted to a depth equal to nose-ear distance was anatomically closest to the brain base but separated from it by tissue layer 2.2 cm thick. Conclusions: T_nasoph measured at a position anatomically closest to the brain reflects epidural brain temperature more closely than T_rectum. When measured at nose-ear distance it is unaffected by breathing or head turning. Measuring T_nasoph within a feeding tube and standardizing the measuring position is crucial for its use as brain temperature analogue.