Comparison of Nasopharyngeal Temperature Measured at Fossa of Rosenmuller and Blindly Inserted Temperature Probe with Esophageal Temperature: A Cross-Sectional Study.

INTRODUCTION: Monitoring body temperature and maintaining normothermia are now essentially the standard-of-care during anesthesia. This study was designed to compare the temperature measured by nasopharyngeal temperature probes inserted by landmark method and fiberscope-guided method with esophageal temperature. We hypothesized that placing the temperature probe at the level of fossa of Rosenmuller will reflect core temperature as it is in close relationship to the brain. SUBJECTS AND METHODS: Sixty-five patients aged 18-60 years were enrolled in this cross-sectional study. Two methods were used in our study to place the temperature probes. In landmark-based method, we inserted temperature probe through nostril for a depth equal to philtrum-tragus distance. In fiberscope-guided method, the temperature probe was inserted into nostril and its tip was positioned at fossa of Rosenmuller under fiberscope guidance.
RESULTS: The nasopharyngeal temperatures were recorded at seven time intervals along with esophageal temperature. Mean temperatures were calculated at three different sites. The degree of agreement between two methods at seven time intervals was also calculated. Both methods had good correlation with esophageal temperature. Depth of insertion of temperature probes was documented. There was difference in depth of insertion of temperature probe of around 4.26 cm between two methods, probe length from philtrum to tragus (D1) being longer than distance from fossa of Rosenmuller to nares (D2).
CONCLUSIONS: Nasopharyngeal temperature measured at fossa of Rosenmuller with probe inserted by fiberscope-guided method and that measured by landmark-based method with probe inserted according to philtrum-tragus distance shows good correlation with esophageal temperature.

INTRODUCTION

Humans are homeothermic and require a nearly constant internal body temperature. The human thermoregulatory system maintains core body temperature around 36°C–37°C. Thermoregulation refers to the ability of an organism to keep body temperature within certain boundaries even when the surrounding temperature is different. Processing of thermoregulation occurs in three phases; afferent thermal sensing, central regulation, and efferent responses.[12] Information from thermoreceptors found throughout the body are integrated at several levels within the spinal cord and brain and finally at hypothalamus. When there is a deviation from the normal body temperature, metabolic functions are likely to deteriorate. Anesthetized patients are relatively poikilothermic, wherein the body temperature is determined by the environment. Inadvertent hypothermia which is defined as body temperature <36°C occurs in 6%–90% surgical patients.[3] The major cause of hypothermia in most patients undergoing anesthesia is an internal core to peripheral redistribution of body heat and cold operating room environment.[45] During general anesthesia, most commonly used site for temperature monitoring is nasopharynx. We hypothesized that placing the temperature probe at the level of fossa of Rosenmuller (nasopharyngeal fossa) will reflect core temperature [Figure 1]. This area is supplied by septal branch of sphenopalatine artery which is a branch of external carotid artery and the parapharyngeal branch of internal carotid artery.[67]

Figure 1

Fossa of Rosenmuller

Our primary objective was to compare the temperature measured by nasopharyngeal temperature probes inserted by landmark method and fiberscope-guided method with esophageal temperature. Secondary objective was to quantify the differences in depth of insertion of nasopharyngeal temperature probes by two methods.

SUBJECTS AND METHODS

This cross-sectional study was commenced after approval from departmental dissertation committee and hospital ethics committee. Sixty-five patients of either gender aged between 18 and 60 years belonging to the American Society of Anesthesiologists physical Status I and II, undergoing elective surgery under general anesthesia, were enrolled in the study. Patients with history of nasal surgery/nasal trauma, evidence of nasal deformity, epistaxis, surgery requiring prone position, history of esophageal disorders, and bleeding tendency were excluded from this study. There were two observers in our study. Observer 1 was the anesthesiology resident who evaluated patients preoperatively and recorded the observations. Observer 2 was the consultant anesthesiologist who inserted the three temperature probes. Two methods were used in our study for the insertion of temperature probe, landmark-based method, and fiberscope-guided method. In landmark-based method, temperature probe was inserted blindly according to philtrum-tragus distance, whereas in fiberscope-guided method, temperature probe was inserted under vision until its tip reaches the level of fossa of Rosenmuller. Preoperative evaluation of the patient was done on the day before surgery, written informed consent obtained, and premedications were administered as per concerned consultant anesthesiologist's order. Patients were kept nil oral, 6 h for solids, and 2 h for clear fluids. Inside the operating room, intravenous (i.v.) access was secured. Preinduction monitors included noninvasive blood pressure, pulse oximeter, and 5-lead electrocardiography. It was ensured that operating room temperature was maintained approximately at 22°C. Patients were placed supine with head in neutral position and a pillow under occiput. After preoxygenation, anesthesia was induced with i.v. propofol 2–2.5 mg/kg and fentanyl 1–2 mcg/kg. Vecuronium 0.1 mg/kg was used to facilitate intubation after checking for the ability to ventilate with bag and mask. Anesthesia was deepened with 1.5%–2% isoflurane in oxygen to attain minimum alveolar concentration of 1–1.3. After 3 min of bag and mask ventilation, laryngoscopy and endotracheal intubation performed with appropriately sized endotracheal tube. The cuff was inflated to ensure the absence of leak around the cuff of tube. Heat and moisture exchanger was used. Anesthesia was maintained with isoflurane and nitrous oxide in 50% oxygen. The temperature probes intended to be used for the study were tested for accuracy by dipping their tip in prewarmed sterile saline at 37°C for 5 min and noting that differences in temperatures measured by three probes differ by no >0.1°C after an equilibration time of 5 min. Distance between philtrum and tragus was measured using flexible ruler along the facial curvature (D1). Three adult central temperature probes (Datex Ohmeda – length: 1.5 m, sensor diameter: 4 mm, time response: 6.9 s, and accuracy: ±0.2 at 0°C–25°C, ±0.1 at 25°C–50°C) were taken and marked as probe 1, probe 2, and probe 3. Water-soluble KY jelly was applied on temperature probes. To prevent coiling of esophageal temperature probe (probe 1), Aintree catheter with external markings (catheter length – 56 cm and internal diameter – 4.7 mm) was used as a conduit through which esophageal temperature probe was passed and placed according to depth in the esophagus [Figure 2]. Direct laryngoscopy was performed and the temperature probe 1 whose tip projected by one centimeter beyond tip of Aintree catheter was inserted into the esophagus such that the 24 cm mark lies at the level of vocal cords. The temperature probe 2 was inserted into the right nostril according to the philtrum-tragus distance (D1). Observer 2 inserted fiberscope (Pentax corporation FI-7BS, OD – 2.4 mm) into the left nostril, identified the fossa of Rosenmuller, and placed the tip of temperature probe 3 under vision at that level. The distance from tip of temperature probe 3 to the nares was noted (D2). After 15 min of equilibration time, all three temperatures were noted (T1, T2, and T3 from probes 1, 2, and 3, respectively). The temperatures were noted at serial intervals of 15 min up to 105 min or until the end of surgery if duration of surgery is shorter.

Figure 2

Assembly of esophageal temperature probe inserted into Aintree catheter. A = Tip of temperature probe, B = Tip of Aintree intubation catheter

Statistical analysis

The sample size of 65 patients was required for the study, allowing confidence interval of 95% and expecting a difference of 0.035°C between two methods. Clinically significant difference was considered to be 0.5°C. Statistical analysis was made using SPSS version 16.0 (SPSS Inc., Chicago, IL, USA) software for windows.

RESULTS

Sixty-five patients were included in the study, all of whom completed the study. Demographic data were comparable in our study [Table 1]. The nasopharyngeal temperatures were recorded at seven time intervals along with esophageal temperature. Mean temperatures were calculated at three different sites, namely, T1, T2, and T3. Concept of limits of agreement described by Bland and Altman was used to evaluate the degree of agreement between two methods at seven time intervals [Figure 3]. The limits of agreement is defined as “mean difference between two methods ± 2 standard deviation”. Differences in depth of insertion of nasopharyngeal temperature probes were also calculated. In our study, average philtrum-to-tragus distance, i.e., D1 was 14.665 ± 0.7909 cm. Average distance from the nares to the tip of temperature probe after placing in fossa of Rosenmuller, i.e., D2 was 10.40 ± 0.718 cm. There was difference in depth of insertion of temperature probe of around 4.26 cm between two methods [Table 2]. Correlation coefficient between D1 to height of patients and D2 to height was 0.754 and 0.664, respectively [Table 3]. Temperature recorded at three sites, mean difference, and limits of agreement at seven time intervals were documented [Tables 4 and 5].

Table 1

Demographic data (n=65)

Figure 3

Bland–Altman plots of temperatures at esophageal probe versus nasopharyngeal probes. Horizontal axis depicting average temperature in °C and vertical axis refers to mean difference in temperature (°C) ±2 standard deviation. They were plotted for both the methods at seven time intervals. Plots at 15 min, 45 min, and 105 min are depicted here. The uppermost and lowermost horizontal lines within the graph are estimated Bland–Altman limits of agreement (mean difference ±2 standard deviation). All points are within clinically acceptable range of ±0.5°C. (a) At 15 min, (b) at 45 min, (c) at 105 min

Table 2

Distances (cm) (n=65)

Table 3

Correlation coefficients between height and the distances measured (D1 and D2) (n=65)

Table 4

Temperatures recorded (°C) in three sites at seven time intervals

Table 5

Mean difference in temperatures and limits of agreement at seven time intervals

DISCUSSION

The American Society of Anesthesiologists standards require that “every patient receiving anesthesia shall have body temperature monitored when clinically significant changes in temperature are intended, anticipated, or suspected.” Body temperature monitoring is recommended during any surgical procedure under general anesthesia exceeding 30 min duration and in patients having major operations under neuraxial anesthesia.[28] Inadvertent hypothermia during anesthesia is by far the most common perioperative thermal disturbance. Incidence of hyperthermia during anesthesia is lesser when compared with hypothermia.[9] The body is divided into two thermal compartments, namely, core and peripheral thermal compartments. The core compartment is defined by well-perfused tissues in which temperature remains relatively uniform. The core compartment comprises 50%–60% of the body mass.[10] Tissues in which temperature is nonhomogeneous and variable over time define the peripheral thermal compartment, which mainly consists of the arms and legs. Heat loss occurs primarily from the skin of a patient to environment through radiation (60%), evaporation (20%), conduction, and convection.[11] The interthreshold range is the range over which there is no triggering of autonomic thermoregulatory responses. It is bounded by the sweating threshold at its upper end and by vasoconstriction threshold at the lower end. Most anesthetics increase warm response and reduce cold response thresholds. Interthreshold range is 0.2°C–0.4°C in humans. Both general anesthesia and regional anesthesia widen the interthreshold range.[4] Sites of core temperature monitoring include pulmonary artery, esophagus, urinary bladder, tympanic membrane, and nasopharynx.[12] Nasopharyngeal temperature monitoring is most preferred site as it is easily accessible and it reflects core temperature due to its proximity to the brain.[1314] Adverse consequences of hypothermia such as postoperative shivering, impaired coagulation, increase in myocardial ischemia, wound infection, and delayed emergence are well documented in literature.[151617] Several studies are available in literature which describe the optimal depth of insertion of nasopharyngeal temperature probe.[181920] There has been no study to define the exact site of temperature probe placement in the nasopharynx. We hypothesized that placing the temperature probe at the level of fossa of Rosenmuller will reflect core temperature as it is in close relationship with septal branch of sphenopalatine artery, which in turn is a branch of external carotid artery and the parapharyngeal branch of internal carotid artery.

A total of 65 patients were included in the study. The nasopharyngeal temperatures were recorded at seven time intervals along with esophageal temperature. The primary objective of our study was to compare nasopharyngeal temperature measured by two methods (landmark method and fiberscope method) with esophageal temperature. Mean temperatures were calculated at three different sites named as T1, T2, and T3 which are esophageal temperature, nasopharyngeal temperature measured by blindly inserted temperature probe according to philtrum-tragus distance, and nasopharyngeal temperature measured at level of fossa of Rosenmuller, respectively. Mean difference in temperature was calculated between T1 and T2 and T1 and T3. Mean difference in temperature was marginally lower in fiberscope-guided method compared to landmark-based method, but the difference was clinically insignificant. Limits of agreement were comparable in both the methods [Table 5]. Bland–Altman plots revealed most of observations closer to the mean difference and lying within limits of agreement (mean ± 2 SD).

CONCLUSIONS

Nasopharyngeal temperature measured at fossa of Rosenmuller with probe inserted by fiberscope-guided method and that measured by landmark-based method with probe inserted according to philtrum-tragus distance shows good correlation with esophageal temperature. There is a difference in depth of insertion of temperature probe about 4.26 cm between two methods, the philtrum-tragus distance being longer than distance between nares to fossa of Rosenmuller. Limitations to our study include inability to assess the correlation at lower range of temperatures, pediatric patients, and in surgeries involving major fluid shifts.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.