Evaluation of skin temperature over carotid artery for temperature monitoring in comparison to nasopharyngeal temperature in adults under general anesthesia.

BACKGROUND: Thermoregulation is markedly affected in patients undergoing surgical procedures under anesthesia. Monitoring of temperature is very important during such conditions. Skin temperature is one of the easy and noninvasive ways of temperature monitoring. Common skin temperature monitoring sites are unreliable and did not correlate to the core temperature measurement. AIM: To compare and study the correlation of skin temperature over carotid artery in the neck to that of simultaneously measured nasopharyngeal temperature in adult patients undergoing surgical procedures under general anesthesia. SETTINGS AND
DESIGN: Prospective double-blinded study in a Tertiary Care Center.
MATERIALS AND METHODS: Ninety-seven consecutive American Society of Anesthesiologists I-II patients of age 18-40 years posted for elective surgical procedures under general anesthesia were included. Two temperature sites are monitored: The skin temperature over the carotid artery in the neck with a skin temperature probe T (skin-carotid) and the nasopharyngeal temperature T (naso) with another nasopharyngeal probe. The temperature readings are taken at 0, 15, 30, 45, and 60 min after induction of general anesthesia. STATISTICAL ANALYSIS: Paired t-test, Pearson correlation and Bland-Altman analysis for the rate of agreement.
RESULTS: The skin over the carotid artery in the neck showed statistically significant lower values than simultaneously measured nasopharyngeal temperature. This comparison is done with paired t-test at P< 0.05 significance. Bland-Altman plots showed good agreement between the two sites of temperature measurement.
CONCLUSION: This study has shown that the skin temperature over the carotid artery in the neck was strongly correlated to the nasopharyngeal temperature in adult patients undergoing surgical procedures under general anesthesia.

INTRODUCTION

Anesthesia and surgery significantly affects the internal thermoregulation of the human body. Therefore, temperature monitoring is one of the mandatory parameters to be monitored during anesthesia and to take appropriate measurements to avoid large changes in body temperature.[1] Ideally, temperature monitoring should be the measurement of the core temperature. The core thermal compartment is composed of the highly perfused tissues whose temperature is uniform and high compared with the rest of the body. This core temperature monitoring is done at several sites including nasopharyngeal, esophageal, tympanic, and pulmonary artery[2] which are mostly invasive in nature. Both nasopharyngeal and esophageal temperatures are favored as primary indicators of intraoperative thermal status due to their proximity to the internal carotid artery and great vessels and heart, respectively.[3] The peripheral sites of temperature monitoring like axillary,[4] forehead, and abdominal skin showed poor reliability with the core temperature monitoring sites during intraoperative temperature monitoring. A recent study[5] in infants has shown that skin temperature over the carotid artery T (skin-carotid) is found to be highly correlated to nasopharyngeal temperature T (naso) during perioperative management of infants. The aim of this present study was to compare and study the correlation of skin temperature over carotid artery in the neck to that of simultaneously measured nasopharyngeal temperature in adult patients undergoing surgical procedures under general anesthesia. We hypothesized that the skin temperature over carotid artery did not correlate with the nasopharyngeal temperature during the first 1 h of the intraoperative period in adult patients undergoing a surgical procedure under general anesthesia.

MATERIALS AND METHODS

With the Institute Ethics Committee approval and written informed consent from all patients, this prospective study was carried out in our Department of Anesthesiology in accordance with the ethical standards of Helsinki Declaration over a period of 6 months. We hypothesized that the skin temperature over carotid artery did not correlate with the simultaneously measured nasopharyngeal temperature monitored during the intraoperative period in adult patients undergoing a surgical procedure under general anesthesia. The sample size of 97 based on statistical power analysis was arrived from the previous study.[5] Ninety-seven consecutive American Society of Anesthesiologists (ASA) I–II patients of 18–40 years of age posted for elective surgical procedures under general anesthesia are included in this study. The following patients are excluded: ASA III and above, obesity body mass index >30 kg/m2, pregnancy, emergency surgery, head and neck surgery, major laparotomies, laparoscopic procedures, vascular surgical procedures, peripheral vascular disease, hypo and hyperthyroid disorders, febrile illness, craniofacial abnormalities, and any bleeding or coagulation abnormalities.

The patients were transported from the holding area to the operating room. Standard preinduction monitoring was done with electrocardiogram – II and V lead noninvasive blood pressure, temperature, and pulse oximetry (Phillips IntelliVue MP50). Ringer lactate solution is started at maintenance infusion rate just before induction in all patients. The nasopharyngeal probe and the skin probe are applied before the induction for it to equilibrate with the surrounding tissue. The temperature readings are recorded only after induction of general anesthesia, and oral endotracheal intubation is completed. The nasopharyngeal probe (Phillips nasal temperature probe model no. 21075) is introduced into nasopharynx through one of the nasal apertures to a precalculated depth. The depth of insertion is determined before insertion by measuring the distance externally between the tragus and the nasal aperture. Simultaneously another skin surface temperature probe (Phillips skin temperature probe model no. 21078A) is attached to the anterior part of the neck, centered to the site of maximum intensity of carotid artery pulsation at the level of thyroid prominence [Figure 1]. The skin probe is secured in place and covered with a padded dressing to avoid the effect of external objects affecting the temperature reading. The vital parameters including blood pressure, heart rate, SpO2, and temperature at both sites were recorded.

Figure 1

Schematic diagram showing the placement of the skin temperature probe

After preoxygenation with 100% oxygen for 3 min, patient were induced with injection midazolam 1 mg intravenous (IV), injection fentanyl 2 µg/kg IV, injection thiopentone 5 mg/kg IV, ventilated with sevoflurane 2% in 100% oxygen through a closed circuit, checked for mask ventilation following which vecuronium 0.1 mg/kg administered IV. At the end of 3 min, oral endotracheal intubation was done with 7.5 mm in female patients and 8.5 mm cuffed endotracheal tube in male patients. EtCO2 maintained within 40 mmHg during the study period. The nasopharyngeal T (naso) and skin over carotid artery temperature T (skin-carotid) were measured at the following intervals: Soon after endotracheal intubation (T0), 15, 30, 45, and 60 min. The patient's temperature was monitored and recorded up to 60 min after intubation. The operating room temperature was thermostatically maintained in the range of 22–23°C and relative humidity of 50%. All the patients received warm air blanket (Bair Hugger™ model no. 750) applied over the lower extremity without disturbing the monitoring site.

All the patients were ventilated with N2O:O265%:35% through closed circuit system and anesthesia were maintained with sevoflurane of 2–4%. All the study parameters collected and documented by the same observer who is blinded for the study protocol.

The hemodynamics is maintained within normal range throughout the study period. The significant hemodynamic response that would warrant intervention is defined in the study as a change in heart rate or blood pressure with a 20% change from baseline. Drop in blood pressure or heart rate is treated with injection ephedrine 6 mg IV boluses.

Statistical analysis

Sample size calculation was based on the previous study.[5] A sample size of 96 was required as per correlation coefficient of 0.52 with an alpha error of 5% and power of 80%. Statistical analysis was done with SPSS 16.0 for Windows (SPSS Inc., Chicago, IL, USA), P < 0.05 was considered as significant. To describe the data descriptive statistics mean and standard deviation (SD) were used for continuous variables. To find the significant difference between the two temperature sites paired sample t-test was used. Pearson's coefficient was calculated and studied for the correlation between the two measuring sites. The Bland–Altman method was used for evaluation of the limits of agreement between the two temperature monitoring sites. In the Bland–Altman method, the 95% confidence interval (mean ± 1.96 SD) should include zero, and the distribution of the values should be within confidence limits.

RESULTS

The nasopharyngeal and skin temperature over the carotid artery in the neck were recorded and compared in 97 adult patients undergoing surgical procedures under general anesthesia. The patient group had a mean weight of 55 ± 6.1, the height of 170 ± 10.63, and the age of 30.7 ± 7.2 gender ratio of 27/33 male: female. The normal distribution of the variables is checked and confirmed with Shapiro–Wilk test (P > 0.05). All the patients were afebrile before the procedure. The intraoperative hemodynamic parameters were maintained as per the study protocol.

There was statistically significant difference between the corresponding temperature readings of the two monitoring sites at the study intervals of 0, 15, 30, 45, and 60 min (P = 0.001 at all-time intervals). This comparison was done by paired sample t-test. Thus, T (skin carotid) is significantly lower than the nasopharyngeal temperature as shown in Table 1.

Table 1

Comparison of temperature measurements between the two monitoring sites

Table 2 shows the correlation between the temperature measurements at the two monitoring sites. There is a strong positive correlation between the two monitoring sites at all the study time intervals except for 0 min where the Pearson's coefficient is 0.21, but still it is positively correlated.

Table 2

Correlation of temperature measurements between the two monitoring sites

Figures 2–6 shows the Bland–Altman plot which plots the mean temperature difference among the two monitoring sites at each time interval against the average temperature of the two sites. Bland–Altman plot showed the distribution of mean temperature difference between the two sites of measurement lying within ± 1.96 SD at all study time interval. Thus, Bland–Altman plots showed strong agreement between the two temperature monitoring sites - nasopharyngeal and skin over carotid artery in the neck.

Figure 2

Bland–Altman plot for nasopharyngeal and skin over carotid artery temperature at 0 min

Figure 6

Bland–Altman plot for nasopharyngeal and skin over carotid artery temperature at 60 min

Bland–Altman plot for nasopharyngeal and skin over carotid artery temperature at 15 min

Bland–Altman plot for nasopharyngeal and skin over carotid artery temperature at 30 min

Bland–Altman plot for nasopharyngeal and skin over carotid artery temperature at 45 min

Therefore, skin temperature measured over carotid artery in the neck is significantly lower than the corresponding nasopharyngeal temperature but there are a strong correlation and agreement between the temperature readings of the two sites.

DISCUSSION

In this study, we compared and evaluated the correlation of skin temperature monitored over carotid artery in the neck to that of simultaneously measured nasopharyngeal temperature in patients undergoing a surgical procedure under general anesthesia. We found that the skin temperature over the carotid artery in the neck was significantly lower, but it is highly correlated to a nasopharyngeal temperature at all study time intervals.

Common skin temperature monitoring sites are axilla, forehead, back, chest, anterior abdominal wall, fingers, toes, and the antecubital space. Previous studies have shown poor correlation between common skin monitoring sites and core temperature.[678] Skin temperature monitoring sites such as axilla, forehead, and great toe had less precision and accuracy compared to core temperature monitoring sites during the intraoperative period.[9] Most of the investigations have found that skin temperature does not accurately reflect core temperature. Skin surface temperature is typically 2–4°C less than core temperature.[610] Mean skin temperature is the area-weighted average temperature of the skin surface which has shown to have greater accuracy compared to individual skin monitoring sites.[11] However, the application of this mean skin temperature to monitoring during the intraoperative period is not investigated.

Ideally, the temperature monitoring site should be close to a major vessel or core tissues to reflect the core temperature. For example, the nasopharyngeal probe is placed as close to the internal carotid artery to reflect the core temperature.[12] We sincerely considered that the skin over the neck region is very closely related to the carotid artery. Hence, we designed this study to evaluate and compare skin temperature measured over the carotid artery in the neck with that of nasopharyngeal temperature. Nasopharyngeal temperature is a reliable site to reflect the core temperature even in conditions of extreme thermal perturbation.[213] Hence, we compared these skin monitoring site to that of nasopharyngeal temperature which strongly reflects the core temperature.

Earlier study[5] has shown that the skin temperature over the carotid artery in the neck provides an accurate noninvasive estimate of nasopharyngeal temperature in infants and young children undergoing elective surgery. They also showed that a fixed correction factor of + 0.52°C provides an accurate estimate of nasopharyngeal temperature in infants and children. Our study also reflected the same findings in adult surgical patients during the intraoperative period.

The limitations of our study are: we did not compare the skin temperature over carotid artery in the neck to other skin monitoring sites. The previous studies[8] comparing the various skin monitoring sites have shown a poor correlation of other skin sites to nasopharyngeal temperature. The other limitation is that we have included only the first 1 h after induction of general anesthesia. When we see the pattern of hypothermia[14] during general anesthesia, there is a rapid fall in the core temperature during the 1sth of general anesthesia exposure. Hence, we sincerely felt that the two temperature monitoring sites should be compared and studied during this period. The other limitation can be that we did this study in patients with superficial surgeries without much of fluid shifts or hemodynamic disturbances, hence should be checked in extreme clinical conditions such as a cardiopulmonary bypass, massive fluid shifts where there will be extreme temperature perturbations.

Like the previous study,[5] this study has substantiated the idea of monitoring skin temperature over the carotid artery in the neck in adult patients coming for surgical procedures under general anesthesia. This skin temperature monitoring site may overcome the inaccuracy and unreliability associated with other skin temperature monitoring sites. This is the scope of further research to compare the other skin temperature monitoring sites to that of skin temperature over carotid artery. Hence, skin temperature monitoring site over the carotid artery in the neck can be used for intraoperative temperature monitoring.

CONCLUSION

This study has shown that the skin temperature over the carotid artery in the neck was strongly correlated to the nasopharyngeal temperature in adult patients undergoing surgical procedures under general anesthesia.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest