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OBSTETRIC ANESTHESIOLOGY

Intraoperative Forced Air-Warming During Cesarean Delivery Under Spinal Anesthesia Does Not Prevent Maternal Hypothermia

From the Department of Anesthesia, Stanford University School of Medicine, Stanford, California.

Address correspondence to Brendan Carvalho, MBBCh, FRCA, Department of Anesthesia, H3580, Stanford University School of Medicine, 300 Pasteur Drive, Stanford, CA 94305. Address e-mail to bcarvalho{at}stanford.edu.

Abstract

BACKGROUND: Prewarming and intraoperative warming with forced air-warming systems prevent perioperative hypothermia and shivering in patients undergoing elective cesarean delivery with epidural anesthesia. We tested the hypothesis that intraoperative lower body forced air-warming prevents hypothermia in patients undergoing elective cesarean delivery with spinal anesthesia.

METHODS: Thirty healthy patients undergoing cesarean delivery with spinal anesthesia were randomly assigned to forced air-warming or control groups (identical cover applied with forced air-warming unit switched off). A blinded investigator assessed oral temperature, shivering, and thermal comfort scores at 15-min intervals until discharge from the postanesthetic care unit. Umbilical cord blood gases and Apgar scores were also measured after delivery.

RESULTS: The maximum core temperature changes were similar in the two groups (–1.3°C ± 0.4°C vs –1.3°C ± 0.3°C for the forced air-warming group and control group, respectively; P = 0.8). Core hypothermia (35.5°C) occurred in 8 of 15 patients receiving forced air-warming and in 10 of 15 unwarmed patients (P = 0.5). The incidence and severity of shivering did not significantly differ between groups. Umbilical cord blood gases and Apgar scores were similar in both groups (P = NS).

CONCLUSIONS: We conclude that intraoperative lower body forced air-warming does not prevent intraoperative hypothermia or shivering in women undergoing elective cesarean delivery with spinal anesthesia.

Patients undergoing cesarean delivery with spinal anesthesia are at increased risk of developing core hypothermia during the perioperative period compared with those who have epidural anesthesia (1). Core hypothermia may be associated with a number of adverse outcomes in nonpregnant patients, including shivering, wound infection, coagulopathy, increased blood loss and transfusion requirements, decreased metabolism, and prolonged recovery (2–4). Perioperative shivering can occur in up to 85% of patients undergoing cesarean delivery under regional anesthesia (5). Shivering can result in interference with monitoring, increased tension on wound edges, and increased oxygen consumption (6). Patient discomfort and surgical disruption as a result of hypothermia and shivering may also present problems during the perioperative period.

Forced air-warming units are commonly used to prevent intraoperative hypothermia. A previous study has shown that patients undergoing cesarean delivery with epidural anesthesia experience less hypothermia and shivering if forced air-warming is used in the preoperative and intraoperative periods (7). In the same study, neonates of mothers receiving forced air-warming during cesarean delivery had improved umbilical venous pH and were less hypothermic. However, elective cesarean delivery is commonly performed under spinal anesthesia (8). To our knowledge, no previous studies have evaluated the use of forced air-warming during cesarean delivery under spinal anesthesia.

The primary objective of this study was to test the hypothesis that intraoperative lower body forced air-warming prevents hypothermia in patients undergoing cesarean delivery with spinal anesthesia. Secondary objectives were to assess if forced air-warming is associated with less perioperative maternal shivering and more favorable neonatal outcomes.

METHODS

Study Design and Patient Population
After obtaining local ethics committee approval and written informed consent, 30 healthy ASA physical status I or II pregnant women presenting for scheduled cesarean delivery under spinal anesthesia were enrolled in this randomized, placebo-controlled study. All parturients between 18 and 40 yr of age with a singleton pregnancy and gestation longer than 37 wk scheduled cesarean delivery were eligible to participate. Exclusion criteria included severe uncontrolled medical conditions (including diabetes mellitus, pregnancy-induced hypertension, coagulation disorder, and significant cardiovascular disease) and body mass index >40 kg/m².

Study Protocol
After gaining written informed consent, patients were randomly assigned to one of the two groups: forced air-warming unit with lower body warming cover (Bair Hugger®; Augustine Medical, Eden Prairie, MN) using a Model 501 warming unit set at 43°C, or control (identical cover applied with forced air-warming unit switched off). Group assignments were determined by a computer-generated random number sequence and were contained in sequentially numbered opaque envelopes to ensure blinding.

All patients were premedicated with metoclopramide 10 mg and ranitidine 50 mg and received a room temperature colloid initial administration of 500 mL 6% hydroxethylstarch solution (Hespan; Hospira, Lake Forest, IL) via an 18-gauge IV cannula. Spinal anesthesia was performed in the sitting position at the L3–4 interspace with a 25-gauge Whitacre needle by an anesthesiologist not involved in the study, with hyperbaric bupivacaine 12 mg, fentanyl 10 µg, and morphine 200 µg. Block height was tested using pinprick and surgery commenced after obtaining a sensory block T4 dermatome. After induction of spinal anesthesia and urinary catheter placement, an anesthesiologist not involved in the study placed the lower body forced air-warming cover over the patient (the adhesive strip was secured on the upper thighs just distal to the inguinal fold), and set-up the forced air-warming unit (switched on or off depending on study group allocation). During application of the warming cover, the study investigator temporarily left the operating room to maintain blinding. A warmed cotton blanket was placed over the forced air-warming cover of patients in both study groups, and a second warmed cotton blanket was placed over the upper body with arms positioned on arm rests. The forced air-warming cover was removed at the end of surgery before the patient was transferred to the recovery area.

Maternal data and observations, including core temperature, degree of shivering, thermal comfort, pain, nausea, heart rate, mean arterial blood pressure, and block height were recorded before initiating anesthesia in the operating room (baseline) and at 15-min intervals for a maximum of 135 min. The study period ended when the patient was discharged from the postoperative recovery room. The data were collected by an investigator who was blinded to group assignment. Core temperatures were recorded using an oral digital thermometer (Medichoice, Portsmouth, VA) placed under the tongue for at least 60 s. The thermometer contains a thermistor to measure temperature, and electronic circuitry within the thermometer measures the thermistor's electrical resistance. The temperature difference between the highest and lowest temperature readings (T) was calculated for each patient, and these values were averaged within each group. The operating room ambient temperature was maintained near 23°C, and both operating room and recovery room temperatures were measured at the start of the study period using a Mon-a-therm thermocouple temperature unit (Mallinkrodt Medical Inc., St. Louis, MO).

Shivering was graded using the following scale devised by Wrench et al. (9): 0 = no shivering, 1 = one or more of the following: piloerection, peripheral vasoconstriction, peripheral cyanosis without other cause, but without visible muscular activity; 2 = visible muscular activity confined to one muscle group; 3 = visible muscular activity in more than one muscle group; and 4 = gross muscular activity involving the whole body. If the shivering score was 3, treatment with IV meperidine (12.5–25 mg) was offered to the subjects to alleviate shivering-associated discomfort. Patients were assessed for thermal comfort using a verbal numerical scale, similar to that devised by Horn et al. (7): 0 mm as worst imaginable cold, 50 mm as thermoneutral, and 100 mm as insufferably hot.

Pain intensity during the cesarean delivery was measured using a verbal numerical scale (0–100, with 0 = no pain and 100 = the worst pain imaginable), and nausea was measured using a verbal numerical scale (0–100, with 0 = indicating no nausea and 100 = the worst nausea imaginable). IV fluids were administered at room temperature. The volume of intraoperative IV fluids used and the estimated blood loss were also recorded for each patient.

Umbilical artery and umbilical vein blood were sampled from a double-clamped segment of umbilical cord for blood gas analysis immediately after birth. A pediatrician blinded to the group assignment determined Apgar scores at 1 and 5 min after birth.

At the end of the study period, patients were asked to provide a satisfaction score for the quality of care using a verbal numerical scale (0–100, with 0 = extremely unsatisfied and 100 = extremely satisfied). In addition, patients in both groups were asked to select a response to describe their experience of the lower body cover and forced air-warming (very uncomfortable/uncomfortable/neither comfortable nor uncomfortable/comfortable/very comfortable).

For purposes of this study, hypothermia was defined as 35.5°C (3). For ethical reasons, active warming (with the forced air-warming unit set at 43°C) was instituted if any patient in the control group developed core hypothermia during the study period.

An a priori sample size calculation based on data from a previous study that used forced air-warming during cesarean delivery under epidural anesthesia (7) revealed that 14 patients per study group were required to detect a 1°C change in maternal temperature (power = 0.9; P = 0.05; two-tailed).

Descriptive statistics were used to summarize demographic, outcome, and side effect data. Data are expressed as mean ± sd, median (range) and numbers or percentages as appropriate. Outcome measures of interest between the two groups were compared using the Student's t-test or Mann–Whitney U-test. Normal distribution was determined using QQ plots and the Kolmogorov-Smirnov test. Associations among discrete variables were investigated using Pearson's ² test or Fisher's exact test. Longitudinal data analysis was performed using repeated measures analysis of variance to determine differences in outcome measure values over time. Analyses were performed with SPSS 11.0 statistical package (Chicago, IL) and Microsoft Excel; P < 0.05 was considered statistically significant.

RESULTS

All 30 patients completed the study according to the protocol and were included in the analysis. No spinal block failures or surgical complications requiring conversion to general anesthesia were encountered. Maternal demographic and obstetric data were similar between the groups (Table 1). The median duration of surgery was similar and short in both groups (41 min vs 52 min; forced air-warming versus control, respectively). Surgical and anesthetic characteristics are summarized in Table 2.

There were no significant differences between the study groups with respect to core temperature values during the study period (Fig. 1). The mean ± sd maximum core temperature change (T) was –1.3°C ± 0.4°C compared with –1.3°C ± 0.3°C for the forced air-warming group and control group, respectively (P = 0.8). We found no differences between the groups in the number of patients who became hypothermic (35.5°C) (8 patients vs 10 patients in the forced air-warming group and control group, respectively; P = 0.5). Sixty-six percent (10 of 15 patients) of patients in the control received lower body forced air-warming as they became hypothermic during the study period.

View larger version (11K): [in this window] [in a new window]   Figure 1. Intraoperative and postoperative temperatures in patients assigned to forced air-warming or a control group with no active warming over the study period. Data are expressed as mean ± sd; P = NS between the groups for all time intervals from time of spinal anesthesia (t = 0 min). #Mean intrathecal injection-incision interval for both study groups (P = NS between groups); *Mean intrathecal injection-end surgery interval for both study groups (P = NS between groups). The interval between these two points (line) approximates the period of intraoperative active or placebo warming.

There were no significant differences in the incidence of shivering (shivering scores 1) (Table 3), shivering intensity (Fig. 2), or thermal comfort scores at any time observed between the groups. One patient in each group received meperidine (12.5 mg) to treat shivering. There were no differences in arterial blood pressure and heart rate (data not shown), or in the incidence of nausea or vomiting between groups. Patient satisfaction scores were not different between the groups (Table 3). Furthermore, 67% of patients in both groups described the warming cover (with and without forced air-warming) as "comfortable," and no patient described the warming device as "uncomfortable." We observed no differences in neonatal outcome as measured by Apgar scores at 1 and 5 min, and umbilical blood gas values (Table 4).

View larger version (14K): [in this window] [in a new window]   Figure 2. Shivering intensity between patients receiving forced air-warming and controls. Shivering intensity for each patient was determined using a shivering scale: 0 = no shivering, 1 = one or more of the following: piloerection, peripheral vasoconstriction, peripheral cyanosis without other cause, but without visible muscular activity; 2 = visible muscular activity confined to one muscle group; 3 = visible muscular activity in more than one muscle group; and 4 = gross muscular activity involving the whole body). P = NS between groups.

Post hoc power analysis indicated 100% power in both the forced air-warming and control groups to detect a decrease of 1 degree in maternal temperature from a normothermic baseline of 36.5°C ± 0.35°C.

DISCUSSION

In our study, intraoperative lower body forced air-warming did not reduce the incidence of perioperative hypothermia in patients undergoing cesarean delivery with spinal anesthesia compared with a control group receiving passive insulation.

Intraoperative core hypothermia during neuraxial anesthesia occurs because of a number of processes that impair thermoregulation. Core-to-peripheral redistribution of heat by vasodilation is the major cause of core hypothermia in the first hour of neuraxial anesthesia (10,11). Neuraxial anesthesia also reduces thermoregulatory vasoconstriction and shivering thresholds by approximately 0.5°C, respectively (12–14). Although vasoconstriction and shivering can subsequently occur above the upper level of dermatomal block, these thermoregulatory mechanisms are ineffective in preventing decreases in core temperature after neuraxial anesthesia for cesarean delivery (1). Hypothermia may continue if metabolic heat production is unable to match environmental heat loss. Patients are also more likely to tolerate core hypothermia due to an increase in apparent lower body skin temperature with neuraxial anesthesia (1,13).

Active warming devices, such as forced air-warming, are commonly used to maintain normothermia during the intraoperative period. Forced air-warming has been shown to reduce hypothermia for nonobstetric patients undergoing general anesthesia (15) and for those recovering from hypothermia in the postoperative period (16). The results of our study contrast with those from an earlier study in patients undergoing cesarean delivery, which reported that patients receiving forced air-warming had higher core temperatures and fewer shivering episodes compared with a group receiving passive insulation (7). However, there are significant differences between the two studies including: epidural versus spinal anesthesia, upper body versus lower body forced air-warming, prewarming (15 min forced air-warming before initiation of epidural anesthesia) versus no prewarming period, warmed versus unwarmed IV fluids, and a much lower total active warming period (more than 2 h compared with <1 h in our study). The shorter period of active warming in our study is the most likely explanation for different findings of the two studies, although other protocol differences may have also contributed, including lower body versus upper body forced air-warming and mode of anesthesia.

To our knowledge, no previous studies have compared the efficacy of lower body versus upper body forced air-warming for patients undergoing neuraxial anesthesia. Brauer et al. investigated the effects of both forced air-warming modalities using a sophisticated copper manikin, with a similar combined heat exchange coefficient for radiation and convection as reported in volunteers (17,18). The results from these studies suggested that heat transfer with forced air-warming over the lower body is higher than over the upper body (67 vs 58 W, respectively). Motamed et al. compared both forced air-warming modalities for patients undergoing prolonged abdominal surgery with general anesthesia (19). Normothermia was attained more quickly with lower body forced air-warming (120 min vs 180 min); however, a significant initial decrease in temperature was observed in both groups (presumably from core-to-peripheral heat redistribution). Other studies have also shown that forced air-warming does not compensate for the initial hypothermia after induction with general anesthesia (15,20,21). In current obstetric practice, spinal anesthesia is the preferred technique for patients undergoing elective cesarean delivery (8); we ensured that the study protocol was similar to our institutional anesthetic practice. Previous work has shown that spinal anesthesia decreases initial core temperatures more rapidly than epidural anesthesia for patients undergoing cesarean delivery (1). Therefore, we speculate that the profound initial effect on core temperature with spinal anesthesia, as well as the short anesthetic induction time and surgical duration, are likely to counter any potential warming effects with either upper body or lower body forced air-warming.

Previous studies have shown that prewarming (mean 81 min) is beneficial in reducing intraoperative hypothermia for surgical patients receiving epidural or general anesthesia (7,22). However, long periods of active prewarming for patients before surgery may lead to significant organizational delays in hospitals with high cesarean delivery rates, and it is unclear whether patients would tolerate prolonged periods of forced air-prewarming. However, instituting forced air-warming for hypothermic patients in the postoperative period may increase the rate of rewarming and reduce the duration of core hypothermia (16,23).

The incidence of shivering and shivering intensities was similar between the groups, suggesting that lower body forced air-warming may not influence shivering activity for patients undergoing cesarean delivery with spinal anesthesia. Spinal anesthesia has been shown to reduce the shivering threshold by impairing autonomic thermoregulation, with a resultant increase in apparent leg warming by blocking sensory input from the legs (1). A previous study assessing the thermoregulatory responses to hypothermia in women undergoing spinal anesthesia showed that, when upper body skin temperatures were kept constant, the shivering threshold was significantly reduced compared with a control group (14). Indeed, forced air-warming has also been shown to be ineffective in reducing the duration of shivering (compared with passive insulation) in patients recovering from general anesthesia, despite improving patient thermal comfort and reducing oxygen consumption (24). We acknowledge that measurement of the degree of peripheral arteriovenous shunt vasoconstriction would have been beneficial in confirming the thermoregulatory origin of the shivering activity in our study.

Our results may have been influenced by the effects of intrathecal opioids on thermoregulation. The effects of intrathecal opioids on core temperature and shivering activity for patients undergoing cesarean delivery are unclear. Intrathecal morphine has been shown to produce a more pronounced hypothermic effect in patients during cesarean delivery compared with a control group receiving no opioids (25). Combining intrathecal fentanyl and morphine has been shown to decrease the incidence of shivering compared with fentanyl alone (26). Furthermore, Hong and Lee reported that intrathecal pethidine produced fewer and less intense shivering episodes than intrathecal morphine, although core hypothermia occurred to a similar degree in all study groups (27). Further investigations are warranted to study the effects on maternal temperature and shivering intensity of single and combination intrathecal opioids during spinal anesthesia for cesarean delivery.

Our neonatal outcome measures (umbilical cord blood gases and Apgar scores) remained within normal limits; however, the effects of maternal hypothermia on fetal well-being remain unclear. A study of the effects of core hypothermia in pregnant ewes showed that fetal temperature did not decrease as much as maternal temperature (28), and that fetal homeothermy may be preserved by maternal and uteroplacental vascular mechanisms in the setting of maternal hypothermia.

We acknowledge that there are some limitations in our study. Core temperature values were measured with oral thermometers, and there may be variability between oral and other modes of temperature measurement (e.g., tympanic). However, our measurement technique was consistent for all patients, and the thermometers used in the study have good measurement accuracy (0.1°C between 34°C and 42°C; Medichoice, Portsmouth, VA). For ethical reasons, we instituted forced air-warming if any patient in the control group developed a core temperature of 35.5°C. Therefore, we appreciate that intergroup comparisons of final core temperature values are somewhat limited as a result of this aspect of our study protocol. We aimed to maintain patient, pediatrician, and investigator blinding throughout the study, however, this proved difficult due to the noise the forced air-warming unit produced when active. Nonetheless, our primary end point (oral temperature) is an objective measure and therefore is unlikely to be affected by observer bias.

In our study, IV fluids were administered at room temperature according to our routine practice. However, the use of warmed IV fluids has been shown to improve perioperative core temperature (as a unimodal warming approach or with forced air-warming) (29,30). Further work is necessary to assess the efficacy of warmed fluids in maintaining normothermia during the short perioperative period associated with cesarean delivery.

In summary, we compared the effects of lower body forced air-warming on core temperature and shivering in patients requiring scheduled cesarean delivery under spinal anesthesia. Under conditions of the study, intraoperative forced air-warming did not prevent perioperative hypothermia compared with a control group without forced air-warming. In addition, the incidence and intensity of shivering episodes were not significantly decreased. Therefore, the use of intraoperative lower body forced air-warming as a single modality to prevent core hypothermia for patients undergoing scheduled cesarean delivery under spinal anesthesia is not supported. Further research is required to assess whether multimodal approaches to maintain peripartum normothermia can alter maternal and neonatal outcomes in patients undergoing cesarean delivery.

Footnotes

Accepted for publication July 31, 2007.

Financial support: This study was conducted at Lucile Packard Children's Hospital and Stanford University School of Medicine, Stanford, CA. This study was funded internally by the Department of Anesthesia, Stanford University Medical Center. The authors involved in this study and the preparation of the manuscript received no external funding. Dr. Carvalho's work is supported by a Building Interdisciplinary Careers in Women's Health Research grant from the Office of Research on Women's Health and National Institute of Child Health and Human Development of the National Institutes of Health (5K12 HD043452).

The authors had no affiliation or relationship with any company or organization which has a potential interest in the outcome of the study.

Reprints will not be available from the authors.

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a colleague Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via Web of Science (5) Citing Articles via Google Scholar Google Scholar Articles by Butwick, A. J. Articles by Carvalho, B. Search for Related Content PubMed PubMed Citation Articles by Butwick, A. J. Articles by Carvalho, B. Related Collections Obstetrics Equipment Regional Anesthesia