Hypothermia Treatment Using Radiant Heat Hypothermia Critique

Hypothermia Treatment Using Radiant Heat

Hypothermia

Critique of Postoperative Hypothermia Treatment Using Radiant Heat by Yang et al. (2012)

Critique of Postoperative Hypothermia Treatment Using Radiant Heat by Yang et al. (2012)

Perioperative hypothermia has been shown to increase the risk of post-surgery complications, thereby threatening the health of patients and increasing the cost of care (reviewed by Hooper et al., 2009). Guidelines for active warming measures have been suggested by a number of experts, which attempt to address the health risk perioperative hypothermia represents. Should patients arrive in the post-anesthetic care unit (PACU) in a hypothermic state, the conventional approach is to cover patients with pre-warmed hospital blankets; however, forced air-warming has been shown to be more effective across patient populations.

Yang and colleagues (2012) recently published the results of a study examining the relative efficacy of warmed blankets and a radiant heat source for achieving a normothermal state in the PACU. This report will examine all aspects of this study in detail and provide a critical analysis, thereby determining whether their findings are sufficiently rigorous to complement the existing evidence base supporting the implementing of active warming measures in the PACU.

Substantive and Theoretical Dimensions

Hypothermia has long been considered a significant health risk to postoperative patients (Pikus and Hooper, 2010); so much so that the American Society of PeriAnesthesia Nurses (ASPAN) issued formal guidelines in 1998 that have been updated as research findings have become available (Hooper et al., 2009). This continues to be an active area of research and therefore investigating the relative effectiveness of warm blankets and radiant heaters is clinically relevant. Should the researchers discover a better method of warming patients postoperatively, then the benefits to surgical patient outcomes could be significant. This explains why Yang and colleagues (2012) tracked the cost of care and hospital length of stay for patients enrolled in their study.

The use of radiant heaters is one method of many suggested by the ASPN, but forced air warming is considered the gold standard for perioperative treatment of hypothermia (Hooper et al., 2009). Yang and colleagues (2012) mentioned forced air warming as a common method for achieving normothermia in the PACU, but viewed this option as problematic because of the contamination issues such devices create. In countries with a relatively underdeveloped health care system, forced air warming may not be a viable choice because the devices require so much attention. Although not discussed directly by the authors, radiant heat would be one method of warming that would help minimize contamination risks and require little maintenance. The authors of this study may have also felt unconstrained to try methods they have experience with, since this research took place in Taiwan. Radiant heat is therefore a clinically relevant method for patient warming because it reduces the contamination risk of forced air warming and is more conducive to a care environment lacking the resources common to Western socities.

The research methods employed in this study are straightforward and directly address the question of whether radiant heat is more effective than warmed blankets for hypothermic postoperative patients. The primary outcome measure was changes in patient tympanic temperature. Therefore, the independent variables were warmed hospital blankets and radiant heat, while the dependent variable was tympanic temperature. The independent and dependent variables are directly relevant to the question being asked and have important clinical significance.

In framing their question, Yang and colleagues (2012) reviewed the relevant literature and presented the findings from selected studies in the introduction. Their introduction helped frame the research question being investigated sufficiently for the reader to easily comprehend both the rationale for the study and why the study needed to be performed. However, it was far from comprehensive. A recent multi-study review of postoperative warming techniques (Pikus and Hooper, 2010) was not cited, nor was several relevant studies mentioned in this review. In fact, the studies cited by Yang and colleagues (Giuffre et al., 1991 and Weyland et al., 1994) were not cited by Pikus and Hooper, and vice versa (Brauer et al., 2004 and Villamaria et al., 1997). Pikus and Hooper limited their analysis to studies conducted since the release of the ASPAN guidelines in 1998 and therefore would not have included the articles cited by Yang and colleagues; however a good explanation for why Yang and colleagues did not cite the same articles that Pikus and Hooper did is lacking. The literature review by Yang and colleagues is therefore missing at least two relevant studies that should have been discussed.

Methodological Dimensions

Yang and colleagues (2012) chose to use a quasi-experimental design for their study, therefore patients were not randomized when assigned to the two treatment modalities (Hooper et al., 2009). In addition, patient treatment modality was not blinded to physicians and nursing staff. A quasi-experimental design can result in significant skewing in group characteristics, which could render any significant results questionable and therefore of little scientific value. Yang and colleagues seem to avoid this problem by using a fairly large patient sample (N = 130) and recording relevant demographic and medical information that could differ significantly between groups. For example, tympanic temperature was monitored for all study subjects from time of admission to PACU discharge. This data reveals no significant differences between the two study groups. Another potentially confounding factor is whether active warming methods were employed during surgery and again there was no significant difference between the two groups. Female and older have been suggested to be risk factors for intraoperative hypothermia (Hooper et al., 2009) and the two groups recruited by Yang and colleagues are essentially equivalent in terms of gender and age. There does appear to be a significant difference between the two groups in terms of comorbidity, in the direction of healthier radiant heat-treated patients, but the variance is so large as to render any differences statistically insignificant. Radiant heat-treated patients also received significantly less solution during surgery, which could have an impact if the solutions are cooler than body temperature.

Yang and colleagues (2012) did not explain their choice of a quasi-experimental study design. They did, however, describe the abundant supply of patients undergoing back surgery at a single hospital in Taiwan. Why these patients could not be randomized is not clear, but based on the above discussion, there are a few significant differences that could have had an impact on the outcome.

The patients were all adults (> 18 yrs) undergoing spinal surgery at a massive 3,477-bed medical center in Northern Taiwan (Yang et al., 2012). Most patients (65%) had no comorbidities and another 32% suffered from only one other medical condition. This suggests that the patient population selected for the study, with an average age between 61 and 65 years, was relatively healthy. All patients were also operated on dorsally. The inclusion criteria also limited patient study participation to patients remaining in surgery between 3 and 6 hours and having a postoperative temperature of 34.0 to 35.5°C. The patients were therefore fairly uniform in terms of medical condition, age, and surgical procedure, which limit the relevance of the study's findings to other patient demographics. This limitation was mentioned by the authors.

The dependent variable, tympanic temperature, was collected every 10 minutes by PACU nurses (Yang et al., 2012). This methodological approach may be problematic because the radiant heater is placed above the head, to make use of the dense capillary structure in the face and head. No other temperature readings or vitals were collected. By comparison, Weyland and colleagues (1994) collected information on urinary bladder temperature and the temperatures of four subcutaneous locations for laparoscopic patients in Sweden. They also collected VO2, HR, and blood pressure. Given the close location of the radiant heat source to the head of the patient in the study by Yang and colleagues (2012), the use of a second location for taking the patient's temperature away from the head would have provided a more rigorous experimental approach. This is particularly important, because these patients were being discharged from the PACU based in part on their tympanic temperature.

To summarize, the use of a quasi-experimental design necessarily limits the study's internal validity. The study's findings may have also been biased by apparent differences between the two patient groups in terms of comorbidity and amount of solution administered during surgery. The lack of a second outcome measure is troubling, given the close proximity of the radiant heat source to the tympanic membrane of patients. Finally, the relevance of these findings to other patient populations would be limited, because of the uniformity of the patient sample in terms of type of surgery, length of surgery, and postoperative tympanic temperature.

Ethical Dimensions

The research study was approved by the relevant institutional review board for the hospital where the study took place (Yang et al., 2012). All patients were required to sign an informed consent form before inclusion in the study. Physicians and other medical care personnel responsible for the patients' care were not blinded to either their enrollment status or postoperative hypothermia treatment modality. How confidentiality was maintained beyond this point was not mentioned. In terms of patient vulnerability, this patient group would not be more vulnerable than other patients undergoing a surgical procedure under general anesthesia. Although the average age was between 61 and 65 years, this patient group was otherwise quite healthy.

Interpretive Dimensions

Yang and colleagues (2012) found that radiant heat warmed postoperative patients significantly (p < .001) faster (1.83° C/hr) than warmed cotton blankets (1.03° C/hr). This difference is quite large and represents a substantial improvement over warming rates found by other research groups (Yang et al., 2012; Pikus and Hooper, 2010). Yang and colleagues also divided the study groups into three temperature ranges, to better understand whether warming rates were a function of tympanic temperature upon admission into the PACU. What they found was that patients with lower body temperatures warmed at a slower rate (1.5° C/hr) than patients who were within 0.5° of the discharge temperature (36° C) upon admission to the PACU.

Most other research groups discovered warming rates well below those found by Yang et al. (2012), therefore it seems reasonable to question the use of tympanic temperature when the radiant heat source is placed next to the head. If there is a causal relationship between heat source placement and tympanic temperature, then this has important safety implications in terms of PACU discharge decisions. The use of tympanic temperature determined with an infrared probe is not recommended by ASPAN guidelines for perioperative warming (Hooper et al., 2009). Based on these guidelines, in situations when the core temperature cannot be determined without putting patient safety at risk, then oral temperature should be used to approximate core temperature. Since the patients in this study had back surgery, obtaining an oral temperature would have been feasible.

What Yang and colleagues (2012) claimed to have shown is that patients were warmed faster when subjected to radiant heat, compared to patients warmed with hospital blankets. This finding is clinically relevant because the need for active warming systems for perioperative patients has already been recognized by the ASPAN (Hooper et al., 2009), therefore much of the research being done in this area concerns the relative efficacy of the different active warming approaches. While the use of warmed blankets remains the conventional approach for treating hypothermic surgery patients, forced air warming remains the recommended approach. Yang and colleagues (2012) dismiss forced air warming as too much of a contamination risk and therefore did not include it in their study. In essence, they used the conventional warmed blankets as the control for assessing whether radiant heat was more effective. To ask this question in 2012, when active warming measures have already been shown to be more effective than warmed blankets, seems a bit redundant. When combined with the questionable outcome measure of tympanic temperature, the clinical relevance of these findings is limited.

The other outcome measures included in the study were wound infection rate, length of hospital stay, and hospitalization costs (Yang et al., 2012). Wound infection rates are important because hypothermia can induce vasoconstriction, thereby causing localized tissue hypoxia and increasing the risk of wound infection (reviewed by Hooper et al., 2008). Neutrophil function can also be impaired, which would interfere with immune surveillance of the wound. The length of hospital stays, and therefore the costs of care, can be increased by as much as 20% if the wound does not heal appropriately. Hypothermia has also been shown to increase the risk of pressure ulcers, reduce the efficacy of drugs, and increase the need for blood transfusions in peroperative patients.

The study by Yang and colleagues (2012) revealed that no infections were detected in any of the patients enrolled in their study. The length of hospital stay did differ between the two groups, but did not reach statistical significance. There was a significant amount of variability between the two experimental groups, which may have prevented a significant finding. Increasing the sample size might reveal that radiant heat is associated with a shorter hospital stay. Medical costs were also lower for radiant heat patients, but again the difference was not statistically significant. These results suggest that postoperative patients recovering in the PACU may benefit more from radiant heat than pre-warmed blankets, in terms of length of hospital stay and hospitalization costs.