The third enrolled patient was assigned to use the circulating-water sleeve and had an operation that unexpectedly lasted 10 hours. Immediately after surgery, study personnel removed the warming sleeve and noticed that she had a second-degree burn (blistering) covering much of her hand and forearm. Soon thereafter we noticed that a patient from the previous day, who was also assigned to circulating-water sleeve warming, also had several small blisters, although none had been present immediately after surgery. The smaller burn healed completely without intervention. The other was treated by a plastic surgeon and, after several weeks, healed completely without scarring.
Enrollment was stopped, and the IRB at the Cleveland Clinic and the sponsor were informed. Extensive analysis revealed that the specific circulating-water sleeve device used with these 2 patients had been incorrectly assembled by the sponsor's Food and Drug Administration–approved contract manufacturer; specifically, the inflow and outflow tubes were inserted backwards, which resulted in the device operating at a temperature about 2°C higher than design specifications.
Because the burns were thought to result from a manufacturing error rather than an intrinsic design flaw, the study was restarted with precautions that included (a) a change in device manufacturing instructions and routine inspection; (b) a reduction in operating temperature from 42°C at −10 mm Hg to 41°C at −5 mm Hg; and (c) a study case duration limit of 4 hours with circulating-water sleeve warming. Restarting the study was approved by the IRB at the Cleveland Clinic and participating surgeons; the IRB at the Vienna General Hospital (which had yet to start enrollment) was informed of the injuries at the clinic, as were all subsequent patients. In the subsequent 34 circulating-water-sleeve patients, we did not observe any cutaneous complications.
RMA with time as a continuous factor showed that temperature increased over time for the combined groups, with slope (SE) of 0.12°C (0.01°C) per hour, P < 0.001. The group-by-time interaction was highly significant (P < 0.001 for time as either a continuous variable or a categorical variable), indicating that noninferiority needed to be assessed separately at the various time points instead of overall, because the group effect was not consistent across the times (Fig. 6). Ignoring the interaction, mean core temperature was not different between groups, with an estimated mean (SE) difference of 0.037°C (0.099°C) lower in patients assigned to the circulating-water sleeve (P = 0.71) in a 2-tailed test for superiority. As with any nonsignificant superiority test, this result cannot be used to claim noninferiority.
Because of the group-by-time interaction, we assessed for noninferiority at 1, 2, 3, and 4 hours after start of measurements. Using the RMA model, at each time the numerator for the 1-tailed t test test statistic was the circulating-water sleeve mean minus forced-air mean +0.5°C, and the denominator was the SE of the difference in means. Using a Holm-Bonferroni correction for multiple comparisons, the significance criterion for the smallest to largest P values are 0.006, 0.008, 0.012, and 0.025, respectively. The observed P values from smallest to largest were P < 0.001 (hour 1), P < 0.001 (hour 2), P = 0.011 (hour 3), and P = 0.016 (hour 4). Therefore, for this primary analysis, noninferiority was detected at all 4 times, because all P values are less than the respective significance criteria. Table 2 gives these results, including the difference in means and 95% CI for each time point. The lower bound of the 95% CI for the difference between groups at each time point being above the noninferiority δ of −0.5°C corresponds to the claim of noninferiority at each time.
Because noninferiority was concluded at each time point, tests of superiority were conducted using the Holm-Bonferroni method; unsurprisingly, given the proximity of the means, no superiority was detected. The results thus indicate that the circulating-water sleeve is noninferior to forced air during surgery, defined by core temperature being no >0.5°C lower.
The efficacy of clinical warming systems depends on their ability to transfer heat to patients. For example, surgical patients are often positioned above a circulating-water mattress. Although these systems permit unrestricted access to the anterior surfaces of patients, they are inefficient warmers and the combination of heat and reduced local perfusion from the patient's own body weight restricts capillary bloodflow, which can lead to burns and pressure-heat necrosis.16–18
Recently developed systems allow circulating-water garments to cover a larger surface area of the body and thus transfer more heat than do traditional water mattresses that only heat the back and legs.6,14,19 Additionally, there are circulating-water systems using efficient “energy transfer” pads that transfer far more heat per square meter of body surface area than do conventional mattresses.20,21 However, both systems are considerably more expensive than forced air, which remains by far the most commonly used intraoperative warming system. Furthermore, most systems require contact with a fairly large body surface area. We thus evaluated a novel circulating-water system in which heating was restricted to a single hand and forearm.
Our study was designed to test noninferiority. That is, we sought statistical power for concluding that the mean core temperature for the circulating-water sleeve was no >0.5°C less than the forced-air mean. This is a far stricter test than simply showing lack of a significant difference, which often simply results from inadequate power and is the proper way of demonstrating comparable or noninferior performance. We met this goal for the first 4 hours and thus conclude that mean core temperatures for the circulating-water sleeve were significantly not lower (by >0.5°C, our a priori designated δ) than means for forced air during this period. Although the observed mean core temperature was slightly lower for the circulating-water sleeve during the later surgical period (hours 3 and 4), the lower bound of the confidence interval for the difference between methods was within the noninferiority region (above −0.5°C), mean core temperatures differed at most by about 0.2°C, and confidence intervals for the difference were within ±0.5°C for the first 3 hours. Thus, there was no suggestion of any clinically important difference. We thus conclude that performance of the systems was comparable. Our results are similar to those reported by Trentman et al.,22 who found that the circulating-water sleeve system kept all but 1 of 36 patients normothermic during unilateral total knee arthroplasty; furthermore, they found that core temperature in patients warmed with the circulating-water sleeve system were only 0.4°C cooler than those warmed with forced air after 2 hours of surgery.
Heat loss is substantial during open abdominal surgery, and these patients inevitably become hypothermic without active warming. The circumstances of our study thus constituted a strict test of the heating systems. Although we did not actually quantify heat flux, it is reasonable to assume that heat transfer was comparable with forced air and the circulating-water sleeve system because core temperatures were nearly identical.
Core temperatures during open abdominal surgery were similar in patients assigned to forced air or to the circulating-water sleeve system, although the circulating-water sleeve was applied just to 1 hand and forearm. Similar clinical efficacy might appear curious because the surface area covered by the circulating-water sleeve system is only about one third of that covered by an upper-body forced-air cover with 1 arm tucked (≈5% vs. ≈15% of the total).23 However, heat transfer per square centimeter under optimal circumstances is slightly greater with circulating-water pads than with forced air.24 Transfer is likely to be considerably better with a system that maintains tight device–skin contact, perhaps explaining how the smaller surface area of the circulating-water sleeve system could nonetheless transfer comparable amounts of heat.
The sleeve system with circulating water and vacuum was found to be effective. In contrast, circulating-water mattresses perform poorly.25 That the circulating-water sleeve system was effective is somewhat counterintuitive because heat transfer with any given type of warming is usually a linear function of surface area, and the surface area of the back is substantial. However, there are 2 other factors to consider: insulation intrinsic to the heating system and insulation provided by skin and subcutaneous tissues.
Circulating-water mattresses, by virtue of the pressure to which they are exposed, need to be relatively thick. Although a few millimeters of plastic may seem inconsequential, it substantially impedes heat transfer. Heat flow is further impeded by the sheet that is usually present between the mattress and a patient. But heat transfer does not just depend on characteristics of the warmer; it is also limited by the ability of skin and subcutaneous tissues to absorb and dissipate heat to the rest of the body. The ability of tissues to absorb and dissipate heat is, in turn, a function of perfusion. A limitation of circulating-water mattresses is that the weight of patients compresses capillaries and reduces perfusion of subcutaneous tissues of the back, thus making the back a better insulator and reducing heat dissipation.
The circulating-water sleeve system we tested descended from a device (ThermaStat, Aquarius Medical, Scottsdale, Arizona) that reportedly increased core temperatures at the remarkable rate of 13.6°C ± 2.1°C per hour.26 The theory behind the system was that vacuum with a thermal load would open arteriovenous shunts in the fingers and thus provide a “pipeline to the core.” Why this mechanism would be effective during anesthesia when arteriovenous shunts are already dilated by the central effects of anesthetics27,28 remained unclear. In fact, subsequent work with the original device showed that the system was essentially ineffective.29,30 The ThermaStat system used a fairly high-level vacuum in a rigid shell. The result was that tissues experienced “negative pressure” and were thus prone to edema. The important difference of the circulating-water sleeve that we evaluated is a small vacuum applied to a flexible plastic shell, which pulls the heating element towards the underlying hand and forearm. The result is a small amount of pressure on tissues that maintains good contact between the circulating-water heating element and the skin. Good contact is a critical feature of the device because even tiny air gaps are highly insulating and limit flow of heat into tissues.
The circulating-water-sleeve device that we tested also differs from the ThermaStat in another important way: the previous version used a chemical heat pack that inadequately contacted a limited amount of skin, then delivered its specified heat for only a short period before degrading. In contrast, the circulating-water sleeve uses thin-walled, low-pressure fluid pads and precisely controls both the delivered temperature and vacuum levels. The combination of a low-resistance exchange system and good skin contact proved effective.
We did not record anesthetic dosing. However, anesthetic dose is unlikely to affect outcome because even small doses of most anesthetics induce thermoregulatory vasodilation,31 which is the only physiological response likely to influence core temperature under the circumstances of this study.32
Two of the 37 patients assigned to circulating-water sleeve warming were burned, although thermal injury appears to have resulted from a manufacturing defect rather than from an intrinsic design flaw. We caution, though, that our study was not powered to evaluate safety. Furthermore, the total clinical experience with the circulating-water-sleeve system was limited to <100 surgical patients at the time of our study. Additional study is thus required to confirm safety of the system, especially in patients with thin or sensitive skin, and during prolonged surgery.
In summary, mean core temperatures during 4 hours of open abdominal surgery were similar (and significantly noninferior) with the warm-water sleeve and upper-body forced-air warming. Both appear suitable for maintaining normothermia.
Supported by Dynatherm Medical, Inc. (Fremont, California). The study was designed by the investigators in collaboration with the sponsors; however, the sponsors were not involved in data collection, analysis, or interpretation of the data; the manuscript was written by the investigators, and a copy was shared with the sponsors as a courtesy.
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© 2011 International Anesthesia Research Society
32. Sessler DI. Perioperative heat balance. Anesthesiology 2000; 92:578–96