The benefits of active rewarming after cardiac operations: a randomized prospective trial

Forced-air warming: a source of airborne contamination in the operating room?

Forced-air-warming (FAW) is an effective and widely used means for maintaining surgical normothermia, but FAW also has the potential to generate and mobilize airborne contamination in the operating room.

We measured the emission of viable and non-viable forms of airborne contamination from an arbitrary selection of FAW blowers (n=25) in the operating room. A laser particle counter measured particulate concentrations of the air near the intake filter and in the distal hose airstream. Filtration efficiency was calculated as the reduction in particulate concentration in the distal hose airstream relative to that of the intake. Microbial colonization of the FAW blower's internal hose surfaces was assessed by culturing the microorganisms recovered through swabbing (n=17) and rinsing (n=9) techniques.

Particle counting revealed that 24% of FAW blowers were emitting significant levels of internally generated airborne contamination in the 0.5 to 5.0 µm size range, evidenced by a steep decrease in FAW blower filtration efficiency for particles 0.5 to 5.0 µm in size. The particle size-range-specific reduction in efficiency could not be explained by the filtration properties of the intake filter. Instead, the reduction was found to be caused by size-range-specific particle generation within the FAW blowers. Microorganisms were detected on the internal air path surfaces of 94% of FAW blowers.

The design of FAW blowers was found to be questionable for preventing the build-up of internal contamination and the emission of airborne contamination into the operating room. Although we did not evaluate the link between FAW and surgical site infection rates, a significant percentage of FAW blowers with positive microbial cultures were emitting internally generated airborne contamination within the size range of free floating bacteria and fungi (<4 µm) that could, conceivably, settle onto the surgical site.

Efficacy of forced-air warming systems with full body blankets

PURPOSE

Postoperative hypothermia after cardiac surgery is still a common problem often treated with forced-air warming. This study was conducted to determine the heat transfer efficacy of 11 forced-air warming systems with full body blankets on a validated copper manikin.

METHODS

The following systems were tested: 1) Bair Hugger 505; 2) Bair Hugger 750; 3) Life-Air 1000 S; 4) Snuggle Warm; 5) Thermacare; 6) Thermacare with reusable Optisan blanket; 7) WarmAir; 8) Warm-Gard; 9) Warm-Gard and reusable blanket; 10) WarmTouch; and 11) WarmTouch and reusable blanket. Heat transfer of forced-air warmers can be described as follows: Q = h x DeltaT x A. Where Q = heat flux (W), h = heat exchange coefficient (W x m-2 x degrees C-1), DeltaT = temperature gradient between blanket and manikin surface (degrees C), A = covered area (m2). Heat flux per unit area and surface temperature were measured with 16 heat flux transducers. Blanket temperature was measured using 16 thermocouples. The temperature gradient between blanket and surface (DeltaT) was varied and h was determined by linear regression analysis. Mean DeltaT was determined for surface temperatures between 32 degrees C and 38 degrees C. The covered area was estimated to be 1.21 m2.

RESULTS

For the 11 devices, heat transfers of 30.7 W to 77.3 W were observed for surface temperatures of 32 degrees C, and between -8.8 W to 29.6 W for surface temperatures of 38 degrees C.

CONCLUSION

There are clinically relevant differences between the tested forced-air warming systems with full body blankets. Several systems were unable to transfer heat to the manikin at a surface temperature of 38 degrees C.

Effectiveness of a circulating-water warming garment in rewarming after pediatric cardiac surgery using hypothermic cardiopulmonary bypass

OBJECTIVE

To evaluate the effectiveness and safety of the ALLON 2001 microprocessor-based thermoregulation system in pediatric patients undergoing cardiac surgery requiring hypothermic cardiopulmonary bypass compared with the routine thermal care.

DESIGN

Prospective randomized clinical study.

SETTING

Single tertiary academic medical center.

PARTICIPANTS

Infants (0-1 year) who underwent congenital heart surgery requiring hypothermic cardiopulmonary bypass (n = 18). Patients with open wounds and/or patients treated with an investigational drug or device within 30 days of surgery were excluded.

INTERVENTIONS

Randomized use of thermoregulation system (warming garment, n = 9) or routine thermal care (control, n = 9) after separating from cardiopulmonary bypass until the arrival to the pediatric intensive care unit (PICU).

MEASUREMENTS AND MAIN RESULTS

There were no statistically significant differences in the demographic data, cardiopulmonary bypass time, operating room time, incidence of deep hypothermic circulatory arrest, and cooling temperature between the groups. The nasopharyngeal temperature was significantly higher in the warming garment group after separation from cardiopulmonary bypass. Nasopharyngeal temperature at 20 minutes was 36.5 degrees C versus 35.01 degrees C (p = 0.0047), at 40 minutes was 36.98 degrees C versus 35.30 degrees C (p = 0.034), and at admission to the PICU was 36.09 degrees C versus 35.31 degrees C (p = not significant). There was no difference in the core-to-peripheral temperature gradient (nasopharyngeal-to-skin temperature) between the 2 study groups at any time point. No adverse events related to the use of the warming garment thermoregulation system were observed.

CONCLUSION

The investigated thermoregulation system was effective in preventing the after-drop of temperature that occurs after cardiopulmonary bypass in small infants compared with routine warming methods.

Perioperative hypothermia in the high-risk surgical patient

Perioperative hypothermia is common in high-risk surgical patients. Anaesthesia impairs central thermoregulation, allowing re-distribution of body heat. Cool ambient temperatures and high-volume fluid administration accelerate loss of heat to the environment. Randomized, controlled trials have proven that mild hypothermia increases the incidence of wound infection and prolongs hospitalization, increases the incidence of morbid cardiac events and ventricular tachycardia, and impairs coagulation. Other complications include enhanced anaesthetic drugs effects, prolonged recovery room stays, shivering, and impaired immune function. There is compelling animal evidence for cerebral protection by mild hypothermia. However, evidence for protection in surgical patients is not yet available. The most effective means of preventing perioperative hypothermia is active pre-warming. High ambient temperatures, warmed intravenous fluids and active cutaneous warming are useful intra-operatively, while active cutaneous warming and intravenous pethidine abolish post-operative shivering. Proper thermal management may reduce complications and improve the outcome in high-risk surgical patients.

Oxygen consumption after hypothermic cardiopulmonary bypass: the effect of continuing a propofol infusion postoperatively

OBJECTIVE

To evaluate the effect of a fixed rate of infusion of propofol on total body oxygen consumption during the postoperative rewarming phase after cardiopulmonary bypass.

DESIGN

Prospective, randomized, controlled study.

SETTING

Cardiac intensive care unit, university hospital.

PARTICIPANTS

Twenty-four male and female patients undergoing elective first-time coronary artery bypass graft surgery.

INTERVENTIONS

Total body oxygen consumption was measured using a pulmonary artery catheter and thermodilution during postoperative rewarming. Twelve patients had propofol infused at 2 mg/kg/h for 4 hours or until rewarmed.

MEASUREMENTS AND MAIN RESULTS

Total body oxygen consumption was reduced in the propofol group compared with the control group. Oxygen consumption was a median of 30.0 mL/min/m(2) less in the patients receiving propofol (p = 0.01). One patient receiving propofol shivered compared with 4 in the control group (p = 0.14).

CONCLUSION

Administration of propofol during postoperative rewarming reduces total body oxygen consumption and may reduce shivering.

Association between postoperative hypothermia and adverse outcome after coronary artery bypass surgery

BACKGROUND

We examined the effect on outcome of mild hypothermia (< 36 degrees C) upon intensive care unit (ICU) admission on patient outcome after coronary artery bypass grafting (CABG) with cardiopulmonary bypass (CPB).

METHODS

We performed a retrospective database analysis of 5,701 isolated CABG patients requiring CPB, operated upon from January 1995 to June 1997. Patients were classified as either hypo- (< 36 degrees C) or normothermic (> or = 36 degrees C) upon ICU admission. ICU admission bladder core temperature (BCT) versus outcome was evaluated. Outcome measures included mortality, resource utilization (mechanical ventilation time, ICU and hospital length of stay, and postoperative packed red blood cell transfusion), and major morbidity (cardiac, renal, neurologic, or major infection).

RESULTS

Overall, patients admitted to the ICU with BCT < 36 degrees C had a significantly greater mortality (p = 0.02), prolonged mechanical ventilation (p = 0.007), packed red blood cell transfusion (p = 0.001), ICU (p = 0.01), and hospital (p = 0.005) length of stay.

CONCLUSIONS

BCT of less than 36 degrees C, upon ICU admission, has a significant association with adverse outcome after CABG with CPB. M An __ Tl QA_7_t-0

Third-degree burns due to intraoperative use of a Bair Hugger warming device

We report the case of a 3-year-old boy who underwent correction of transposition of the great arteries who developed burns from use of a patient warming device. His repair had been delayed because he was from a developing country, and he was offered surgery as part of a humanitarian effort. Postoperatively he was noted to have second- and third-degree burns from use of a Bair Hugger (Augustine Medical, Eden Prairie, MN) warming system after cardiopulmonary bypass.

The effects of hypothermia on coronary artery bypass graft surgery

This article will discuss how induced hypothermia affects the patient undergoing coronary artery bypass graft surgery. Nursing interventions differ greatly for cardiac procedures done on cardiopulmonary bypass versus off-bypass procedures, such as "keyhole." Induced hypothermia, cardiopulmonary bypass, cardioplegia, and various delivery techniques of cardioplegia, and postoperative hypothermia will be discussed.

Forced-air warming decreases vasodilator requirement after coronary artery bypass surgery

Postoperative hypothermia is common and associated with adverse hemodynamic consequences, including adrenergically mediated systemic vasoconstriction and hypertension. Hypothermia is also a known predictor of dysrhythmias and myocardial ischemia in high-risk patients. We describe a prospective, randomized trial designed to test the hypothesis that forced-air warming (FAW) provides improved hemodynamic variables after coronary artery bypass graft. After institutional review board approval and written informed consent, 149 patients undergoing coronary artery bypass graft were randomized to receive postoperative warming with either FAW (n = 81) or a circulating water mattress (n = 68). Core temperature was measured at the tympanic membrane. A weighted mean skin temperature was calculated. Heart rate, mean arterial blood pressure, central venous pressure, cardiac output, and systemic vascular resistance were monitored for 22 h postoperatively. Mean arterial blood pressure was maintained by protocol between 70 and 80 mm Hg by titration of nitroglycerin and sodium nitroprusside. The two groups had similar demographic characteristics. Tympanic and mean skin temperatures were similar between groups on intensive care unit admission. During postoperative rewarming, tympanic temperature was similar between groups, but mean skin temperature was significantly greater in the FAW group (P < 0.05). Heart rate, mean arterial pressure, central venous pressure, cardiac output, and systemic vascular resistance were similar for the two groups. The percent of patients requiring nitroprusside to achieve the hemodynamic goals was less (P < 0.05) in the FAW group. In conclusion, aggressive cutaneous warming with FAW results in a higher mean skin temperature and a decreased requirement for vasodilator therapy in hypothermic patients after cardiac surgery. This most likely reflects attenuation of the adrenergic response or opening of cutaneous vascular beds as a result of surface warming. IMPLICATIONS Forced-air warming after cardiac surgery decreases the requirement for vasodilator drugs and may be beneficial in maintaining hemodynamic variables within predefined limits.

Postanesthetic vasoconstriction slows peripheral-to-core transfer of cutaneous heat, thereby isolating the core thermal compartment

Forced-air warming during anesthesia increases core temperature comparably with and without thermoregulatory vasoconstriction. In contrast, postoperative forced-air warming may be no more effective than passive insulation. Nonthermoregulatory anesthesia-induced vasodilation may thus influence heat transfer. We compared postanesthetic core rewarming rates in volunteers given cotton blankets or forced air. Additionally, we compared increases in peripheral and core heat contents in the postanesthetic period with data previously acquired during anesthesia to determine how much vasomotion alters intercompartmental heat transfer. Six men were anesthetized and cooled passively until their core temperatures reached 34 degrees C. Anesthesia was then discontinued, and shivering was prevented by giving meperidine. On one day, the volunteers were covered with warmed blankets for 2 h; on the other, volunteers were warmed with forced air. Peripheral tissue heat contents were determined from intramuscular and skin thermocouples. Predicted changes in core temperature were calculated assuming that increases in body heat content were evenly distributed. Predicted changes were thus those that would be expected if vasomotor activity did not impair peripheral-to-core transfer of applied heat. These results were compared with those obtained previously in a similar study of anesthetized volunteers. Body heat content increased 159 +/- 35 kcal (mean +/- SD) more during forced-air than during blanket warming (P < 0.001). Both peripheral and core temperatures increased significantly faster during active warming: 3.3 +/- 0.7 degrees C and 1.1 +/- 0.4 degrees C, respectively. Nonetheless, predicted core temperature increase during forced-air warming exceeded the actual temperature increase by 0.8 +/- 0.3 degree C (P < 0.001). Vasoconstriction thus isolated core tissues from heat applied to the periphery, with the result that core heat content increased 32 +/- 12 kcal less than expected after 2 h of forced-air warming (P < 0.001). In contrast, predicted and actual core temperatures differed only slightly in the anesthetized volunteers previously studied. In contrast to four previous studies, our results indicate that forced-air warming increases core temperature faster than warm blankets. Postanesthetic vasoconstriction nonetheless impeded peripheral-to-core heat transfer, with the result that core temperatures in the two groups differed less than might be expected based on systemic heat balance estimates.

IMPLICATIONS

Comparing intercompartmental heat flow in our previous and current studies suggests that anesthetic-induced vasodilation influences intercompartmental heat transfer and distribution of body heat more than thermoregulatory shunt vasomotion.

Forced-air warming is no more effective than conventional methods for raising postoperative core temperature after cardiac surgery

OBJECTIVE

To determine whether postoperative forced-air warming of cardiac bypass patients in the intensive care unit (ICU) results in faster rate of warming and improved outcomes compared with more conventional ICU warming methods.

DESIGN

Prospective randomized effectiveness study.

SETTING

Three hundred fifty-bed university-affiliated hospital.

PARTICIPANTS

Sixty consenting randomized patients from a consecutive series of 84 patients undergoing routine adult cardiac surgery.

INTERVENTIONS

One group of patients received usual patient care, which includes warm blankets and overhead heat lamps. Patients in the other group were placed under forced-air warming devices on arrival in the ICU. Sixty consenting patients (30 in each group) were randomly assigned to one or the other method of warming. The remaining 24 patients refused randomization and self-selected a treatment group.

MEASUREMENTS AND MAIN RESULTS

Results are presented for the randomized groups. Core temperature, measured by pulmonary artery catheter thermistor, increased in both groups at the rate of 0.25 degree C per hour. No statistically or clinically significant differences were found between the group for whom the warming device was used and the standard care group in the incidence of postoperative cardiac arrhythmia, duration of time in the ICU, or any other clinical variable.

CONCLUSIONS

There is no evidence from this study to warrant use of forced-air warming devices for the care of postoperative cardiac surgical patients in the ICU.

Determinants of prolonged mechanical ventilation after coronary artery bypass grafting

BACKGROUND

Early extubation of cardiac surgical patients enhances ambulation, improves cardiopulmonary function, and can lead to savings in health care costs.

METHODS

We retrospectively examined the role of 48 variables in determining the period of ventilatory support in 507 patients having coronary artery bypass grafting.

RESULTS

Fifteen (< 3%) of 507 patients required ventilatory support in excess of 24 hours. Among the remaining patients, extubation was achieved early (< or = 8 hours) (mean time, 5.65 +/- 1.31 hours) in 53% and late (> 8 hours) (mean time, 13.7 +/- 3.4 hours) in 47%. Logistic and linear multivariate regression analyses implicated increased age, New York Heart Association functional class IV, intraoperative fluid retention, postoperative intraaortic balloon pump requirement, and bank blood transfusions as predictors of late extubation. Also, the linear regression linked lower body weight and number of anastomoses (or grafts) to increased mechanical ventilatory support.

CONCLUSIONS

Analysis of the fluid balance and cardiopulmonary bypass data suggests that earlier extubation may be achieved by actively reducing fluid retention (eg, by hemoconcentration) and time on bypass (eg, normothermia). Finally, intensive care unit stay and postoperative length of stay were significantly lower in the early versus late extubation groups without an increase in pulmonary complications.