Anesth Analg 2004;99:788-792
© 2004 International Anesthesia Research Society
doi: 10.1213/01.ANE.0000129995.42008.A4
TECHNOLOGY, COMPUTING, AND SIMULATION
Flow Rates and Warming Efficacy with Hotline and Ranger Blood/Fluid Warmers
Peter E. Horowitz, MD,
Miguel A. Delagarza, MD,
Jaime J. Pulaski, MD, and
Robert A. Smith, PhD
Department of Anesthesiology, College of Medicine, University of South Florida, Tampa, Florida
Address correspondence and reprint requests to Peter E. Horowitz, MD, 12901 Bruce B. Downs Blvd., MDC Box 59, Tampa, FL 33612-4799. Address e-mail to phorowit{at}hsc.usf.edu
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Abstract
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The heating capabilities of a water bath blood/fluid warmer, Hotline, have proven superior to those of other devices. The dry heat warmer Ranger has not previously been compared with the Hotline. We evaluated these devices in terms of flow rates and efficacy of warming. We delivered room temperature (21°C) saline and 10°C packed red blood cells (RBCs) by using 90 mm Hg (gravity equivalent) and 300 mm Hg bag pressure and various sizes of IV catheters. The outflow from each device was connected to an inline thermistor, and simultaneous measurements of outflow temperature and flow volume per minute were recorded. Additional data points were obtained with a roller pump that delivered flows of 1–6 L/h through each device. We calculated the effect of these flow rates and outflow temperatures on the mean body temperature (MBT) of a 70-kg patient. The Hotline and Ranger had similar flow rates at 90 and 300 mm Hg pressure infusion when studied with various sizes of IV catheters. Hotline was able to deliver warmer RBCs and saline at slower flow rates (1–4 L/h), but because changes in MBT were almost identical, there was no clinically important advantage, and almost no heat was transferred at these slower flow rates. At more rapid flow rates (>4 L/h), the Ranger warmed RBCs and saline better and produced smaller decreases in MBT than the Hotline. The use of the Hotline for rapid infusions, especially of cold RBCs, is not recommended because of low outflow temperatures and decreases in MBT that were three times larger than those seen with the Ranger.
IMPLICATIONS: We compared the ability of a new dry heat blood/fluid warmer (Ranger) with the water bath warmer Hotline in terms of flow rates and outflow temperatures by using room temperature saline and 10°C diluted packed red blood cells (RBCs). Flow rates were similar, but the Ranger provided higher outflow temperatures at flows >4 L/h, especially during rapid pressurized infusions of both RBCs and saline.
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Introduction
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Blood/fluid-warming devices are used intraoperatively to reduce the decrease in mean body temperature (MBT) produced by the IV administration of unwarmed blood and fluids. Studies have reported perioperative complications related to even mild hypothermia in select groups of patients (1–5). The Hotline (SIMS Level 1, Inc., Rockland, MA) uses water bath technology and has previously outperformed other fluid-warming devices (6–8). The Ranger (Arizant Healthcare, Eden Prairie, MN) uses dry heat technology. No previous study has compared these devices.
The purpose of this study was to compare the flow rates achieved by each blood/fluid warmer by using similar infusion pressures and different sizes of IV catheters. We also compared the heating ability of these two blood/fluid warmers by recording the outflow temperatures during a variety of flow rates, and, with the use of a mathematical formula, we determined the changes in MBT that would be produced in a 70-kg patient after red blood cell (RBC) or saline administration with each of these devices.
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Methods
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We compared the commercially available dry heat Ranger and water bath Hotline HL90 blood/fluid warmers. Flow rates and outflow temperature measurements were obtained for the Hotline and Ranger by delivering 21°C saline and 10°C diluted packed RBCs to each warming device with 90 and 300 mm Hg bag infusion pressures. A standard Y blood administration set and the disposable warming set designed for each warming unit were used: L-70 for the Hotline and the standard disposable warming set for the Ranger. The outflow tubing of each heating device was connected in sequence to a 6-in.-long IV extension tube, a 2-in.-long wide-bore tube containing a calibrated thermistor, and IV catheters of various gauges (18, 16, and 14 gauge x 1
in.; B. Braun Medical, Inc., Bethlehem, PA). A thermistor (VHA Plus; Sims Respiratory Support Products Inc., Irvine, CA) was inserted into wide-bore tubing to avoid flow obstruction, and the entry point was sealed with epoxy cement. The thermistor was confirmed to agree with a mercury thermometer within 0.2°C over a range of 15°C–40°C.
Outdated human RBCs were diluted to a hematocrit of 30% with normal saline and placed in a 10°C water bath for 30 min. One-liter bags of normal saline or 500-mL bags of diluted RBCs were placed in a pressurized infusion bag (Medex Medical, Dublin OH), which was connected to an automated tourniquet (A.T.S. 2000; Zimmer Healthcare, Dover, OH) and positioned at the same level as the warming unit. Flow commenced with an infusion bag pressure of either 90 or 300 mm Hg, and temperatures were recorded after reaching equilibration for 30 s. Flow rates were determined by using a graduated cylinder and stopwatch for a 60-s collection. Saline or RBC bags were replaced when they became half empty, to ensure constant flow rates. Room temperature was monitored continuously with a separate thermistor and was maintained at 21.0°C ± 0.2°C throughout the study.
Additional data were obtained by using a roller head pump (Masterflex Model 7550-90; Cole Parmer Instrument Co., Chicago, IL) to deliver flows of 1–6 L/h in 1-L increments through each warming device while we recorded the outflow temperature at equilibration after at least 30 s. Roller pump calibration was determined by using a graduated cylinder and stopwatch and was accurate ±1% over the range of flows studied. All measurements were made in duplicate at each flow condition. The two results were averaged and were both repeated if they did not agree within 10%. Dispersion of the measurements of temperature and volume was quantified by using the coefficient of variation (SD divided by mean) and was found to range between 0.2% and 0.6% for all observations.
To demonstrate the clinical effect of the observed differences in flow rate and temperature output with each blood/fluid warmer, we calculated the decrease in MBT for a 70-kg patient receiving 10°C RBCs or 21°C saline with the Hotline, the Ranger, or no warming device at 1–6 L/h and at maximum flow rates, as would occur in a major fluid resuscitation, with the following formula (5,9):
where 
MBT indicates the change in MBT, TF indicates the temperature of the fluid infused, TPt indicates the patient’s core temperature (37°C), SF indicates the specific heat of fluid infused [0.87 kcal · L–1 · °C–1 for blood (10) and 1.0 kcal · L–1 · °C–1 for saline], Vol indicates the volume of fluid infused (in liters), SPt indicates the specific heat of human tissue (0.83 kcal · L–1 · °C–1), and Wt indicates the weight of the patient (in kilograms). We also used the formula to calculate the MBT that would result, in any patient, from the administration of 21°C saline 10 mL/kg or 10°C RBCs 10 mL/kg.
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Results
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The Hotline and Ranger blood/fluid-warming devices (Fig. 1) provided similar flow rates for diluted RBCs and saline with 90 and 300 mm Hg infusion pressure gradients when studied with various IV catheter sizes (Figs. 2 and 3). Maximum flow with RBCs was obtained at 300 mm Hg through a 14-gauge catheter and was 221 mL/min for the Hotline and 234 mL/min for the Ranger. The maximum flow of saline occurred at 300 mm Hg through a 14-gauge catheter and was 244 mL/min for the Hotline and 295 mL/min for the Ranger.
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Figure 1. The blood/fluid warmers tested in this study: Hotline HL-90 (countercurrent water bath) and Ranger (dry heat).
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Figure 2. Comparison of flow rates of saline and red blood cells (hematocrit, 30%) from the Ranger and Hotline, infused through catheters of varied gauges (ga) with a driving pressure of 300 mm Hg.
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Figure 3. Comparison of flow rates of saline and red blood cells (hematocrit, 30%) from the Ranger and Hotline, infused through catheters of varied gauges (ga) with a driving pressure of 90 mm Hg.
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Figure 4 combines data obtained from both the pressure infusion and calibrated pump (1–6 L/min) portions of the study. The Hotline had a higher outflow temperature at flows of 1–4 L/h. At a flow of 1 L/h, the Hotline delivered saline at 40.5°C, versus 36.2°C for the Ranger. The Hotline delivered RBCs at 39.0°C, versus 34.6°C for the Ranger. The Ranger had higher outflow temperatures at flows more than 4 L/h. The outflow temperature of the Hotline at maximum flow with RBCs was 24.7°C, versus 33.2°C for the Ranger. The outflow temperature of the Hotline at maximum flow with saline was 30.1°C, versus 35.0°C for the Ranger.
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Figure 4. Comparison of outflow temperatures of red blood cells (hematocrit, 30%) and saline from the Ranger and Hotline with varied flow rates.
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The changes in MBT of a 70-kg patient that resulted from various flow rates of RBCs and saline through the Hotline and Ranger are illustrated in Table 1. In a 70-kg patient, maximum flow of 10°C RBCs for 1 h through a Hotline would reduce MBT by 2.6°C, compared with 0.9°C with a Ranger and 5.7°C with no warmer. In a 70-kg patient, maximum flow of 21°C saline for 1 h through a Hotline would reduce MBT by 1.7°C, compared with 0.6°C with a Ranger and 4.1°C with no warmer. The administration of 10 mL/kg of 10°C RBCs would reduce MBT by 0.28°C, whereas 10 mL/kg of 21°C saline would reduce MBT by 0.19°C.
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Table 1. Estimated Change in Mean Body Temperature in a 70-kg Patient After One Hour of Varied Infusion with Saline or Diluted Red Blood Cells (Hematocrit, 30%)
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Discussion
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We chose 21°C saline to represent the delivery temperature of crystalloid in a standard operating room. We chose RBCs with a hematocrit of 30% at 10°C to reflect the common practice of diluting refrigerated (4°C) packed RBCs with room temperature saline to improve rheology during administration. We chose a 90 mm Hg bag pressure delivered at the level of the warming unit to approximate the hydrostatic pressure of a bag of RBCs or saline hung 4 feet above the patient’s IV site because the pressure conversion of 48 in. H2O equals 89.7 mm Hg (11).
In previous studies, the heating capabilities of a water bath blood/fluid warmer such as the Hotline have proven superior to those of other dry heat warming devices (1–5). The Ranger is a new dry heat blood/fluid warmer that had not been compared with the Hotline.
Flow rates for diluted RBCs and saline through the Hotline and Ranger were similar with 90 mm Hg (gravity flow) and a 300 mm Hg infusion pressure with several sizes of IV catheters. The Ranger had a lower outflow temperature than the Hotline at a flow of 1 L/h because, at lower flows, the fluid in the Ranger’s 40-inch exit tubing spends more time exposed to room air. This allows cooling of the fluid after it exits the warming unit (12). The Hotline circuit provides heating up to the connection to the patient’s IV. At flow rates more than 4 L/h, outflow temperatures with the Ranger are significantly higher, likely because of the larger surface area of the heating cartridge, which provides the Ranger with a higher heating capacity. At higher flows, the Hotline begins to exceed its ability to adequately warm RBCs and saline.
The clinical significance of the differences in outflow temperatures at slow and rapid flows is determined by the calculated MBT for each set of flow/temperature data, and this is shown in Table 1. For a 70-kg patient, the difference in MBT between the Hotline and Ranger was small in the 1–5 L/h flow range. The difference reached clinical significance only when the devices were used at their maximum flow rates (Table 1). The Hotline is suitable for delivering normothermic fluids and blood at gravity flow rates up to 5 L/h (13). However, use of the Hotline for pressure-driven, rapid infusion of fluids and blood is not recommended, as confirmed in this study. The Ranger was also unable to deliver normothermic saline and RBCs at a 300 mm Hg infusion pressure, but even after the administration of 14 L of RBCs or 17 L of saline with the Ranger, the decrease in MBT will be <1°C in a 70-kg patient.
Macario and Dexter (14) recognized the need to define the clinical benefit of several active warming devices and recommended studies to determine which patients require these devices to avoid hypothermia. The question of which patients need a blood/fluid warmer is best answered by results obtained with the MBT formula. Because the MBT equation predicts a decrease of 0.19°C for every 10 mL/kg of 21°C saline and 0.28°C for every 10 mL/kg of cold (10°C) RBCs, a decision to use a blood/fluid warmer can be based on a simple calculation of the anticipated temperature decrease that will result from the blood or fluids to be administered to a given patient during the surgical procedure.
Clinically, a warmer should be used only if the MBT from fluid administration is likely to decrease more than 0.5 to 1.0°C, because, in most instances, this degree of hypothermia can be tolerated or reversed with a forced-air heater alone. Adult patients probably do not require a blood/fluid warmer at slower flow rates (0–2 L/h), because of the minimal effect on MBT. Even the smallest pediatric patients can tolerate a continuous room temperature infusion of blood or fluids to a total of 30 mL/kg because only a 0.6°C–0.8°C decrease in MBT will occur.
The dry heat technology used by the Ranger avoids the potential for cross-contamination between the water bath and patient circuit that can occur with the Hotline (15). The manufacturer of the Hotline presently recommends a setup procedure to test for circuit integrity for every patient: this involves circulating heated water through the warming circuit and checking for leakage into the patient fluid pathway before priming the IV channel (12). There are also concerns about nosocomial infections resulting from contaminated Hotline water baths (16). The manufacturer recommends a monthly cleaning of the water bath by circulating an alcohol solution through the circuit for 30 minutes. The success of the prescribed regimen to maintain water bath asepsis requires further study.
In summary, when the clinical decision is made to use a blood/fluid warmer, the dry heat Ranger and Hotline are able to deliver similar flow rates for saline and RBCs for gravity or pressure infusions. Although the Hotline is able to deliver warmer blood and saline at slower flow rates (1–5 L/h), there is no clinically important difference with the Ranger because almost no heat is transferred at these slower flow rates. Ranger is able to deliver warmer blood and saline at the more rapid flow rates required for rapid volume resuscitation. The use of the Hotline for pressurized infusion, especially of cold RBCs, is not recommended because of low outflow temperatures and decreases in MBT that are three times larger than those seen with the Ranger.
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Acknowledgments
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The authors thank John B. Downs, MD, for his assistance in the preparation of this manuscript; Chris Jackson for drawing the illustration; and Colleen Masters, MS, SBB (ASCP), for blood banking assistance.
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Footnotes
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Presented in part at the annual meeting of the American Society of Anesthesiologists, Orlando, FL, October 15, 2002.
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References
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Accepted for publication April 5, 2004.
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Abstract
Introduction
