Authored by Essam Abdelrazek*
Abstract
In this observational study, we assessed the efficiency of the use of a 40° C warming cabinet method for heating intravenous (IV) fluids administered to patients during surgery. 24 bags of Ringers lactate were placed in a warming cabinet set at 40° C. Fluid temperatures at two points of the delivery system (bag and patient end) were measured every 3 minutes for 15 minutes after being removed from the warming cabinet.
Results: The bag temperature (34-31 °C) was significantly lower than 40°C but remained above room temperature throughout the study period. The patient–end temperatures were significantly lower than bag temperatures for all time-periods (p<0.01) and were similar to room temperatures within 3 minutes. We conclude that the quality of the warming cabinet technique used in operating theatres to warm fluids prior to IV infusion, is clinically inefficient and fails to deliver the IV fluids at a warm enough temperature and may therefore put patients at risk of unanticipated hypothermia. We recommend that the practice of the studied method of fluid warming needs to be revised and replaced by another method which could be more clinically effective and reliable in keeping the patient’s temperature within normal limits under anaesthesia.
Keywords: Hypothermia; Prevention; Hypothermia; Physiology effects
Introduction
Hypothermia is a well-known complication of general as well as regional anaesthesia [1,2]. It could have detrimental effects on the patient’s hemodynamic, respiratory, neurological, and metabolic conditions [3]. Post-operative shivering in recovery is usually very unpleasant and distressing experience for patients as well as the staff in the recovery room [4]. This can be a cause of delayed recovery from general anaesthesia. It may increase the patient’s feeling of pain in the immediate post-operative period, which could be difficult to control without improving their postoperative temperature [5]. Intraoperative administration of pre warmed intravenous (IV) fluids (37- 40° C) has been shown to be one of the useful methods used to counter the anaesthesia/surgery induced hypothermia effects in patients undergoing surgery [6]. Different methods have been suggested to warm IV fluids given to patients undergoing surgery, with variable degrees of efficiency [7-10]. One of these methods is the use of electrically operated warming cabinets. This is considered to be a simple, easy, available, and relatively cheap method of warming fluids given to patients in the operating theatre. Therefore, it has become popular globally. However, evidence of the efficiency of this technique remains unclear [7-9].
At the Royal Bournemouth Hospital (RBH) using warming cabinets to warm the intravenous fluids given to patients under anaesthesia is a standard technique. However, clinical observation has suggested that the warming cabinet is not fully reliable because the warmed fluids appear to lose heat quickly on traversing down the giving set. This study was designed to assess the efficiency of using the warming cabinet technique as a method of pre warming IV fluids and by inference, it’s appropriateness for maintenance of patient temperature during surgery.
Methods
Approval of an Ethics Committee was not necessary as there were no patients involved. Based on the results of an initial pilot study, a mean difference of 10° C (standard deviation±2.25) between the starting temperature on leaving the warming cabinet and two points A (the fluid bag) and B (the distal part of the giving set - i.e. patient end) was deemed statistically significant. A sample size of 24 fluid bags would be required based on significance of p<0.05 and 90% power. According to the standard practice at our institution, the warming cabinet (W157, LEEC Limited, Nottingham, U.K.) was used to warm the fluid bags. Twenty-four fluid bags of equal volume (1 L) were placed in the warming cabinet set at 40° C 12 hours prior to testing. Using the wall mounted room temperature sensor (Sauter Ltd. Company®, Basil, Switzerland), the room temperature was kept at 21ºC±0.23ºC. The fluid sample temperatures were measured using the C21 Comark thermometer (Comark instruments company, Norwich, U.K.). This is a batteryoperated digital thermometer designed to measure temperature of wide range between -50 to 150° C with reliable accuracy.
Once removed from the warming cabinet, the tested fluid bag was attached to a standard line giving set with total capacity of 20 ml fluid volume (Fresenius Kabi, Homburg, Germany). The fluid temperature was checked at two different points
I. Temperature point A (The bag end): Via a 14 G cannula inserted into the bag and connected to a three-way tap for easy and frequent aspiration of 20 ml fluid sample to enable measurement of bag-fluid temperature every three minutes for fifteen minutes.
II. Temperature point B (The patient end): Via a three-way tap placed at the distal end of the infusion giving set to enable easy and frequent aspiration of 20 ml fluid sample at the patient delivery point every three minutes for fifteen minutes.
All the fluid samples were collected using a 20 ml syringe and placed into a plastic cup for immediate temperature measurement performed by blinded investigators (JW and JVR).
Statistical analyses were performed using IBM SPSS Statistics 22.0. The mean value of the two recordings made at each point and time was used in the analysis. Unless otherwise stated, reported statistics are mean (SD). Comparisons were made using one sample t-tests and Repeated Measures ANOVA with post hoc comparisons where appropriate and the measurements were compared at each time point:
I. Temperature difference between Point A and Point B.
II. Temperatures at Point A versus the warming cabinet temperature
III. Temperatures at Point B versus the warming cabinet temperature.
IV. Temperature difference between Point A and room temperature.
V. Temperature difference between Point B and room temperature.
Results
Throughout the study period, the cabinet temperature was maintained at 40° C and the ambient room temperature ranged from 20.7 to 21.5 ºC with a mean of 21±0.23ºC. All temperatures recorded at each sampling location for each sampling time are presented in Table 1 and Figure 1.
Differences in temperature between point A and point B (Table 1 and Figure 1)
There was an overall significant difference between the temperatures taken at Point A versus Point B (F1,46 = 4382.63, p < 0.001). Post hoc comparisons showed significant differences between Point A and B at each sampling time (p < .001). Temperatures were significantly lower at Point B for all time points with a minimum difference occurring at time 0 (7.8±0.9ºC); and a maximum difference at 3 minutes (9.21±0.73ºC). At the end of the study period, the difference between point A and Point B was 8.44±0.74ºC.
Bag temperature (point A) compared to warming cabinet temperature
The starting fluid bag temperatures at point A were significantly lower (p < 0.001) by a mean of 5.84 ºC (0.87) from the starting temperature of 40ºC after being attached to the giving set. However, throughout the study period, the mean bag temperature remained above 30 ºC (Figure 1).
Distal end delivery point temperature (point B) compared to warming cabinet temperature
The starting distal end delivery point temperatures at point B were significantly lower (p<0.001) than the warming cabinet temperature (40 ºC) by a mean of 13.6 (0.81) ºC. Throughout the study, the mean temperature measured at point B continued to decrease. At the end of the study period, the mean difference between the two temperatures was 17.21 (0.43) ºC (Figure 1).
Differences between Point A temperature and room temperature
The bag temperature was significantly greater (p<0.01) throughout the study period. It was 13.0±0.87ºC greater at time 0 and this difference gradually decreased to 10.0±0.66ºC greater than room at the end of the study period (Figure 1).
Differences between Point B temperature and room temperature
The Point B temperature was 5.2±0.81ºC greater than room temperature at time 0 but this difference quickly decreased with no significant difference (1.6±0.43ºC) at the 3-minute sampling time (Figure 1).
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