Pocket Warming of Epidural Medication to Shorten Onset of Labor Analgesia

Studies have shown that warmed epidural medication resulted in a mean analgesic onset of 9.2 minutes as compared to a mean onset of 16 minutes for the same medication at room temperature. Onset time for pain medication during labor is important to a great many participants, thus techniques to shorten analgesic onset are relevant to daily clinical practice. Combined-spinal epidural (CSE) techniques have been used to improve the onset and reliability of labor analgesia compared to epidural techniques; however, CSEs have risks associated with their usage. Firstly, is not always possible to administer a spinal dose despite successful loss-of resistance with CSE techniques, providing less satisfactory labor analgesia, and secondly, the CSE technique has been associated with a greater incidence of non-reassuring fetal heart tones (FHT), uterine hyperactivity, maternal pruritis, and greater incidence of neurologic sequelae compared to epidural analgesia. Further, CSE labor analgesia is more likely to result in prolonged FHT decelerations if there is FHT abnormalities prior to the neuraxial procedure. Given these potential limitations of CSE techniques for labor analgesia, epidural medication warming may represent an effective alternative for safe and satisfactory labor analgesia.

Previous studies in obstetric and non-obstetric patients undergoing surgery have demonstrated a more rapid onset of sensory blockade in patients receiving body temperature (37 °C) epidural medication compared to room temperature medication, though other reports have found no significant difference in the onset of sensory blockade when comparing body temperature to room temperature epidural medications. To the knowledge of the investigators, there are few studies examining the effect of warmed epidural medications for patients receiving epidural labor analgesia.

In the experience of the investigators' colleagues, the close proximity to the body has a warming effect on the epidural medication and ultimately leads to quicker onset of pain relief once administered. Pocket warming does not warm the medications to the same extent as an incubator, but is certainly less expensive and is readily available to every obstetric anesthesia practice.

The investigators have previously measured the temperature of five 10mL syringes of normal saline at room temperature and at baseline the average was 21.7 °C (range 21.5-21.9 °C), and increased to an average of 29.7 °C (range 29.1 - 30.2 °C) after 1.5 hours of pocket warming. It has not been studied and is unknown if this degree of warming would be effective in enhancing the onset of labor analgesia, but such information is valuable given that an approximate 30 °C temperature may be accomplished by simple pocket warming and is within manufacturer recommended storage temperatures of 15-30 °C.

The investigators also assessed the potential for epidural medication cooling by measuring the temperature of one of the investigator's 10mL saline syringes for twenty minutes after removal from the warm pocket environment. The initial temperature of 30.0 °C had cooled to 27.0 °C by five minutes, 24.7 °C by ten minutes, 22.9 °C by fifteen minutes, and had returned to baseline room temperature by twenty minutes. The time between medication removal and dosing is important to consider given that significant cooling may occur and negate any potential benefits of warming the medication. This cooling effect makes use of a centralized warmer less promising, as it could take 10-20 minutes to position the patient and complete placement of the epidural catheter prior to dosing the medication. The use of a bedside incubator or a pocket warming technique would be useful in this regard, because the medication could be administered immediately after removal from the warm environment.