Heart Rate Response to Atropine Doses Less Than 0.1mg IV to Anesthetized Infants

Overview

An infants heart rate is very important because it ensures that blood is pumped to all organs in the body. Heart rate may decrease during anesthesia and surgery, and this is why the anesthesiologist will often give a medication to prevent this from happening. The most common drug for this purpose is called atropine. The dose of most drugs given to babies is based upon the baby's weight, but some believe that the dose of atropine should not be less than 0.1mg. However there is no evidence to support this minimum dose. A larger dose of atropine may cause a very fast heart rate instead. Anesthesiologists routinely dose the atropine based upon the baby's weight without regard for a minimum dose. The purpose of the present study is to measure the heart rate after doses of atropine in neonates and infants who receive less than 0.1 mg.

60 infants, ASA physical status I and II, undergoing elective surgical procedures will be enrolled after signed informed parental consent. All children will be fasted according to institutional guidelines and unpremedicated. After arriving in the operating room, EKG, pulse oximeter and blood pressure monitors will be applied (Datex-Ohmeda Aisys). Anesthesia will be induced with 66% N2O in O2 and 8% sevoflurane. Respiration will be supported by a properly sized face mask through which he/she is allowed to breath spontaneously. Respiration will continue spontaneously through a facemask at 2 MAC sevoflurane in 66% N2O. All children will be positioned supine, warmed with a forced air warmer and given 20 ml/kg IV balanced salt solution over 30 minutes after the IV has been established. The end-tidal pCO2 will maintained 35-45 mmHg and oxygen saturation \>96%. After a 22 or 24G IV cannula is inserted, 0.005 mg/kg atropine will be administered intravenously over 5 seconds through a fast-flowing IV and followed by 5 ml of normal saline to flush it in through the IV deadspace. The study (ECG recording) period will extend from 30 seconds before atropine administration to 5 minutes after injection. During this time the heart rate and rhythm (through lead II) will be monitored and recorded continuously using an analogue interface system. EKG will be recorded on paper continuously for the 330 seconds of the study. The recording will be analyzed for the heart rate (based on the R-R interval) and arrhythmias by a physician blinded to the study. Blinding means that the individual is unaware of the hypothesis of the study and what medication was administered to account for any changes in heart rate. Bradycardia is defined as a 20% reduction from baseline heart rate while tachycardia is a heart rate \> 160 beats/minute(6). Any heart rate \< 100/minute will be considered a bradycardia in this age group. Arrhythmia is any disorder of rhythm or rate observed. These will be summarized for each child. All heart rate responses will be recorded and reviewed. The time to record the heart rate after atropine, which will occur before surgery, will add less than 5 additional minutes to the anesthetic since it will overlap the time taken for other surgical preparatory events. The blood pressure will be monitored non-invasively immediately before receiving atropine and at one and five minutes after it is given. The primary outcome is the incidence of bradycardia during the first 5 minutes after atropine will be determined by reviewing the electrocardiogram. All continuous data will be reported as means +/- standard deviation.