Case 4: Air Today; Gone Tomorrow?
A patient in the intensive care unit (ICU) required the use of a transthoracic pacer. A defibrillator/pacer, normally kept in the unit, was used. Disposable pacing electrodes, which can double as defibrillator paddles, were placed in the classical transthoracic posi
tion (over the heart and on the patient’s back). However, the pacer would not capture. The pacing current was increased but would still not capture. The defibrillator-pacemaker was changed to a second unit with no better results. Clinical Engineering was called in to assess the “failure” of both pacers as the staff began preparing the patient for the insertion of a temporary transvenous pacing catheter. Our records indicated that both units had been recently inspected and were working properly. A quick test in the ICU confirmed this. I returned to the patient’s cubicle and looked around. I noticed that the patient had a chest-drainage tube in place. I spoke to the medical staff who informed me that the patient was being treated for a pneumothorax that had occurred a short while before the pacer was required. The pneumothorax would allow air to enter the chest cavity. If some of this air had been present between the heart and chest wall, where the anterior pacing electrode was placed, it might account for the lack of capture by the transthoracic pacer. Even at high pacing currents, the insulating air prevented enough current from crossing the myocardium and causing capture. The increasing pacing current found a route around the chest wall with very of the current passing through the heart. The electrodes were repositioned to sites that reliably allowed the pacer to capture the patient’s heartbeat. This clinical engineering intervention might have averted a catastrophe had the need for defibrillation arisen. The catheter insertion in this patient in the midst of an ongoing episode of bradycardia might have triggered arrhythmias or even ventricular fibrillation. If an attempt had been made to defibrillate the patient, the same air pocket over the heart may also have prevented successful defibrillation.