To the Editor:
A novel method of phonocardiography based on the capture of heart sounds from the cuff of a modified endotracheal tube (ETT) is presented. The ETT (Leonides Y. Teves, U.S. Patents 5,029,591 and 5,315,991) is constructed using a cuff inflation line with a larger bore inner diameter than is conventionally used (4.2 mm vs 0.90 mm) (Fig. 1). The ETT cuff is connected to a special leak-free microphone using a stopcock after cuff inflation to 20 mm Hg pressure. The microphone assembly comprises a miniature electret type microphone (e.g., Radio Shack model 30-3013), a shortened 3-mL plastic syringe with a Luer lock end, and epoxy glue to secure the microphone into the barrel of the syringe. An audio amplifier amplifies the phonocardiogram signal to a level suitable for recording. The signal can also be monitored with a set of earphones.
Following institutional approval, the system was evaluated clinically in 10 surgical patients undergoing general anesthesia with positive pressure ventilation. In all cases, the phonocardiogram was easily heard, along with easily heard breath sounds synchronous with ventilation. Two control recordings taken using a conventional ETT with a small-bore cuff inflation line were completely inaudible.
The phonocardiographic recording system presented here eliminates the need for precordial and esophageal stethoscopes in intubated patients. It is an atraumatic, simple, and reliable method for obtaining recordings of heart sounds (and breath sounds) in intubated patients. Disadvantages of the system include the need for a special microphone and endotracheal tube.
D. John Doyle, MD, PhD, FRCPC