Respiratory Gas Exchange Monitors
Prior to the routine availability of gas exchange monitors, anesthesiologists used a precordial or esophageal stethoscope to ensure that the lungs were being ventilated in the event that the circuit became disconnected. Likewise, the heart tones could be auscultated to confirm a beating heart. Although less essential today because other modalities are available, the finger on the pulse and auscultation remain front-line monitors, especially when tech-nology fails. Chest auscultation remains the primary method to confirm bilateral lung ventilation in the operating room, even if end tidal CO 2 detectionis the primary mechanism to exclude esophageal intubation.
Instrumentation of the esophagus should be avoided in patients with esophageal varices or strictures.
A precordial stethoscope (Wenger chestpiece) is a heavy, bell-shaped piece of metal placed over the chest or suprasternal notch. Although its weight tends to maintain its position, double-sided adhe-sive disks provide an acoustic seal to the patient’s skin. Various chestpieces are available, but the child size works well for most patients. The bell is con-nected to the anesthesiologist by extension tubing.
The esophageal stethoscope is a soft plastic cath-eter (8–24F) with balloon-covered distal openings (Figure 6–1). Although the quality of breath and heart sounds is much better than with a precordial stethoscope, its use is limited to intubated patients. Temperature probes, electrocardiogram (ECG) leads, ultrasound probes, and even atrial pacemaker electrodes have been incorporated into esophageal stethoscopes. Placement through the mouth or nose can occasionally cause mucosal irritation and bleed-ing. Rarely, the stethoscope slides into the trachea
instead of the esophagus, resulting in a gas leak around the tracheal tube cuff.
The information provided by a precordial or esoph-ageal stethoscope includes confirmation of ventila-tion, quality of breath sounds (eg, stridor, wheezing), regularity of heart rate, and quality of heart tones (muffled tones are associated with decreased cardiac output).The confirmation of bilateral breath sounds after tracheal intubation, however, is made with a binaural stethoscope.