There are two types of smooth muscle contraction detectable in the lower oesophagus:
Spontaneous lower oesophageal contractions (SLOC)
Arise spontaneously and can be induced by emotion and stress in the awake individual. It is believed that SLOC are under the control of a central oesophageal motility centre, the activity of which is influenced by higher centres. SLOC arise spontaneously, and can be detected by a pressure transducer.
Provoked lower oesophageal contractions (PLOC)
These result from sudden distension of the oesophagus, as if due to the arrival of a food bolus. PLOC are induced by the rapid inflation of a balloon catheter in the lower oesophagus. This causes smooth muscle contraction and is detected by a more distally placed pressure transducer. The dose-response curve for PLOC is more shallow than that of SLOC.
Spontaneous and provoked lower oesophageal contractions both reduce in latency and amplitude during general anaesthesia. These are measured using a balloon in the oesophagus; however, published evidence of its use as a depth of anaesthesia monitor is limited.
One way of improving the available information is by combining the measurement of SLOC frequency with PLOC amplitude, leading to the derivation of the oesophageal contractility index (OCI):
OCI = 70 x (SLOC rate + PLOC amplitude)
The OCI is easy to interpret and can be used in the presence of muscle relaxants; however, consensus opinion is against this method being a reliable measure of anaesthetic depth.
[i] Fundamentals of Anaesthesia, 2nd Edition, Pinnock, Lin, Smith; Pages 866-867
[ii] Lower oesophageal contractility and detection of awareness during anaesthesia. Br J Anaesth 65: 319-24