Selective oxygen leak. A potential cause of patient hypoxia

[Anesthesia equipment: fresh gas delivery systems. I. Mecanical systems with rotameters and calibrated vaporizers]

OBJECTIVES

To analyse the design, performance, failures, the checking before use of mechanical fresh gas delivery units (FGDUs) equipped with bobbin or ball-flowmeters delivering a continuous gas flow and calibrated vaporizers, marketed in France in 1999.

DATA SOURCES

Articles were obtained from a Medline review (1960-1999; search terms: anaesthetic machine, flowmeter, vaporizer), textbooks and personal files; specific data were provided by manufacturers.

STUDY SELECTION AND DATA EXTRACTION

The articles were considered for performance data, benefits and drawbacks, and characteristics, as well as the risks carried by mechanical FGDUs.

DATA SYNTHESIS

Seven anaesthetic machines out of 11 are equipped with mechanical FGDUs, including rotameters delivering a gas mixture up to 30 L.min-1, calibrated vaporizers and an O2-flush valve delivering at least 500 mL O2 per second (30 L.min-1). These units allow closed circuit anaesthesia. They carry a risk for barotrauma as three out of them can deliver at the gas outlet of the FGDUs a gas mixture at a pressure reaching 3.5 bars and four others at a pressure of 150-200 mmHg. They also carry a risk for hypoxia, either from a preferential leak of oxygen at the corresponding rotameter or the O2-flush valve, or from a leak of fresh gas mixture either in a vaporizer or the selectatec manifold. The vaporizers carry a risk for vapor delivery at a concentration differing notably from the value set on the concentration dial. Therefore their accuracy must be checked periodically and the FGDUs checked for a leak after the addition to or the removal of a vaporizer from the selectatec manifold. The optimal technique for leak detection is the negative pressure test.

Association of anaesthetist's checklist for anaesthetic machines. Problem with detection of significant leaks

Eight experienced anaesthetists performed a 'cockpit drill', following instructions in the Association of Anaesthetist's checklist, on an anaesthetic machine that had a significant leak (3 l.min-1 at a pressure of 16 kPa). Only one anaesthetist detected the leak and this was by audible means rather than by any of the protocol's set manoeuvres. We demonstrated that a leak of 3 l.min-1 from the flowmeter block resulted in an inspired oxygen concentration of 6% when the anaesthetic machine was used with a minute volume divider ventilator.

Potential leaks from anaesthetic machines. Potential leaks through open Rotameter valves and empty cylinder yokes

Twenty anaesthetic machines were examined to detect leakage of fresh gas through open Rotameter flow control valves and empty cylinder yokes. Significant leakage was demonstrated in most machines. Any leakage in machines which comply with ISO 5358, where oxygen is the last gas into the common gas manifold, is selective for nitrous oxide and, conversely, it is selective for oxygen in machines where oxygen is the first gas into the common gas manifold. Leakage is augmented by back pressure from ventilators that function as minute volume dividers. In ventilated patients the leakage described may contribute to hypoventilation and patient hypoxia or to pollution of the operating theatre and patient awareness.