[Pros or cons of accessory anesthetic circuits. I. Arguments for their use]

Historical vignette – The Mapleson G, an original pediatric anesthesia circuit

A previously unpublished pediatric anesthesia circuit is presented here. It was invented and constructed by Dr Bernard-François Gribomont (hence called BFG circuit) around 1965 as a response to the important pediatric case load in the university hospital of Lovanium, near Leopoldville (now Kinshasa, DRC). The original objective was to find a simple solution that would enable the manual ventilation (assisted or controlled) of young children during ENT surgery, remaining very close to the child to reduce dead space while at the same time keeping far enough away from the surgeon in order to avoid obstructing their work. It includes a short coaxial single piece circuit devoid of any mechanical valve connected to an in-line fresh gas ventilation bag; it does not fit into any existing Mapleson category. Hence, the authors propose to classify it in a new Mapleson G class. Its main advantages are conceptual simplicity, inherent safety, very low dead space accounting for minimal rebreathing and thus reduced fresh gas flow, small size and weight, and ease of use even during prolonged manual ventilation in small children. Its main drawback is difficult scavenging of expired gases. For logistical reasons it was abandoned in the nineties but could be of renewed interest in low-income countries.

[Risk of barotrauma when using non-reinhalation Waters valves: a comparative study on bench test]

OBJECTIVE

Manual ventilation is delivered in the operating room or the intensive care unit to intubated or non-intubated patients, using non-rebreathing systems such as the Waters valve. New generation Waters valves are progressively replacing the historic Waters valve. The aim of this study was to evaluate maximal pressure delivered by these 2 valves.

TYPE OF STUDY

Bench test.

MATERIAL AND METHOD

Thirty-two different conditions were tested, according to 2 oxygen flow rates (10 and 20L/min), without (static condition) or with manual insufflations (dynamic condition) and 4 valve expiratory opening pressures. The primary endpoint was maximal pressure measured at the exit of the valve, connected to a model lung and a bench test.

RESULTS

Measured pressures were different for most evaluated conditions. Increasing oxygen flow from 10 to 20L/min increased maximal pressure for both valves. Increasing valve expiratory opening pressure induced a significant increase in maximal pressure for the new generation valve (from 4 to 61cmH2O in static conditions and from 18 to 68cmH2O in dynamic conditions). For the historic valve, maximal pressure increased significantly but remained below 15cmH2O in both static and dynamic conditions.

CONCLUSION

Use of new generation Waters valves should be different from historic Waters valves. Indeed, barotrauma could be caused by badly adapted valve expiratory opening pressure settings.

[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.