Massive arterial air embolism during cardiopulmonary bypass: antegrade blood cardioplegia delivered by the pump--an accident waiting to happen

Sudden pump stop may cause air release in oxygenators, 'The Hammer Effect ': An in vitro evaluation

BACKGROUND

Oxygenators, as used in cardiopulmonary bypass (CPB) circuits, are components with good air removal properties. However, under some conditions the semipermeable characteristics of hollow fibers allow air to accidentally enter the blood side of the CPB circuit. This may occur when a fluid in motion is stopped suddenly by which the rapid change in momentum may cause a relative negative pressure drop, the so-called hammer effect. The hammer effect is not yet described in literature related to CPB. The aim of this in vitro study was to reproduce the hammer effect.

METHODS

The in vitro setup consisted of a CPB circuit with a fully occluded roller pump and one of four test oxygenators. The hammer test was performed by a sudden pump stop. The pressure wave was measured and after the test the residual air present in the oxygenator was forced into the arterial line and measured with a bubble detector.

RESULTS

We showed that a sudden pump stop could lead to the hammer effect, represented as a relative negative pressure drop in the arterial line. This hammer effect resulted in air release through the semipermeable fibers as we showed in two of the four tested brands of oxygenators.

CONCLUSIONS

We conclude that the hammer effect may occur before connection of the CPB system to the patient, and this may result in air release into the arterial blood side of the oxygenator. The hammer effect can be caused by clamping of the tubing in combination with a centrifugal pump, or by suddenly stopping the roller pump. With this study we would like to raise awareness of the hammer effect.

Successful treatment of accidental air embolism in warm heart surgery

Normothermic cardiopulmonary bypass has recently been proposed as a superior technique for maintaining body metabolism. However, its use remains controversial since the degree of cerebral protection provided might be inferior to that conferred by conventional hypothermic techniques. We report a case of accidental massive air embolism during coronary artery bypass surgery under normothermia, which was successfully managed with induced hypothermia at 20 degrees C and retrograde cerebral perfusion.

Australasian perfusion incident survey

An anonymous postal survey about perfusion accidents, incidents and safety procedures was conducted retrospectively in all identifiable cardiac surgery units in Australia and New Zealand over an 18-month period from January 1994 to July 1995. Responses were received from 69% of all eligible perfusionists representing 39 to 42 operating units. The most frequent incidents reported were heater/cooler failure (43% of respondents), urgent return to bypass following circuit disposal (38%), air embolus in a circuit not reaching the patient (24%), accidental cannula displacement (28%), protamine-induced circuit clotting postbypass (20%), hospital power failure (31%) and oxygenator membrane leaks (24%). There were 11 serious injuries and 10 deaths reported, giving an overall rate of serious injury or death of one in 1300 cases. However, the perfusion-related injury rate was lower, at one injury or death per 2500 perfusions. The use of safety equipment was widespread with low-level alarms in use by all respondents, bubble detectors by 74%, arterial line filters by 82% and written or computerized checklists by 80%. While the rate of injury was lower than that reported in surveys done elsewhere, the rate of reporting of incidents was greater and this may reflect a changing attitude of perfusionists to accountability in the delivery of their services. The data suggest that the level of safety in perfusion in Australasia is high but that improved incident reporting may help to further improve practices.