Cardiac arrest is a rarity in the chamber, as most arrhythmias seem to improve under hyperbaric conditions. Anecdotally, it can be noted that one patient with a myocardial infarction, who was being treated with hyperbaric oxygen as part of a research study, suffered 30 cardiac arrests during the 48 hours he was being treated with the chamber. The schedule being followed called for two hours at pressure in the chamber followed by one hour on the surface. This cycle was repeated for two days. It can be seen that the patient spent only 1/3 of his time breathing air on the surface. During the study, the patient suffered 28 cardiac arrests while breathing air on the surface, but only two arrests while at pressure in the chamber. The patient eventually recovered and returned to work. (Thurston, J. Westminster Hosp, London, Personal Communication, 1973.)
The cardiovascular assessment of a critical care patient receiving hyperbaric oxygen therapy is similar to any cardiovascular assessment of a critical care patient. Cardiac rhythm should be assessed and monitored during the treatment. If the patient would experience a cardiac emergency during HBO treatment, the staff should be trained to handle such an emergency. It is important to remember that a patient at depth is well oxygenated and will remain so for 5 to 8 minutes. The safest way to bring a patient that has had a cardiac arrest to the surface is to bring them up at 5 psig (fastest rate on a monoplace chamber), while the staff prepares to deliver emergency care. Performing a rapid ascent using the emergency ascent button on monoplace chambers places the person at grave risk for air embolism due to expansion injuries. Once the patient has surfaced, move them to the point farthest away from the open chamber, remove the hyperoxygenated clothing, and if appropriate, defibrillate. Cold oxygen will fall to the floor and dissipate in 30 seconds, so do not place the area at an increased fire risk with the use of a defibrillator. The staff needs to be able to respond quickly and appropriately to such an emergency, so it is our recommendation that the staff be Advanced Cardiac Life Support (ACLS) certified.
Hyperbaric oxygen exposure can produce significant hemodynamic changes. An increase in systemic afterload due to hyperoxic vasoconstriction in well perfused tissues can lead to a decrease in left ventricular function and a decrease in ejection fraction in some patients. When this decrease in left ventricular function occurs in the setting of pulmonary arterial vasodilatation due to improved alveolar oxygenation with increased left atrial and left ventricular filling, acute left ventricular dysfunction and pulmonary edema can result. Cases have been reported in patients with a history of pulmonary edema or low left ventricular ejection fractions or in patients with sudden fluid shifts from volume overload. Acute pulmonary edema appears to be more common in monoplace than multiplace treatment settings, perhaps because of the requirement for patients to be in a more supine position in the monoplace chamber rather than the sitting position with legs dependent available in the multiplace chamber.