7.2.5 Heat exchanger

In current systems, the heat exchanger is coupled to the oxygenator and the venous reservoir. It consists of a countercurrent exchange system between blood and a coil containing water, the temperature of which is managed by a device external to the bypass machine (see Figures 7.6 and 7.16). Temperature management is based on a number of recommendations [1].

• Temperature gradient between water and blood should never exceed 10°C, and water temperature should not go above 38°C or below 12°C.
• During cooling, temperature gradient between inlet and outlet of the heat exchanger/oxygenator must never exceed 10°C.
• During reheating when temperature is < 30°C, temperature gradient between inlet and outlet of the heat exchanger must never exceed 10°C.
• During warming up when temperature is > 30°C:
  • Temperature gradient between inlet and outlet of heat exchanger must remain ≤ 4°C;
  • Heating rate should remain ≤ 0.5°C/min.
• Blood temperature leaving the heat exchanger should never exceed 37°C to avoid cerebral hyperthermia.
• When weaning off, the most appropriate temperature measurement is the nasopharyngeal T°; the pulmonary catheter T° is reliable as soon as pulmonary flow is restored.
• The gradient between rectal/vesical and oesophageal temperature should remain below 10°C; rectal or bladder T° is 2-4°C below brain temperature during rewarming.

Risk of gas embolism becomes prohibitive if thermal gradient limit (10°C) is exceeded, because gases dissolved in blood become gaseous as local temperature increases [2]. Physics reminds us that gas solubility in a liquid decreases with increasing temperature or decreasing pressure, but increases with hypothermia and overpressure.It is William Henry's law (1803): at constant temperature and equilibrium, the amount of gas dissolved in a liquid is proportional to the partial pressure of that gas on the liquid.

Heat exchanger contamination with Mycobacterium chimaera is an unusual complication that has been occurring in Europe in recent years; seldom, but results in a postoperative mortality of around 50% [3].

 © CHASSOT PG, GRONCHI F, last update December 2019

References

1. ENGELMAN R, BAKER RA, LIKOSKY DS, et al. The Society of Thoracic Surgeons, the Society of Cardiovascilar Anesthesiologists, and the American Society of Extracorporeal Technology: clinical practice guidelines for cardiopulmonary bypass - Temperature management during cardioplmonary bypass. J Cardiothorac Vasc Anesth 2015; 29:1104-13
2. GEISSLER HJ, ALLEN JS, MEHLHORN U, et al. Cooling gradients and formation of gazeous microemboli with cardiopulmonary bypass: an echocardiographic study. Ann Thorac Surg 1997; 64:100-8
3. NINH   A, WEINER M, GOLDBERG A. Healthcare-associated Mycobacterium chimaera infection subsequent to heater-cooler device exposure during cardiac surgery. J Cardiothorac Vasc Anesth 2017; 31:1831-5