The findings of this study show that oesophageal temperatures monitored with the TOE probe during CPB reliably approximate the arterial outlet temperatures at the membrane oxygenator, especially during rewarming. The performance of the TOE probe was considerably better than the nasopharyngeal probe during both the cooling and rewarming periods.
In humans, the basal cerebral metabolic requirement for oxygen (CMRO2) is 3.5 ml/100gm/min, of which 60% is used for electrophysiological function (functional CMRO2), and 40% is for maintenance of structural integrity (structural CMRO2) . While anaesthetics reduce only the functional CMRO2, temperature affects both functional and structural CMRO2; and total CMRO2 reduces by 6–7% per degree centigrade drop in temperature . Apart from neuroprotective effects through reduction of metabolism, the institution of hypothermia has multiple pleiotropic benefits such as free radical suppression, inhibition of destructive enzymatic reactions, and inhibition of synthesis and release of putative excitatory neurotransmitters . However, hypothermia is associated with risks like coagulopathy, prolongation of bypass duration due to an increase in the time for rewarming, postoperative shivering due to after drop of temperatures, and infections. Rewarming to reverse the hypothermia is equally critical. Cerebral hyperthermia may account for 50–80% of the neurophysiologic decline following cardiac surgery [11, 12].
Although the nasopharyngeal site is commonly monitored during CPB , nasopharyngeal temperature lags behind standard reference sites during rewarming [7, 8, 14, 15]. Nussmeier and colleagues , found that the nasopharyngeal temperature lags around 1.6 (1.2)0C at 15 minutes of rewarming compared with jugular bulb temperature. Similarly, Johnson and colleagues , found a poor correlation between the arterial outlet temperature probe and the nasopharyngeal temperature probe. In general, the mean arterial line temperatures were 3.60 C higher than the nasopharyngeal temperatures . In fact, they suggested that the sole reliance should not be on nasopharyngeal temperature probe only . Furthermore, nasopharyngeal temperatures also do not correlate well with brain temperatures during induction of deep hypothermia as well as its reversal . In addition, massive epistaxis has been reported by the use of reusable nasopharyngeal temperature probes in a patient undergoing cardiac surgery .
The correlation with brain temperature is worse for other sites such as the bladder and rectum. At a rectal or bladder temperature of 370C, the brain temperature is 2-40C higher . This puts the brain at higher risk for strokes since cerebral embolization occurs when the brain is warm or rewarming, after removal of the aortic cross-clamp. Other sites like the tympanic membrane are not commonly used for temperature management. A significant amount of wax may preclude accurate temperature measurements with probes inserted near the tympanic membrane [16, 18]. Studies have also shown that the tympanic membrane temperatures are lower than hypothalamic temperatures and can change in opposite directions . Although the arterial outlet temperatures have been shown to correlate well with jugular bulb temperatures, in a survey of cardiac surgeons in the United Kingdom, only 33% rely on the arterial outlet temperature, and thus, it is not universally used . While pulmonary artery catheters and jugular bulb catheters have excellent intra-class correlation (> 90%) and are considered to approximate brain temperatures best, they are invasive catheters and are currently not the standard of care .
Oesophageal temperatures are commonly measured with a thermistor or thermocouple placed approximately 45 cm from the nose. Oesophageal temperatures have a good correlation with brain temperatures , but the degree of accuracy is less in patients undergoing aortic arch surgeries . Other authors have found that oesophageal temperatures also lagged behind jugular bulb temperatures by around 1.3 (1.2)0 C during rewarming . This may be because oesophageal temperature probes being passive thermistors, probably are not designed to track surrounding temperatures rapidly. The use of oesophageal probes for monitoring core temperature is not very common (around 6.3% in a Canadian survey) , and probably even less so in the present context because of the routine use of TOE probes intraoperatively.
The intra-class correlation of the core temperatures recorded by the TOE probe with the arterial outlet temperatures in this study were at best modest (> 70% at 20 and 30 minutes of cooling and 5 and 10 minutes of rewarming). Despite this, its performance as a temperature monitor was superior to the nasopharyngeal probe. Although cold cardioplegia solutions may impact the temperatures and thus the accuracy of the oesophageal temperatures vs. the brain temperatures during cooling , we did not find a difference between the upper and mid-oesophageal positions of the probe, reflecting the fact that leaving the probe in any position would correlate with the arterial outlet temperature.
A possible reason for the poor intra-class correlation between the TOE and arterial outlet temperatures at 30 minutes following rewarming could be due to the fact the correlation is measured across individual values; and there may have been more variations in the individual measurements, despite similar average temperatures and bias measurements.
During the time the echocardiography machine is kept in standby mode, the TOE probe will reflect the temperature of the surroundings, in this case, the perfusate temperatures. The probe can be affected by the surrounding temperature, even without being used. In a patient with chronic liver disease, atrial fibrillation, poor hemodynamics, and hyperthermia (axillary temperatures 390C), immediate shut down of the TOE probe on insertion into the oesophagus was reported by the authors . At the time of the shutdown, the echocardiography monitor showed a temperature of 42.50C .
Following the removal of the aortic cross-clamp after adequate rewarming, the TOE probe can be used as a cardiac monitor to evaluate the presence of intracardiac air as well as the results of surgery. The machine in standby mode also allows the prebypass images to be analysed without impacting the temperature readings.
One possible disadvantage is that in many centers, the aortic cross-clamp is removed at 32–340 C, at which time, operators often use the TOE probe to “look” at the heart. Once used, the probe will immediately start warming and not be reflective of the arterial perfusate temperatures anymore. However, this can be avoided by allowing the heart to rewarm completely along with the rest of the body, before any cardiac interrogation is performed. Another disadvantage is that after termination of CPB, the nasopharyngeal probe can continue to monitor the core temperatures, and thus allow for treatment of “after drop”, while the TOE probe would be mostly used as a cardiac probe and thus, would not detect the drifts in the body temperature.
Temperature monitoring was not continuous but conducted mostly over 5-minute epochs in our study. Comparisons with oesophageal probes were not carried out as it is impractical to use both an oesophageal temperature probe and a TOE probe in the same patient. Our results are valid for only moderate hypothermia and more research needs to be conducted on the performance of the TOE probe during deep hypothermia. We used the membrane oxygenator arterial outlet temperatures and not jugular bulb temperatures, which has been considered the gold standard by many workers. But given the observational nature of the study, this additional intervention of inserting a catheter into the jugular bulb was not possible. We did not correlate the various factors that could have influenced the magnitude of temperature differences like age, body weight, hematocrit, CPB duration etc. Finally, the results of this study are specific for the GE E95 probe and needs to be validated with TOE probes of other vendors and even different models for the same vendor.
To conclude, the TOE probe can be used to monitor arterial perfusate temperatures during CPB. The performance of the TOE probe as an oesophageal temperature probe fared better than the nasopharyngeal probe during both cooling and rewarming on CPB.