Temperature monitoring in cardiac surgery with transesophageal echocardiography probe: magic bullet or underutilization of a powerful tool?

Temperature monitoring remains crucial in modern cardiac surgery with cardiopulmonary bypass, particularly for targeted systemic cooling and rewarming in complex procedures. Hypothermia is considered an effective strategy for lowering cerebral metabolic rate and allowing safe circulatory arrest of the brain, with the maximum tolerated duration related to the degree of hypothermia [1]. Postoperative hyperthermia, in contrast, even when mild is extremely harmful and clearly associated with worse neurological performance after cardiac surgery [2, 3].

Among the many possible sites where temperature can be measured in the human body, the lowest difference to brain temperature was recorded in the proximal oesophagus (at 24 cm from the dental arch), the nasopharynx (both with a mean difference of 0.4°C) and distal oesophagus (0.5°C), making them the best surrogates of cerebral cortical temperature [1]. Core body temperature is a different entity and refers to the temperatures of internal organs, such as the liver, stomach, bladder and rectum [4]. It is traditionally measured with intracardiac catheters, such as the pulmonary artery catheter, or rectal probes. Core body temperature and cerebral temperature differ under normothermic conditions, but also during induced cooling and rewarming in the setting of cardiopulmonary bypass. The latter is explained—among others—by the cannulation technique of the aorta, which provides blood flow directly to the brain via the carotid and vertebral arteries and thus impacts cerebral, nasopharyngeal and oesophageal temperature first [3, 5].

Special emphasis is given to temperature measurement at the oxygenator arterial outlet of the cardiopulmonary bypass circuit. The latest Clinical Practice Guidelines for Cardiopulmonary Bypass-Temperature Management during Cardiopulmonary Bypass published in 2015 recommend that arterial outlet blood temperature should be used as a surrogate for cerebral temperature during cardiopulmonary bypass (class I, level C) [6]. This wording has been slightly modified in the current EACTS/EACTA/EBCP guidelines on cardiopulmonary bypass in adult cardiac surgery published in 2020 to mention arterial outlet temperature monitoring as a class I, level C recommendation, but not its direct surrogate to cerebral temperature [7]. The highest level of clinical recommendation and the lowest level of research evidence suggest that this recommendation is more of a general agreement between the guideline’s authors than solid evidence. It can also be concluded that this statement is aimed rather at a technical recommendation—in the sense of a temperature control by the perfusionist and avoidance of too high-temperature gradients during cooling and rewarming—than at a clinical monitoring of the accurate temperature at the site of (anatomical) interest. Although, of course, the arterial outlet temperature, due to its direct coupling to the aorta and to the vessels supplying the brain, is able to equalize the temperature in the brain quickly and thus shows the smallest average discrepancy of all temperature sites relative to the jugular bulb temperature which is considered a reasonable approximation of the global cerebral temperature [8].

In the current issue of the European Journal of Cardiothoracic Surgery, Misra et al. [9] prospectively investigated the performance of the transoesophageal echocardiography probe for continuous monitoring of the oesophageal temperature against measurements in the nasopharynx and at the oxygenator arterial outlet, which was also taken as the reference for cerebral temperature and all comparative temperature readings. In thirty adult patients undergoing cardiac surgery with mild hypothermia, the authors were able to statistically demonstrate that the correlation with arterial outlet temperature was best with oesophageal temperature measured with the transoesophageal probe in stand-by mode and placed at the upper or middle oesophageal position (50% of patients each) on cardiopulmonary bypass during both cooling and rewarming. The results led the authors to prefer transoesophageal echocardiography-derived oesophageal temperature over nasopharyngeal temperature for measuring core body temperature in cardiac surgery.

We would like to congratulate the authors for their proper and clear methodology and the communication of their results in this journal. Nevertheless, this study raises some questions and these should be subject to discussion. First, an exact distinction of the temperature measurement (cerebral vs core) seems important in studies to make the statements comprehensible. In this sense, the temperature measurement in the nasopharynx or in the oesophagus—as stated above and in accordance to the literature—cannot be designated as ‘core temperature’. These are modalities that allow to remotely measure a surrogate temperature for the brain. For both, a correlation is present with the cerebral temperature due to their close anatomical proximity and the joint blood supply through branches of the external carotid artery or the thoracic aorta. Second, when considering the advantages and disadvantages of the different sites for temperature measurement, we do not see any singular measurement modality as superior. Depending on the surgical procedure, the sites for temperature measurement should be adapted. For example, in simple cases (e.g. coronary artery bypass grafting in normothermia), measuring the temperature at one site may be sufficient. In complex surgeries, however, several measurement sites are required, each of which provides different information and may trigger different adjustment measures. In this respect, for example, in aortic arch surgery with hypothermic circulatory arrest and selective antegrade cerebral perfusion, a combination of oesophageal, tympanal and bladder temperature measurement may be useful to measure both cerebral temperature (side separated when typanal measurement is applied!) and core body temperature. Third, transoesophageal echocardiography in cardiac surgery is nowadays not only a ideally suited tool for pre- and post-cardiopulmonary bypass investigation of myocardial contractility or valvular pathologies [10]. It is also a tool to assist or guide surgical intraoperative measures such as (i) placement of the aortic and venous cannula, (ii) retrograde cardioplegia lines, (iii) left ventricular vent or (iv) intravascular guidewires into the aorta or right atrium. It also provides valuable information in minimally invasive or robotic cardiac surgery, in monitoring cerebral perfusion during selective antegrade cerebral perfusion, or in supporting initiation and weaning procedures (e.g. in extracorporeal membrane oxygenation, mechanical circulatory device implantation). Therefore, we consider a fixed positioning of the echocardiography probe—only to measure the temperature in the oesophagus through it—as outdated and no longer appropriate. Finally, temperature monitoring should not be restricted to the cardiopulmonary bypass period. In the post-bypass period, patients' temperature should be measured continuously to avoid coagulopathy and increase of blood viscosity due to remaining hypothermia, to adjust the dosage of volatile anaesthetics (lower blood/gas partition coefficient and thus an increased wash-in and more rapid increase in depth of anaesthesia during rewarming), or to avoid hyperthermia and its devastating consequences on outcome as already described above.

This small but well-conducted study is an example of how observation using existing equipment can provide additional information for temperature management in cardiac surgery with cardiopulmonary bypass. In individual cases or with limited resources, the approach and monitoring strategy proposed by the authors may be acceptable as demonstrated in this study.

For complex cardiac surgery, especially aortic surgery, however, we believe that an echocardiography probe left in situ to measure blood temperature constitutes an underutilization of an otherwise highly powerful diagnostic tool rather than being the new magic bullet of temperature measurement. Particularly in complex cases, echocardiographic guidance of surgical procedures and extensive (and thus more accurate) multilocal temperature monitoring—in the way of temperature measurements at multiple anatomic sites—is necessary to comprehensively assess adequate end organ perfusion and to avoid postoperative complications due to unrecognized hypo- or hyperthermia.

Funding

This work was supported by departmental resources of the corresponding author.

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