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Eur J Cardiothorac Surg 2004;26:580-585
© 2004 Elsevier Science NL
Department of Cardiothoracic Surgery, Queen Elizabeth Hospital, University Hospital NHS Trust, Edgbaston, Birmingham, B15 2TH, UK
Received 15 October 2003; received in revised form 16 April 2004; accepted 5 May 2004.
e-mail: hemanthkaukuntla{at}yahoo.co.uk
* Corresponding author. Tel.: +44-121-4721311; fax: +44-121-6272542
e-mail: robert.bonser{at}uhb.nhs.uk
| Abstract |
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Key Words: Cardiopulmonary bypass Temperature monitoring Hypothermia Hypothermic circulatory arrest
| 1. Introduction |
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As direct brain temperature measurement is not possible other than in some neurological procedures, surrogate temperature monitoring sites are used to estimate the brain temperature [5,6]. The most commonly monitored sites include nasopharynx (NP), oesophageal (O), bladder (B) in addition to arterial inflow (AI) and venous return (VR). Other sites include tympanic membrane, rectum and skin. Various studies have shown that these standard sites may not reflect the true brain temperature [3,7].
We sought to compare the routinely monitored temperature sites to the brain temperature. As 99% of jugular bulb blood flow is derived from the brain circulation, jugular bulb (JB) temperature should reliably predict brain temperature. This has been validated in neurosurgical patients undergoing profound hypothermia [7]. We used JB as a reference temperature to compare the accuracy of surrogate sites. In this study, we investigated whether conventional temperature monitoring sites adequately reflect brain temperature during deep hypothermia and normothermic CPB by comparing jugular bulb venous temperature (JB) with routinely monitored peripheral temperature sites including NP, AI, O, VR, B and orbital skin (OS) temperatures.
| 2. Study design and methodology |
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2.1. Jugular bulb temperature monitoring
After induction of anaesthesia, a triple lumen central line was inserted into the jugular bulb by the Seldinger method. Prior to insertion of the triple lumen catheter, a calibrated thermocouple (Columbus Instruments®, Ohio, USA) was inserted through one of the lumens. After the guide wire was placed in the internal jugular vein, the triple lumen catheter was advanced cranially till resistance was encountered. The catheter was then withdrawn while aspirating on the distal lumen till there was an efflux of venous blood into the aspirating syringe. This represented adequate placement of the catheter in the jugular bulb. In patients who underwent hypothermic circulatory arrest, lateral skull radiographs post-surgery were used to confirm catheter position.
2.2. Peripheral temperature monitoring sites
In addition to JB monitoring, calibrated temperature probes were used to measure the NP, AI, VR, B and OS temperatures. The nasopharyngeal probe was placed 5 cm from the external nares and the bladder temperature was measured using a urinary catheter temperature probe. The arterial and venous temperatures were measured directly from the CPB circuit. All the temperature probes were pre-calibrated to an identical reference and data was collected on to a dedicated laptop computer. All patients underwent neurological examination 2448 h post-procedure.
2.3. Profound hypothermia and circulatory arrest
The cardiopulmonary bypass circuit comprised a single roller pump (Stockert® Shiley caps) and integrated hard shell venous reservoir, oxygenator and heat exchanger unit (Terumo® Capiox SX-18, Stockert CAPS® heater chiller). The circuit was primed with 1500 ml of isotonic electrolyte solution. Oxygenated blood was returned to the patient through an ascending aorta, arch or femoral artery cannula (Sarns® 1824 Fg) to achieve antegrade flow when possible. Pump flows were maintained at 2.4 l/min/m2. A mean blood pressure of 5570 mmHg was maintained using aliquots of
-adrenergic receptor agonist or nitrate infusion. An alpha stat pH strategy was utilised in all cases. Arterial inflow and water bath temperature gradient was always maintained below 10 °C. Cooling was continued until the NP temperature reached 15 °C. Circulation was arrested at this temperature and following the completion of the arch reconstruction, orthograde cardiopulmonary bypass via a prosthetic graft side-arm, was reinstituted and rewarming commenced. CPB was discontinued when the NP temperature had been maintained at 36 °C for 10 min. All the patients had additional topical head cooling by ice packs and all received dexamethasone 100 mg and mannitol 1 g/kg approximately 20 min prior to circulatory arrest.
2.4. Normothermic bypass
Normothermic bypass patients underwent coronary artery bypass surgery or valve surgery using a similar bypass circuit. Normothermia was defined as maintaining a bladder (B) temperature of 3537 °C. Anaesthetic induction and maintenance techniques were identical and mannitol 0.5 g/kg was administered pre-bypass. Dexamethasone was not used. The AI temperature was not allowed to exceed 37.5 °C in either group.
2.5. Statistical methods
Collected data were transferred onto a computer spreadsheet and analysed using SPSS (SPSS for windows, version 11.0, Chicago Inc). Data was tested for normality and compared using a t test or Mann Whitney U test as appropriate. Statistical significance was assigned if P
0.05.
| 3. Results |
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| 4. Discussion |
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While deep hypothermia has an established role in aortic surgery, there has been a renewed interest in normothermia during CPB for cardiac surgery including coronary and valve surgery [14]. Normothermic bypass has been shown to have various systemic advantages [1517] but there have also been reports of increased adverse neurological events [18,19]. One possible mechanism for increased brain injury is the potential development of brain hyperthermia during CPB [4]. Brain hyperthermia may not be apparent if conventional temperature monitoring sites are used as these may be inaccurate and may therefore underestimate true brain temperature.
In cardiac surgery, there is a necessary reliance on indirect peripheral temperature measurements as guide to brain temperature and the most commonly used site is the nasopharynx. Crowder et al. [7] have shown that the core brain temperature is reflected by the JB temperature. Little is known about intra-cerebral shunts and how they change during deep hypothermia and normothermic CPB. Stone et al. [3] demonstrated that there is a significant gradient between the near surface temperature (depth of 1 cm) and that measured at an intra-parenchymal depth of 4 cm, the latter being close to 3 °C warmer than the NP temperature. But this study was done in patients whose cranium was open which could influence the temperature changes.
Our study demonstrates that during the cooling phase of profound hypothermic CPB, nasopharyngeal temperature monitoring is a satisfactory surrogate of brain temperature. Thus if a target NP temperature is chosen for initiation of circulatory arrest, the operator can be confident that true brain temperature is at or below the NP measurement. The findings in our study provide a degree of confidence in the nasopharyngeal temperature site as an index of brain temperature during cooling.
We are not aware of any previous reports of jugular bulb temperature measurement during profound hypothermia and normothermic CPB. The thermocouple we used was chosen due to its large calibrated range of temperature measurement, thus allowing accurate measurement of temperatures as low as 15 °C. Studies on cooling prior to HCA have reported that a cooling duration of 50 min is required to achieve electrocerebral silence [20]. Though we did not use EEG monitoring, which we recognise as a limitation, our mean cooling duration was in excess of 50 min. By taking JB temperature as a marker of brain temperature, we are confident that by achieving a NP temperature of 15 °C with the cooling protocol we have described, we have achieved target brain cooling prior to HCA. A faster rate of cooling might not be able to achieve this [3].
During normothermic CPB, brain temperature may be higher than the temperature observed at any surrogate site particularly towards the end of bypass and brain hyperthermia may occur inadvertently. Unless monitored, arterial inflow temperatures may exceed the corporeal temperature and lead to brain heating. Further, increasing brain and corporeal metabolism during rewarming may lead to additional heat generation. The findings of our study are consistent with previous reports of temperature measurement in moderately hypothermic bypass. Grocott et al. showed that during rewarming from moderate hypothermia, NP underestimates the JB temperature [21] while Bissonnette reported a 12 °C higher JB temperatures when compared to tympanic, rectal or oesophageal temperatures in children and infants having cardiac operations [22]. In these studies, excessive arterial inflow temperature was regarded as the main culprit for increased brain temperatures.
Our study demonstrated that JB temperature can exceed AI temperature. This finding is important as it indicates a clear potential to generate brain hyperthermia despite cautious rewarming strategies. The mechanism for this effect is unclear but possibly reflects active brain metabolism and heat generation during normothermic CPB. This finding was not observed in the deep hypothermic group possibly due to continued suppressed cerebral metabolism [23].
The duration of rewarming and control of thermal gradients may also be important. Slow rewarming to prevent rapid heterogeneous changes in brain temperature may be beneficial.
During warm CPB techniques, arterial inflow temperature should be carefully monitored and a nasopharyngeal temperature ceiling of somewhat less than 37 °C should be a prompt to discontinue rewarming prior to CPB separation if brain hyperthermia is to be avoided.
| Footnotes |
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| Appendix A. Conference discussion |
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Dr Kaukuntla: Well, because in the normothermic group, there was hardly any rewarming involved. The patients would have probably cooled to about 34.5° during the time the mammary artery was being harvested. So there was only a very short period of rewarming. Once the required temperature was attained, there was no definite relationship between the nasopharynx and the jugular bulb. This is a very small group to come to any conclusions of what is the statistical relationship between those two temperatures and a bigger study probably might answer that question.
Dr E. Joubert-Huebner (Hamburg, Germany): How did this jugular bulb temperature correspond to the arterial line temperature, the oxygenator?
Dr Kaukuntla: The temperature we monitored was arterial return temperature just after the oxygenator.
During the rewarming phase or during the cooling phase, the jugular bulb temperatures were a good reflection of the arterial inflow; but they always either lagged behind or lagged in front, if you see, depending upon where in the hypothermic circulatory curve the time point was.
What we found was that in the hypothermic group they tend to even out. But in the normothermic group, towards the end of bypass, the jugular bulb temperature was actually even more than the arterial inflow. This was a very surprising result for us. We did ponder on the reasons of why the jugular bulb temperature is more than the arterial inflow temperature, and we can only speculate the reasons. We think that one of the reasons is - in normothermia there is no heat debt, if I could use that term. So the brain venous outflow temperature is temperature of the blood going into the brain plus the brain metabolic heat which is generated in normothermia. So we think that probably is the reason.
A. Philipp (Regensburg, Germany): Why didn't you correlate the temperature of the jugular venous blood and the temperature detected with a probe in the typanum? Tympanic temperature is just easy to monitor, and, as you certainly know, it correlates very exactly with the brain temperature.
Dr Kaukuntla: Well, there is conflicting evidence. Some authors have, yes, as you've rightly said, say that it reflects the core brain temperature quite well. But at the same time, there are some authors who have said that tympanic membrane probably doesn't represent the true temperature at different parts of the brain. Besides this another reason was that when we set up this study, we didn't have the kit to measure tympanic membrane tempearture in the theaters.
Dr J. Wistbacka (Vaasa, Finland): Doesn't this study and the data from the literature point out that we should use rewarming water temperature at the maximum of 37° all the time? That will leave our patients a little bit hypothermic, but I think that's the price we have to pay in order to avoid the risks of neurologic sequelae.
Dr Kaukuntla: I entirely agree with you. Because we do not have a definite relationship between the temperature monitoring sites and the core brain temperature, we cannot say it is safe to heat up to this nasopharyngeal temperature or this bladder temperature or this arterial inflow. The safest way is, as you rightly said is to never allow any of the temperatures to go beyond 37, so the likelihood of the brain going much higher than 37.5 is rather negligible.
Dr A. Hassouna (Cairo, Egypt): I wanted to know when you have made your correlation between the arterial flow and jugular venous bulb temperatures?
In the graph, we see they cross each other at some points. Did you make a correlation over time or did you make a correlation at a certain time after bypass?
Dr Kaukuntla: No, the correlation was done at the point when the jugular bulb temperature reached its peak for each patient.
Dr Hassouna: Just peak correlation?
Dr Kaukuntla: Yes, peak correlation.
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