| Abstract|| |
A patent ductus arteriosus (PDA) is often present in patients undergoing correction of congenital heart disease. It is well appreciated that during cardiopulmonary bypass (CPB), a PDA steals arterial inflow into pulmonary circulation, and may lead to systemic hypoperfusion, excessive pulmonary blood flow (PBF) and distention of the left heart. Therefore, PDA is preferably ligated before initiation of CPB. We describe acute decreases of arterial blood pressure and entropy score with the initiation of CPB and immediate increase in entropy score following the PDA ligation in a child undergoing intracardiac repair of ventricular septal defect and right ventricular infundibular stenosis. The observation strongly indicates that a PDA steals arterial inflow into pulmonary circulation and if the PDA is dissected and ligated on CPB or its ligation on CPB is delayed the cerebral perfusion is potentially compromised.
Keywords: Cardiopulmonary bypass, cerebral hypoperfusion, patent ductus arteriosus
|How to cite this article:|
Neema PK, Dharan BS, Singha SK, Sethuraman M, Rathod RC. Entropy score, patent ductus arteriosus (PDA), and cardiopulmonary bypass (CPB): Ligation of PDA on CPB can compromise cerebral blood flow. Ann Card Anaesth 2011;14:203-5
|How to cite this URL:|
Neema PK, Dharan BS, Singha SK, Sethuraman M, Rathod RC. Entropy score, patent ductus arteriosus (PDA), and cardiopulmonary bypass (CPB): Ligation of PDA on CPB can compromise cerebral blood flow. Ann Card Anaesth [serial online] 2011 [cited 2020 Jul 9];14:203-5. Available from: http://www.annals.in/text.asp?2011/14/3/203/84017
| Introduction|| |
The entropy  and Bispectral index (BIS) , are used to monitor sedation and the depth of anesthesia. Both the entropy and BIS-monitor process electroencephalographic signals and employ algorithms to generate the entropy / BIS score. Generally, entropy has been studied for quantification of the anesthetic drug effect for various GABA-ergic intravenous induction agents like propofol and volatile anesthetic agents like sevoflurane and overall it has been found to be comparable to the current clinical gold standard BIS.  The entropy monitor has two outputs: response entropy (RE) which uses a higher frequency range that includes electromyography (EMG) signals and provides a fast response time; and state entropy (SE) which uses lower frequencies providing a more stable value but with a slower response time. RE and SE are comparable with the BIS, but show significantly less interference from frontal-EMG and superior resistance against artifacts. Spectral entropy is considered more suitable than the BIS during propofol-remifentanil anesthesia, in cardiac surgery patients.  We describe a decrease in entropy score with the initiation of cardiopulmonary bypass (CPB) and increase in entropy score following patent ductus arteriosus (PDA) ligation in a child undergoing intracardiac repair (ICR) of a ventricular septal defect (VSD) and right ventricular (RV) infundibular stenosis.
| Case Report|| |
The patient, a four-year-old child, weighing 8 kg, underwent PDA ligation and ICR for VSD and RV infundibular stenosis. On examination, the heart rate, arterial blood pressure (ABP), and peripheral saturation (SpO 2 ) were 100 per minute, 100 / 50 mmHg, and 95%, respectivley. Preoperative transthoracic echocardiography showed situs solitus, levocardia, normally related great vessels, an 8 mm VSD, and RV infundibular stenosis. The peak RV systolic pressure was 35 mmHg plus mean right atrial pressure, the left ventricular (LV) internal systolic and diastolic dimensions were 21 / 11 mm, the left atrium / aorta (LA / Ao) dimensions were 18 / 14 mm, the LV ejection fraction was 70%, the RV function was reported as good, and both the pulmonary and tricuspid valves were moderately regurgitant. The laboratory investigation results were normal; hemoglobin (Hb) was 11 gm / dl.
In the operating room, anesthesia was induced with sevoflurane 8% in O 2 , fentanyl 20 μg, and pancuronium 1 mg; the anesthesia was maintained with isoflurane 1 - 3% in O 2 , morphine infusion 40 μg/kg/h, and intermittent pancuronium. The patient was monitored with ABP, SpO 2 , electrocardiogram, central venous pressure, end-tidal carbon-dioxide (EtCO 2 ), transesophageal echocardiography, nasopharyngeal temperature, urine output, and entropy (GE HealthCare; Model No D-FPD-15-00; F-CU8-12-VG1, Helsinki, Finland). The SE score during the prebypass period ranged between 60 and 70. The PDA was dissected before initiation of CPB, but not ligated because of low SpO 2 (72 - 80%). The ABP before initiation of CPB showed 80 / 46 mmHg. Red blood cells (RBC) were added to the CPB prime, to achieve a circulating HB of about 10 gm/dl. The patient was anticoagulated with heparin 300 IU / kg, and after achieving an ACT of > 400 seconds, aortobicaval cannulation initiated CPB. With the initiation of CPB, the mean ABP decreased to 28 mmHg and gradually, over one minute, the SE score decreased to 45. After ligation of the PDA, the mean ABP increased to 36 mmHg and the SE score abruptly increased to 87; these changes in the SE score occurred over three to four minutes [Figure 1]. During this period, mechanical ventilation was continuing and the inspiratory isoflurane was 1.5%. Additional fentanyl 20 μg was administered to deepen the anesthesia. Once the PDA was ligated and total CPB was established, mechanical ventilation was discontinued and anesthesia was maintained by sevoflurane added to the CPB gas inlet. The ABG analysis during CPB showed Hb of 9.4 gm / dl and hematocrit of 29%. The patient was cooled to 28 o C and under an aortic clamp and cold blood cardioplegic myocardial protection, the VSD was closed with a Gore-Tex patch, and the RV infundibulum was resected through the right atrium. After surgical repair, the patient was warmed to 37 o C nasopharyngeal temperature. The patient was then separated from the CPB, with the support of epinephrine infusion 0.05 μg/kg/minute. The remaining post bypass and postoperative periods were uneventful.
|Figure 1: First arrow from left shows decrease in entropy score at the initiation of cardiopulmonary bypass; second arrow from left shows increase in entropy with the ligation of patent ductus arteriosus. The time line indicates a period of about three to four minutes|
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| Discussion|| |
A PDA is often present in patients undergoing correction of congenital heart disease. Pulmonary circulation is a low-pressure circulation; therefore, during CPB, if a PDA, Blalock-Tausig shunt or aortopulmonary collaterals are present, a significant proportion of the arterial inflow is stolen in the pulmonary circulation and may lead to excessive pulmonary blood flow (PBF) and distention of the left heart. Therefore, the PDA is preferably ligated before initiation of CPB. However, in patients where PBF is PDA-dependent, it is ligated only after initiation of CPB. The proportion of the stolen arterial inflow depends on the relative resistance of the systemic and pulmonary vascular circuits and blood viscosity (extent of hemodilution). The arterial inflow steal can result in cerebral hypoperfusion and acute neurological insult. Here it is important to realize that the initiation of CPB diverts most of the venous blood to the reservoir,which reduces RV ejections and pulmonary arterial pressure to very low levels, which, in turn, leads to a greater steal as compared to when the RV is full and ejecting.
Several case reports indicate that intraoperative events unrelated to the titration of anesthetic agents can produce rapid changes in BIS scores (e.g., cerebral ischemia or hypoperfusion). , A decrease in the cerebral perfusion (hypotension, cardiac arrest, or increased ICP) is reflected as a fall in the BIS, with a delay of approximately 80 seconds. This may be caused by the 30 to 60 second delay inherent in the calculation time of the BIS monitor and the lag time between decreases in the cerebral blood flow and neuronal dysfunction.  In the present patient, the SE score and ABP decreased with the initiation of CPB and increased after the ligation of PDA. A decrease in SE score could be due to an increase in the depth of anesthesia, acute hemodilution or acute decrease in cerebral perfusion. An increase in the anesthetic depth was unlikely as isoflurane concentration was not changed and no other anesthetic agent was added to the CPB prime. Similarly, acute hemodilution was unlikely as RBCs were added to the CPB prime, and the Hb during CPB was 9.4 gm / dl. The time line of changes in the SE score lasted for about three to four minutes and were related to the initiation of CPB and ligation of the PDA. Earlier, Rodriguez et al., had described an increase in the cerebral blood flow velocity by more than 40% in children less than 15 months old, on surgical or coil occlusion of PDA.  Increase in the BIS score with an increase in cerebral perfusion was reported earlier. 
The close relationship between changes in entropy and ABP, with initiation of CPB, and ligation of PDA indicate that the decrease in entropy was secondary to a decrease of cerebral perfusion, due to arterial inflow steal in the pulmonary circulation; and the increase in entropy after PDA ligation was secondary to the increase / restoration of cerebral perfusion. An increase in the entropy score and increase in ABP after ligation of PDA further supports the well-appreciated belief that during CPB, a PDA steals the cerebral blood flow and its ligation increases the cerebral perfusion. However, one must note that BIS and entropy are not the proven surrogate markers of cerebral perfusion. This case report further indicates that almost simultaneous decreases in ABP and entropy, strongly indicate an acute decrease in cerebral perfusion and warrant its restoration. The measures to increase cerebral perfusion in this situation include increases in the arterial inflow and arterial PCO 2 ; the LV distention due to increased pulmonary venous return is prevented by the venting of LV. An increase in entropy following an increase in arterial inflow will confirm the favorable effect of an increased inflow. If we had used the transcranial Doppler, it would have recorded an increase in cerebral blood flow velocity and would have confirmed the changes in cerebral perfusion that occur with PDA closure on CPB and this belief.
The practice of PDA dissection and ligation vary among surgeons; some dissect and ligate it before CPB, some others dissect it before CPB and ligate it immediately after initiation of CPB, and a few dissect and ligate it on CPB. The reasons of its ligation on CPB include desaturation before initiation of CPB, ductal dependent PBF, and the fear of ductal injury during dissection, particularly if the ductus is large and hypertensive.
We decided not to ligate the PDA before initiation of CPB because of severe desaturation; however, it now appears that it would have been prudent if the PDA was ligated before CPB, despite the desaturation.The observation strongly indicated that if the PDA was dissected and ligated on CPB or its ligation on CPB was delayed, cerebral perfusion could be compromised and might result in an acute neurological insult.
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Praveen Kumar Neema
B-9, NFH, Sree Chitra Residential Quarters, Poonthi Road, Kumarpuram, Trivandrum - 695 011, Kerala
Source of Support: None, Conflict of Interest: None