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Monitoring of selective antegrade cerebral perfusion using near infrared spectroscopy in neonatal aortic arch surgery

Hofer, A.*; Haizinger, B.*; Geiselseder, G.*; Mair, R.; Rehak, P.; Gombotz, H.*

European Journal of Anaesthesiology: April 2005 - Volume 22 - Issue 4 - p 293–298
doi: 10.1017/S0265021505000499
Original Article
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SDC

Background and objective: To prevent neurological complications, low-flow antegrade cerebral perfusion (ACP) is used during repair of complex congenital heart defects. To overcome technical problems, continuous monitoring of cerebral blood flow and oxygenation is mandatory. The aim of the study was to evaluate the effect of different ACP flow rates on cerebral oxygen saturation obtained by near infrared spectroscopy.

Methods: Ten consecutive neonates undergoing Norwood stage I were included. In addition to near infrared spectroscopy (Invos 5100; Somanetics Corp., USA) on both hemispheres, mean arterial pressure and transcranial Doppler flow velocity were measured continuously and arterial and jugular venous oxygen saturation intermittently. Cerebral oxygen extraction ratio was calculated. Measurement points were obtained after starting bypass, during ACP with flow rates of 30, 20 and 10 mL kg−1 min−1 and immediately after ACP. ANOVA and Tukey-Kramer multiple comparison test were used for statistics.

Results: The near infrared spectroscopy signal could be obtained in all children at all measurement points, whereas transcranial Doppler failed in 1 neonate at a flow rate of 30 mL kg−1 min−1, in 3 neonates at 20 mL kg−1 min−1 and in 4 neonates at 10 mL kg−1 min−1. With the reduction of flow there was a significant decrease of cerebral oxygen saturation on both hemispheres (right: 78 ± 8 to 72 ± 9 and 66 ± 8, P < 0.001; left: 71 ± 7 to 65 ± 7 and 60 ± 7, P < 0.001), of jugular venous oxygen saturation (94 ± 6 to 89 ± 13 and 83 ± 15, P < 0.001) and a significant increase in oxygen extraction ratio (9.1 ± 8 to 14.8 ± 14 and 21 ± 16, P < 0.001) respectively, for 30, 20, 10 mL kg−1 min−1.

Conclusion: Near infrared spectroscopy reliably detects flow alterations during ACP with profound hypothermia.

*General Hospital Linz, Department of Anaesthesiology and Intensive Care, Ludwig Boltzmann Institute, Linz, Austria

General Hospital Linz, Department of Thoracic and Cardiovascular Surgery, Linz, Austria

University of Graz, Department of Surgery, Graz, Austria

Correspondence to: Anna Hofer, Department of Anaesthesiology and Intensive Care, General Hospital Linz, Krankenhausstrasse 9, A-4020 Linz, Austria. E-mail: anna.hofer@akh.linz.at; Tel: +43 732 7806 73 565; Fax: +43 732 7806 2154

Accepted for publication January 2005

Deep hypothermic cardiac arrest has been performed in neonates with complex congenital heart defects during neonatal cardiac surgery for many years. Due to the increasing risk of cerebral damage with the duration of cardiac arrest the method of antegrade cerebral perfusion (ACP) has been developed to provide both a bloodless surgical field free of cannulas during aortic arch reconstruction and adequate blood flow to the brain [1]. Technical problems may arise because of the small size of cannulas used in these neonates due to obstruction or kinking of the blood vessels [2]. Therefore continuous monitoring of cerebral blood flow and oxygenation is mandatory. The aim of our study was to evaluate the effect of different ACP flow rates on cerebral oxygen saturation obtained by near infrared spectroscopy.

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Patients and methods

After approval by the local Ethics Committee and parents' written informed consent 12 consecutive neonates scheduled for open heart surgery were included in a prospective study. The underlying congenital heart defects were hypoplastic left heart syndrome (n = 10) or tricuspid atresia (n = 2).

Induction of anaesthesia was performed with midazolam 0.1-0.3 mg kg−1, thiopental 5 mg kg−1 and pancuronium 0.1-0.2 mg kg−1. To maintain anaesthesia sufentanil was administered according to the children's needs together with additional doses of midazolam. An arterial cannula was inserted into the right or left radial artery, a central venous line into the right or left femoral vein if needed because of lacking peripheral venous access.

Arterial cannulation included cannulation of the ductus arteriosus and a Goretex shunt sewed on the brachiocephalic artery. Therefore arterial perfusion during cooling was applied via a y-piece. After starting extracorporeal circulation the neonates were cooled to a core temperature of 20°C with a pump flow rate of 2.5L min−1 m−2 using pH-stat acid-base management. After reaching the desired core temperature of 20°C blood gas management was performed using α-stat, the cannula of the ductus arteriosus was removed and the aortic arch vessels were occluded with tourniquets, so that cerebral perfusion could now be performed through the Goretex shunt at a reduced flow rate of 30 mL kg−1 min−1. For study purpose to simulate ACP problems the flow was reduced to 20 mL kg−1 min−1 for a period of 2 min and to 10 mL kg−1 min−1 for a period of 2 min. After having finished study-related measurements ACP was continued with a flow rate of 30 mL kg−1 min−1 during reconstruction of the aortic arch. After a brief period of circulatory arrest for atrioseptectomy (mean duration 3.2 ± 0.56 min, range 0-6 min) full-flow cardiopulmonary bypass (CPB) was reinstituted, neonates were rewarmed and weaned from CPB at a core temperature of 36°C. Standard monitoring during extracorporeal circulation included electrocardiography (ECG), mean arterial pressure (radial artery), central venous pressure, transcutaneous oxygen saturation, and oesophageal and rectal temperatures.

For study reasons near infrared spectroscopy and transcranial Doppler flow velocity were recorded continuously and arterial and jugular bulb saturation were measured intermittently.

Two neonatal near infrared spectroscopy sensors (Invos 5100; Somanetics Corp., USA) were fixed to the head over the frontoparietal region to measure cerebral oxygen saturation on both hemispheres. As there was concern from the manufacturer that the adhesive could irritate the babies' skin, the device was covered with soft material and the sensors were fixed with a bandage. In addition the sensors were protected from ambient light.

Transcranial Doppler flow velocity (MultidopT; DWL, Germany) of the left middle cerebral artery was recorded through the temporal window in a depth of 24-28 mm after having obtained the inverse signal of the anterior cerebral artery. The surgeon advanced a catheter for jugular venous blood samples. Arterial and jugular venous oxygen saturations were measured at 37°C (ABL 700; Radiometer, Copenhagen, Denmark). Cerebral oxygen extraction ratio was calculated using a standard formula [3].

Measurements were performed after starting bypass (CPB-1), during ACP with flow rates of 30 mL kg−1 min−1 (ACP-30), 20 mL kg−1 min−1 (ACP-20), 10 mL kg−1 min−1 (ACP-10) and after the end of ACP (CPB-2). Blood samples were drawn 2 min after changing flow rates. Two-way ANOVA and Tukey-Kramer multiple comparison test were used for statistical analysis. P < 0.05 was considered statistically significant.

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Results

Out of 12 patients, 2 had to be excluded from the study. In the first one no near infrared spectroscopy signal could be obtained due to excessive growth of hair on the forehead. In the second child carotid artery flow could not be measured and due to surgical problems a second period of ACP was necessary resulting in a total duration of 153 min. In the remaining 10 children the measurements could be performed without any adverse events. Patient characteristics data and duration of antegrade perfusion as well as duration of extracorporeal circulation are shown in Table 1.

Table 1

Table 1

The near infrared spectroscopy signal was consistently obtained in the remaining 10 children whereas a transcranial Doppler signal could not be detected in 2 neonates at a flow rate of 20 mL kg−1 min−1 and in 3 neonates at a rate of 10 mL kg−1 min−1. In one additional child no Doppler signal could be detected at all flow rates.

Mean arterial pressure as well as transcranial Doppler flow decreased with the reduction of flow rates and increased after restoration of initial flow rates. There was a wide inter-individual variation of pre- and post-bypass cerebral oxygen saturation values (range - pre-bypass: right 37-63, left 36-66; post-bypass: right 41-72, left 45-72). Cerebral oxygen saturation values of both hemispheres declined with reduced flow rates and returned to baseline values after increasing flow to the initial amount (Fig. 1). The absolute values of regional cerebral oxygen saturation on the left side were slightly but not statistically significantly lower than on the right side (Tukey-Kramer test). Oxygen saturation of the arterial blood remained unchanged, whereas jugular bulb saturation decreased parallel to the reduction of cerebral blood flow with a concomitant increase of calculated cerebral oxygen extraction ratio (Table 2).

Figure 1.

Figure 1.

Table 2

Table 2

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Discussion

Brain oxygenation decreases significantly during circulatory arrest and remains impaired after rewarming and after termination of CPB despite normalization of oxygen availability [4]. In contrast to experimental low-flow CPB, cerebral hyperaemia was found during rewarming followed by a rise in cerebrovascular resistance lasting for more than 8 h after hypothermic cardiac arrest [5]. In children undergoing repair of congenital heart defects, electroencephalogram (EEG) alterations but also clinical seizures have been shown to be more likely to occur in patients after hypothermic cardiac arrest than with low-flow CPB. Furthermore, release of creatine kinase brain isoenzyme as a marker of cell damage was higher in hypothermic cardiac arrest children than with low-flow CPB. Only in children with a hypothermic cardiac arrest duration <35 min at 18°C were neither EEG alterations nor clinical seizures found [6]. In addition, hypothermic cardiac arrest has been associated with a higher risk of delayed motor development and neurological abnormalities at the age of 1 yr when compared to surgery with low-flow bypass [7]. Clancy and colleagues reported a 19% incidence of adverse neurological events with genetic conditions (dysmorphy, chromosomal aberrations), aortic arch obstruction and deep hypothermic cardiac arrest time >60 min as the major contributing factors [8]. The use of allopurinol, a scavenger and inhibitor of oxygen free radicals, resulted in a better outcome in children with hypoplastic left heart syndrome [9].

To try and prevent the above, selective ACP has been established in adult aortic arch surgery and with adequate cerebral perfusion ACP significantly reduced neurological dysfunction and in-hospital mortality [10,11]. For paediatric cardiac surgery with reconstruction of the aortic arch various methods of cerebral protection have been discussed including pH-strategy before hypothermic cardiac arrest to provide a more uniform cooling of the brain [2,12]. However, because of continuing concerns regarding the adverse effects of deep hypothermic circulatory arrest on the neonatal brain, a technique of regional low-flow perfusion that provides cerebral circulation has been introduced by Pigula and colleagues [1].

As interventions based on neurophysiological monitoring may decrease the incidence of postoperative neurological sequelae, monitoring of cerebral perfusion and oxygenation during ACP by using a combination of different techniques is mandatory and must be primarily directed to assure that oxygen delivery to the brain is adequate. This is of special importance in neonates where accidental interruption of cerebral perfusion may occur simply due to technical problems [13].

In paediatric cardiac surgery routine neurophysiological monitoring mainly consists of EEG, transcranial Doppler and near infrared spectroscopy, and has been shown to improve neurological outcome if an intervention is made following a deviation from defined target values [14]. As EEG monitoring for children during deep hypothermia is not feasible, transcranial Doppler and near infrared spectroscopy are methods of choice for monitoring ACP [15].

Changes in cerebral blood flow velocity correlate well with changes in cerebral blood flow [16-18]. However, comparable to our results, in the presence of low-pump flow cerebral perfusion may not be detectable any more because of limited threshold resolution of the Doppler device [19,20]. In neonates undergoing Switch procedures a minimum bypass flow rate of 30 mL kg−1 min−1 was needed to detect cerebral perfusion [21].

Under the condition of sufficient arterial oxygen supply and stable anaesthetic level jugular bulb saturation is a function of cerebral metabolic rate and cerebral blood flow during CPB [22]. As there is a strong correlation between cerebral oxygen extraction and jugular venous saturation, the latter provides an online monitor for cerebral cooling [23]. However, it is important to recognize that the oxygen content and saturation of the jugular venous bulb is the average of many regions of the brain and may not reflect areas of regional hypoperfusion.

Near infrared spectroscopy monitoring is not dependent on a pulsatile signal. Therefore it can be used during extracorporeal circulation even with non-pulsatile flow and provides the only means of cerebral oxygen monitoring during profound hypothermic arrest, especially when cytochrome aa3 is monitored [24]. Recent publications, however, have revealed that the cytochrome signal may depend on total haemoglobin concentration because the absorption spectra of haemoglobin and cytochrome aa3 overlap and the absorption caused by haemoglobin is much greater than that caused by cytochrome aa3 [25]. Near infrared spectroscopy gives reliable results with hypothermia, whereby trend values are considered rather than absolute values [26]. Near infrared spectroscopy monitoring is also very helpful if the insertion of a right radial arterial line for pressure monitoring is impossible. Van Haaren and colleagues showed that accidental interruption of cerebral venous outflow could be detected by near infrared spectroscopy [13]. The difference in absolute values between the two hemispheres may be caused by perfusion being performed through the right innominate artery. However, as the circle of Willis is assumed to be intact in newborns, no long-term consequences are expected. In the postoperative period no adverse reactions caused by hypoperfusion of the left hemisphere were found in our patients. This may be due to the fact that oxygen saturation on the left side has not been lower than the pre-bypass values in any case. As a possible cause of the lower left-sided oxygen saturation, Androupulos and colleagues mentioned in their investigation that surgical practice included retraction of the left innominate vein and potentially venous stasis [27].

Due to the reduced oxygen demand cerebrovascular saturation increases during progressive cooling, but decreases during cardiac arrest. The critical level of oxygenation is unknown, but it is recommended to keep residual saturation well above 30% during hypothermic cardiac arrest at 18°C to prolong neuronal saturation [28,29].

Depending on temperature 20-30 mL kg−1 min−1 of antegrade flow with a right arterial pressure of 22 mmHg is required to reach baseline cerebral oxygen saturation during full-flow CPB cooling [30,31]. In an experimental study the cerebral metabolic rate for oxygen decreased at a flow rate of 2.5 mL kg−1 min−1, which is far below the lowest flow rate in this study. Nevertheless oxygen extraction ratio increased significantly in our patients with flow rates of 10 and 20 mL kg−1 min−1. In dogs cerebral metabolic ratio of glucose to oxygen and the cerebral vascular resistance were lowest when perfusion pressure was 10-30 mmHg. Full-flow (100 mL kg−1 min−1) perfusion caused paradoxical brain acidosis; a flow of 40 mL kg−1 min−1 provided the best results [32].

The period during which we looked closely at the near infrared spectroscopy signal was the period of hypothermic ACP at a core temperature of 20°C and with a constant arterial oxygen saturation of 99%. The statistical analysis of data obtained with three different flow rates showed a rapid and significant drop in cerebral oxygen saturation with reduced flow. There was also a significant drop in jugular venous saturation suggesting ongoing cerebral metabolism during hypothermia [33]. Monitoring cerebral oxygen saturation as a less invasive and continuous method may help to determine the appropriate perfusion rate to meet the ongoing cerebral metabolic demands. There is a difference between individuals in cerebral oxygen saturation values because of the different arterial and venous contribution to cerebral oxygen saturation, which may be due to anatomical variations of arterioles, capillaries and venules.

The limitations of this study are the absence of blinding, the relatively small sample size and the absence of a control group. For ethical reasons a control group of children with deep hypothermic circulatory arrest was not feasible. The consistency among the patients suggests that the results of the study reflect the benefit of near infrared spectroscopy as part of multimodal cerebral monitoring equipment. However with the device used only cerebral oxygen saturation and delivery can be measured. For the determination of the actual tissue uptake of oxygen, cytochrome aa3 should be used. Nevertheless the present analysis also confirms at least partially previous studies and extends the evidence of the benefit of near infrared monitoring in children. On the other hand, the device is not certified for very small children because of the adhesive which may irritate the neonate's skin. Only small areas of the brain underneath the sensors are detected and some deeper areas are excluded from monitoring, though the near infrared light penetration in small children is much better than in adults. As there is no critical cut-off known, only changes of oxygen saturation can be used to detect improper cannulation or compromised cerebral perfusion.

In conclusion near infrared spectroscopy provides a reliable non-invasive monitoring device for the detection of changes in cerebral blood flow in neonates during ACP.

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Keywords:

BRAIN PERFUSION; near infrared spectroscopy; transcranial doppler; BYPASS TECHNIQUE; antegrade cerebral perfusion; CONGENITAL CARDIAC SURGERY; aortic arch reconstruction

© 2005 European Society of Anaesthesiology