The concern for neurological out¬come following cardiac surgery is a major issue. The actual challenge is to monitor and pre¬vent deleterious factors from affecting brain function. Cardiopulmonary bypass (CPB) is known to cause neurological impairment and perfusion is one of the few modifiable risk factors. Cerebral oximetry assessed via near infrared spectroscopy (NIRS) has been proposed as a “standard of care” and the technique is now widely used. A major advantage of this technique is that it allows for noninvasive continuous monitoring of cerebral oxygen¬ation. Algorithms have been described to treat patients with sustained low levels of regional cerebral saturation (rSO2), i.e. patients who are considered to be at risk for impaired neurological outcomes. However, there is little, if any, scientific proof of NIRS’ reliability in car¬diac surgery or of clinical improvement following perioperative NIRS monitoring. Furthermore, the critical cerebral oximetry level and the duration of cerebral hypoxia that results in neurological damage are still unknown. We present our clinical case in which patient in spite of being moni¬tored by cerebral oximetry with near-infrared spectroscopy (NIRS) developed neurological complications. We used the Invos 5100C monitor( Somanetics, Covidien, Mansfield, USA) using SomaSensor disposable transducer capable of producing and detecting optical data from patient.
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A 52 year old (163 cm; 52 kg) male patient diagnosed with asymptomatic severe aortic stenosis (AS) (senile sclerodegenerative) in NYHA class II presented with history of syncope. The general examination revealed pulse rate of 98 per min, regular with blood pressure of 110/84 mm of Hg. On auscultation S2 was single with Ejection Systolic Murmur grade III/IV. Transthoracic echocardiography revealed severe calcific aortic valve (annulus 26mm) with peak systolic gradient of 72 mm of Hg and mean of 43 mm of Hg, mild aortic regurgitation, concentric Left ventricular hypertrophy with ejection fraction of 55% and mild mitral regurgitation. CT aortic angiography showed fusiform aneurysmal dilatation of the ascending aorta and arch of aorta upto the origin of left subclavian artery. In pre-anaesthetic assessment all laboratory investigations were normal, arterial pressure (BP) of 112/86 mmHg, and heart rate of 84 beats per minute. With no other comorbidity, he was accepted in ASA physical grade IV under GA for Bentall debranching procedure.
Preinduction monitoring included electrocardiogram, pulse oximetry, bispectral index and invasive blood pressure in right radial and left femoral artery. NIRS was used for cerebral oximetry. Two disposable SomaSensor transducers( Invos 5100C, Covidien, USA) capable of producing and detecting optical data from patient were applied to left and right side of patients forehead . Femoral vein and femoral artery were prepared for emergency femoro-femoral bypass. After induction of anaesthesia and endotracheal intubation TEE was performed using 2D TEE probe to confirm diagnosis and to detect any additional findings. After heparinisation cardiopulmonary bypass was commenced using high aortic and two stage right atrial venous cannulation. After replacing the Aortic valve and suturing the proximal end of the tube graft, cannulas were placed separately for right and left carotid artery for antegrade cerebral perfusion. The distal end of aortic arch and left subclavian artery were sutured under deep hypothermic circulatory arrest (DHCA) at 20°C. During DHCA the antegrade cerebral perfusion through right and left carotid artery was continued at rate of 800 – 1000ml/min. Total bypass time was 330 minutes, aortic cross clamp time of 243 minutes and DHCA time of 37 minutes. Monitored parameters (Invasive blood pressure IBP mean, cerebral near-infrared spectroscopy NIRS, temperature, Bispectral index BIS) in operation room and intensive care unit (ICU).
Patient developed right hemiparesis postoperative, confirmed with NCCT head/MRI which revealed Lt MCA territory stroke. He was managed conservatively and started recovering with supportive care.
Regional cerebral O2 saturation (rScO2) monitoring with near-infrared spectroscopy (NIRS) was possible when it was shown that light in the near-infrared spectrum penetrates the skull allowing measurement of brain oxy- and deoxyhemoglobin concentrations. rScO2 monitoring for the adequacy of cerebral oxygenation and perfusion during cardiac surgery soon became popular.
Self-adhesive pads applied to the skin of the forehead emit light in the near-infrared spectrum that is measured by sensors at 30 mm and 40 mm from the light emitting diode in the INVOS 5100C monitor. The distance of the sensor from the light source determines the spatial resolution of the emitted light.The light sensor nearer to the light source picks up infrared light reflected from bone and extracerebral tissue as compared to sensor which is away from light source which picks up infrared light from brain tissues. Using the modified Beer- Lambert law, NIRS provides the measurement of the oxygenated hemoglobin in relation to total hemoglobin concentration. Devices uses a proprietary algorithm for subtracting O2 saturation from superficial tissue (ie, bone and extra-cerebral tissue), from that obtained from deeper tissue to yield rScO2 value from the superficial frontal cortex. Because approximately 70% to 80% of cerebral blood is venous blood, rScO2 provides an indicator of the balance between regional O2 supply and demand. However, different individuals may have different percentages of venous blood in the frontal lobe, which explains why regional cerebral tissue oxygen saturation (rScO2) as measured using cerebral oximetry shows inter-individual variability. Since, the cerebral NIRS monitor based on continuous-wave technology is a trend monitor; change from baseline together with the clinical situation instead of the absolute value observed should be monitored.
The duration of rScO2 <55% during aortic arch surgery with selective antegrade cerebral perfusion has been associated with postoperative neurologic events. Patients with postoperative stroke are more likely to have had a reduction in rScO2 during surgery to 65% to 85% of baseline. Stroke after aortic arch surgery with deep hypothermic circulatory arrest is more common in patients with rScO2 <80% of baseline than in those without desaturation. Strokes were confirmed by CT brain imaging to have occurred in the hemisphere with the rScO2 desaturation. While a relationship between rScO2 reductions and these outcomes have been reported, the results have not been consistent. Сerebral oximetry based on NIRS technology has moderate sensitivity (60%) and low specificity (25%) in predicting clinically symptomatic cerebral ischemia in patients undergoing carotid endarterectomy. Another confounding factor in the interpretation of cerebral oxygenation may be the amount of extracerebral contamination of the NIRS signal. The measures from the INVOS cerebral oximeter are significantly modified by changes in extracranial blood flow and oxygenation and this interference may affect the reliability of monitoring in clinical practice. Moreover, there is no universal definition of what decrement in rScO2 from baseline constitutes an abnormal finding during cardiac surgery. Both relative decreases in rScO2 from baseline (eg, 20% decrease) and an absolute threshold rScO2 (eg, < 50%) have been used. A number of RCTs have considered rScO2 below 75% of the baseline value to represent cerebral tissue desaturation. Additional case reports have suggested that some patients with normal rScO2 values actually suffer from hypoperfusion while others with low rSO2 values are in normal states of cerebral perfusion. Overall, the sensitivity of NIRS measured cerebral tissue oxygenation for detecting intracranial and extracranial flow changes was 87.5% and and 0%, respectively. The specificity for these measurements were 100% and 0%, respectively.
It is important to note that cerebral oximetry is primarily applied to the upper forehead, unless the patient is bald, in which case the monitor can be applied on other parts of the head as well. Therefore, it is regarded as a regional monitor. However, evidence shows that cerebral oximetry can be sensitive to acute changes in blood flow in the middle cerebral artery when the monitor is applied to the upper forehead and the parietal regions of the head. This implies that cerebral oximetry primarily monitors the brain territory perfused by the anterior circulation or the internal carotid artery. A neonate who underwent arterial switch, suffered a wide ischemic stroke of the left parietal and occipital areas. But, NIRS values monitored during surgery (Casmed monitor) showed the left and right values were equivalent during the entire procedure. NIRS samples only a limited anterior area of the brain, the maximal depth of which is about 30 to 40 millimeter. This observation confirms a limitation of NIRS for the diagnosis of cerebral ischemia. Moreover, hemodilution, transfusion, hypocapnia and hypercapnia also leads to significant variations in cerebral oximetry. Therefore, the meaning of the variation induced by the initiation of cardiopulmonary bypass with blood-free prime in patients is multifactorial. On the one hand, the increase in arterial saturation is likely to increase cerebral rSO2; on the other hand, hemodilution and a decrease in PaCO2 are likely to decrease cerebral rSO2. High NIRS cerebral oximetry values have been shown during selective cerebral perfusion, the significance of which is still debated. It would be important to be certain, in such instances, that the high NIRS value is not a drawback of selective cerebral perfusion. Conversely, a low rSO2 value is easier to analyse. NIRS values close to 30% at normothermia are considered the threshold for decreased cerebral energy associated with serious clinical consequences. However, the duration of cerebral hypoxia is also a significant risk factor and neurological injury may occur for rSO2 values above 30%. However, these data are questionable because there is some evidence that NIRS monitoring is less accurate at the upper and lower values of the spectrum.
Several authors reported that rScO2 monitoring was useful for adjusting or confirming the correct position of selective cerebral perfusion catheters during surgery. Others reported that monitoring rScO2 was beneficial for supporting decisions to switch from unilateral to bilateral antegrade cerebral perfusion in patients undergoing aortic arch surgery. These data, though, are mostly anecdotal. Data on the specificity of rScO2 monitoring for ensuring cerebral perfusion are currently not available. That is, there is little evidence on whether the absence of acute reductions in rScO2 ensures adequate cerebral blood flow.
In conclusion, many reports in the literature suggest that reductions in rScO2 during cardiac surgery may provide an indication for mechanical mishaps related to CPB cannulas, particularly during aortic surgery, although most are ancedotal. The level of evidence linking decreased rScO2 during cardiac surgery to postoperative neurologic complications is low. Further, our case shows rScO2 desaturation does not predict posterior circulation stroke.
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