The incidence of postoperative cognitive dysfunction (POCD) is about 50% at discharge, 36% at 6 weeks, and 26%-33% at 1 year.^1,2 The etiology of cerebral complications including POCD is multifactorial and various preoperative as well as intra operative factors are associated with development of cognitive dysfunction.^3,4

The most widely accepted mechanisms for cognitive dysfunction are intraoperative hypotension and multiple microembolization. Both these conditions or events lead to cerebral tissue hypoxia. . During OPCAB, cardiac manipulations are needed to facilitate optimum exposure for grafting of each of the 3 main coronary arteries. Exposure, positioning and stabilization of each coronary target are associated with temporary significant hemodynamic changes.⁵ Though it remains uncertain to what extent these might affect cerebral cortical oxygenation.

This leads us to improve neuromonitoring during cardiac surgeries. Studies show that intra operative neuro monitoring helps reduce incidence of POCD, as well as length of hospital stay and cost.^6,7 Studies with Near Infra Red Spectroscopy (NIRS) for Cerebral Oximetry have shown a significant relationship between low cerebral regional oxygen saturation (rSO₂) values and neurologic complications as well as cognitive dysfunction. ^6,8,9 NIRS is a system used for monitoring cerebral activity. It is based on the absorption of infrared light by biological tissues, similar to arterial saturometry. NIRS provides for continuous non invasive measurement of regional cerebral oxygen saturation (rSO₂). Monitoring of rSO₂ helps in early detection of cerebral desaturation and performance of any interventions to prevent prolonged rSO₂ desaturation thus protecting against neurological complications including cognitive dysfunction. The technique of NIRS relies on the application of modified Lambert-Beer law for the calculation of changes in the concentration of tissue chromophores from measured variations in attenuation of light. The light-emitting diode emits near infrared light of two different wavelengths to measure the ratio of oxyhaemoglobin to total Hb. The sensor is composed of one light-emitting diode and two detectors located 30 and 40 mm away from the diode. Two probes attached to the forehead contain the light sources, each providing 2 continuous wavelengths of near-infrared light (730 and 810 nm) that reach the brain area corresponding to the junction between the anterior and middle cerebral artery vascularization territory. The probe is positioned over the right frontal area of the brain to monitor regional cerebral oxygenation.The Cognistat is a ‘screening tool’ meant to alert regarding issues of potential cognitive deficits and to evaluate it further with the use of more sophisticated neuropsychological assessment techniques. The Cognistat can assess the individual’s functioning in 7 basic ability areas.

AIM AND OBJECTIVES

Aim:

The aim of the study was to correlate the values of cerebral oximetry rSO₂ with postoperative cognitive dysfunction using NIRS in patients undergoing OPCAB.

Primary objective:

To compare the incidence of POCD between NIRS group and Control group in patients undergoing OPCAB.

The incidence of postoperative cognitive dysfunction (POCD) is about 50% at discharge, 36% at 6 weeks, and 26%-33% at 1 year.1,2 The etiology of cerebral complications including POCD is multifactorial and various preoperative as well as intra operative factors are associated with development of cognitive dysfunction.3,4 The most widely accepted mechanisms for cognitive dysfunction are intraoperative hypotension and multiple microembolization. Both these conditions or events lead to cerebral tissue hypoxia. . During OPCAB, cardiac manipulations are needed to facilitate optimum exposure for grafting of each of the 3 main coronary arteries. Exposure, positioning and stabilization of each coronary target are associated with temporary significant hemodynamic changes.5 Though it remains uncertain to what extent these might affect cerebral cortical oxygenation. This leads us to improve neuromonitoring during cardiac surgeries. Studies show that intra operative neuro monitoring helps reduce incidence of POCD, as well as length of hospital stay and cost.6,7 Studies with Near Infra Red Spectroscopy (NIRS) for Cerebral Oximetry have shown a significant relationship between low cerebral regional oxygen saturation (rSO₂) values and neurologic complications as well as cognitive dysfunction. 6,8,9 NIRS is a system used for monitoring cerebral activity. It is based on the absorption of infrared light by biological tissues, similar to arterial saturometry. NIRS provides for continuous non invasive measurement of regional cerebral oxygen saturation (rSO₂). Monitoring of rSO₂ helps in early detection of cerebral desaturation and performance of any interventions to prevent prolonged rSO₂ desaturation thus protecting against neurological complications including cognitive dysfunction. The technique of NIRS relies on the application of modified Lambert-Beer law for the calculation of changes in the concentration of tissue chromophores from measured variations in attenuation of light. The light-emitting diode emits near infrared light of two different wavelengths to measure the ratio of oxyhaemoglobin to total Hb. The sensor is composed of one light-emitting diode and two detectors located 30 and 40 mm away from the diode. Two probes attached to the forehead contain the light sources, each providing 2 continuous wavelengths of near-infrared light (730 and 810 nm) that reach the brain area corresponding to the junction between the anterior and middle cerebral artery vascularization territory. The probe is positioned over the right frontal area of the brain to monitor regional cerebral oxygenation.The Cognistat is a ‘screening tool’ meant to alert regarding issues of potential cognitive deficits and to evaluate it further with the use of more sophisticated neuropsychological assessment techniques. The Cognistat can assess the individual’s functioning in 7 basic ability areas. AIM AND OBJECTIVES Aim: The aim of the study was to correlate the values of cerebral oximetry rSO2 with postoperative cognitive dysfunction using NIRS in patients undergoing OPCAB. Primary objective: To compare the incidence of POCD between NIRS group and Control group in patients undergoing OPCAB.

The following are the results of the study:

Table 1 Comparison of pre operative and post operative Cognistat score and the Lowest recorded MAP during surgery among patients

Table1 shows the Pre operative and 7th Post operative day Cognistat score among the two groups. It can be seen that the pre operative cognitive score was comparable in both groups, but the post operative cognitive score falls in both the groups. This fall in cognitive score was significantly higher in the controls group (p=0.006)

 Table 2 Comparison of rSO₂ values at various intervals during surgery among patients of NIRS group

Table 2 shows the trend of cerebral oxygen saturation (rSO₂) at different intra operative stages. It shows that the rSO₂ falls maximum during LAD grafting as compared to other grafts and this fall is highly significant as p<0.001

Thus to summarize, the above tables show the following findings:

• The patients were comparable in terms of preoperative cognitive score
• The post operative cognistat score fell in both the groups as compared to pre operative cognistat score but this fall in cognitive score was statistically significant (p=0.006) in patients of control group as compared to those in NIRS group
• The fall in rSO2 was significantly (p<0.001) more during LAD grafting as compared to other stages of the surgery.

Apart from these, the patients in both groups were comparable in terms of age, sex, co morbidities, ASA status and Ejection Fraction.

There were no significant differences between the two groups in other measured variables like Heart Rate, MAP, Haemoglobin, Cardiac Output, Cardiac Index, sPO2, Temperature.

There were no incidences of post operative Stroke, seizures or Transient Ischaemic Attack (TIA)/ Reversible Ischaemic Neurological Deficit (RIND) in either groups.

There was no statistical difference between the two groups in terms of ICU length of stay and hospital length of stay

In this study we found that using NIRS for monitoring intraoperative cerebral regional saturation and interventions done to maintain it at optimal levels leads to a better neurological outcome and decreased incidence of Post Operative Cognitive Dysfunction.

However, this being a study on a smaller group of patients, we recommend that a larger multicentric randomized controlled blinded study is necessary for establishing the role of NIRS monitoring in prevention of Post Operative Cognitive Dysfunction.

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