USE OF NIRS FOR EARLY DIAGNOSIS OF BT SHUNT UNDERFLOW

20 Jan 2018

Near-infrared spectroscopy (NIRS) is a non invasive device used to monitor regional blood flow in cerebral region which is based on balance between cerebral oxygen delivery and consumption. Apart from intra operative monitoring during adult and pediatric cardiac surgery, this monitor is now being used in diverse clinical fields including neurosurgery and vascular surgery. The use of NIRS monitoring during immediate post operative care of cardiac surgery patients is not explored well. We recently used NIRS monitoring during post operative period of Blalock Taussig shunt.

A newborn baby with pulmonary atresia underwent palliative modified Blalock Taussig on day 3 of life. Cerebral rSO₂ was monitored using NIRS (SenSmart, NONIN).  Baseline SpO₂ recorded in ICU post surgery was 92% and cerebral rSO₂were 72% and 75% in right and left respectively.  At 2 hrs, a drop in rSO₂ was to 63% and 50% on right and left respectively was observed. At that point, SpO₂ was 88%, mean arterial pressure was normal, haemoglobin level was 11.1 gm/dl. A fall in SpO₂ to 50 % occurred few minutes after the fall in rSO₂ and auscultation revealed faint shunt murmur. Echocardiography showed minimal flow present in the BT shunt. A shunt blockage was anticipated and fluid bolus of normal saline 10ml/kg, heparin bolus of 50 IU/kg was given. Hyperventilation was done to decrease the CO₂ (FiO₂ was already set at 100% on ventilator) and norepinephrine was also initiated to increase systemic driving pressure. After these interventions, SpO₂ and rSO₂ improved to their baseline values. Two more similar episodes drop in rSO₂ (> 20% drop) and SpO₂ occurred at 6 hours and 11 hours post operative period. During each episode, drop in rSO₂ preceded the drop in SpO₂. Both these episodes responded to measures to increase pulmonary flow. Follow up 2D Echo done at 18 hours showed adequate flow through the BT shunt.

The baseline high degree of oxygen extraction in brain and the lower tissue  partial pressure of oxygen in brain tissue is responsible for early fall of NIRS as compared to SPO₂ .Cerebral NIRS monitoring post operatively in neonates undergoing BT shunt might be beneficial.