title: The Effects of an Extracorporeal Circulation on Cerebral Perfusion during Paediatric Critical Care and Cardiothoracic Procedures
Creators: name: Finnigan, L
Publisher: name: Liverpool John Moores University
Year: 2022

description: The developing brain is highly vulnerable to physiological and pharmacological insults, which can impact cerebral blood flow (CBF), resulting in neurological deficit. Within the clinical environment, a group of patients at particular risk of neurological insult are those who require an extracorporeal circulation. This has a profound effect on CBF due to the use of non-pulsatile blood flow, pharmacological agents and periods of hypo/hyperperfusion. As a result, effective cerebral monitoring is essential to prevent periods of ischaemia and subsequently, enhance quality of life in survivors. Patients requiring an extracorporeal circulation include those treated for acute respiratory and/or cardiac failure using extracorporeal membrane oxygenation (ECMO), and the use of cardiopulmonary bypass (CPB) during cardiac surgery for congenital heart defects. The overarching aim of this thesis was to examine how an extracorporeal circulation impacts cerebral perfusion throughout the entire treatment process and following recovery in paediatric intensive care. Measurements of cerebral perfusion used throughout the thesis are described in the experimental methods chapter. These include transcranial doppler ultrasound (TCD) and near infrared spectroscopy (NIRS). Another aim of the experimental chapter was to collect TCD data and compare the values to previously published research. Measurements were taken in a ‘healthy’ neonatal population and then compared to previously published age-matched comparisons. Measurements were also taken in clinically ill patients and then compared to values from previously published ventilated population. Values were comparable to previously published data which suggested the TCD operator was proficient in isonating the middle cerebral artery (MCA). An observational study was conducted using TCD to measure CBFv at multiple time points during ECMO with a focus on the weaning period in a paediatric population. Fourteen patients that underwent veno-arterial (V-A) ECMO were enrolled. Eight (mean age 69 days) had central cannulation for post-cardiac surgery support, while six (mean age 84 days) had neck cannulation for respiratory support. CBFv was measured from the MCA during weaning at several time points: full flow ECMO, ¾ flow, ½ flow, ¼ flow, minimum flow, when off ECMO and post decannulation. NIRS, blood pressure, heart rate and arterial oxygen saturation were recorded at the same time points. During the first 5 days of full flow ECMO, CBFv remained relatively stable (p=0.54). During weaning, those that successfully decannulated had on average a higher CBFv of 9.1 cm/s compared to those that failed weaning. From the patients decannulated, those receiving conventional treatment had an average higher CBFv of 9.9 cm/s compared to patients on high frequency oscillatory ventilation. Overall, the relationship between NIRS and TCD was positive but weak. Another study was undertaken to examine CBFv and an extracorporeal circulation during aortic arch repair. Neonates requiring aortic arch repair are unable to maintain adequate oxygenation levels and require surgical intervention. A high percentage of survivor’s exhibit signs of neurological deficit possibly due to inadequate CBF during surgery. The aim was to continuously monitor MCA velocity (MCAv) during surgery. A secondary aim was to examine the impact of temperature on CBF, with cooling ranging from 18 to 25°C based on surgeon preference. MCAv was monitored in 24 neonates (age 19±6 days, body mass 3.6±0.6 kg) undergoing surgery on the aortic arch, alongside NIRS, blood pH, pO2, pCO2, HCO3, lactate, Hb, Htc (%) and temperature (core and rectal). Using general linear models, MCAv was compared at several time points. These included: initial sedation; cardiopulmonary bypass (CBP); cooling at 30°c, 25°c, the lowest temperature; during selective cerebral perfusion; whole body perfusion; rewarming at 25°c, 30°c, 36°c; off cardiopulmonary bypass; and after surgery. During and following surgery, MCAv was lower compared to previously published healthy age-matched controls, except during cooling period. CBFv increased during cooling at 30°c, 25°c and the lowest temperature respectively when compared to CBP (p=0.03). Once off CBP, MCAv returned to pre-surgery values. No significant difference was noted between patients cooled to 20 or 25°c. Overall, this thesis provides evidence of disruptions in cerebral perfusion during different stages of treatments involving an extracorporeal circulation. The current clinical tool for monitoring cerebral perfusion is NIRS, which may not provide sufficiently sensitive data on cerebral perfusion during treatments using an extracorporeal circulation. Taken together, the findings provide important data for clinicians treating paediatric patients requiring an extracorporeal circulation. It provides data identifying specific time periods of reduced cerebral perfusion, while highlighting limitations of current methods of monitoring.