Optimal ABP in neonatal heart surgery
We are using the NIRS based optimal ABP (ABPopt) metric to study newborns during complex heart surgery.[1] It gives a very good signal- roughly 90% of time its shows ABPopt numbers during the surgery. It appears that lower limit of autoregulation (LLA) is 10 mmHg less than ABPopt, which was the same in our animal data for the same age- in the piglets CPPopt – 10 mmHg was the point of CBF inflection, and lactate/pyruvate increase or PBTO2 decrease (figure 1). Also, ABPopt is exactly equal to normal blood pressure for age in these patients, which is by itself very interesting. We have automated data capture during surgery and routine clinical MRI studies on our operated patients. Right now we have collected data on about 150 subjects.
Lower limit of autoregulation
The ABP at LLA is much more interesting during neonatal heart surgery than the ‘optimal’ blood pressure number (ABPopt). During the period of extracorporeal bypass (heart-lung machine) afterload reduction is needed for survival. Therefore, we need to know how low we can push arterial blood pressure and still be acceptable. ABPopt may not be attainable. Right now we are using the assumption that the LLA has a somehow fixed relation to ABPopt, which may or may not be a valid assumption.
Figure 1 shows experimental (animal) recording with local cerebral blood flow (CBF, laser Doppler flow, blue), local tissue oxygenation (PBTO2, green) and NIRS based HVx index vs ABP error bar (black) measurements. The experiment shows that with low ABP around 40-50 mmHg invasive CBF and tissue oxygenation fall significantly indicating the lower limit of autoregulation (LLA). The non-invasive HVx index shows positive numbers around 45 mmHg indicating the lower limit of cerebrovascular reactivity (LLR).
Near future research plans
We plan to examine in neonates:
- How pre-operative injury affects the NIRS based ABPopt number.
- How the brain immaturity score (MRI) affects ABPopt numbers.
- How the dose of hypotension defined by ABPopt influences outcome- MRI post-surgery and neonatal developmental scoring.
- Right or Left-sided heart lesions change patterns of oxygen streaming to the brain. How does this affect ABPopt?
For more information about these studies:
Please contact Dr. Kenneth Brady, Pediatric Cardiovascular Anesthesiologist, BCM, Texas (USA).
✉ kmbrady@bcm.edu
Reference
[1] Brady KM, Lee JK, Kibler KK, Smielewski P, et al. Continuous time-domain analysis of cerebrovascular autoregulation using near-infrared spectroscopy. Stroke 2007 October;38(10):2818-25.