Regardless of whether you are working in the OR or the ICU, the physiologic concepts guiding cerebral autoregulation, are fundamental, basic principles guiding your practice. In essence, the idea is that blood flow is kept relatively constant across a wide range of arterial blood pressures (or cerebral perfusion pressures: CPP= MAP-ICP) between a lower and upper limit. The upper and lower limits are set by maximal arterial vasoconstriction and -dilation respectively. Above or below these limits, brain (organ) blood flow will passively follow blood pressure (or CPP). In today’s PAAD we are going to discuss the lower limits of autoregulation and how we treat (or don’t treat) hypotension in the OR.
An enormous amount of information on this issue has been developed over the past decade, particularly in neonates and infants, patients undergoing cardiac bypass or who have suffered traumatic brain injury. Much of today’s PAAD is based on the research of Dr. Ken Brady (Lurie Children’s Hospital) and Dr. Jenny Lee (Johns Hopkins Hospital) and involves the pressure reactivity index (PRx).[1-4] PRx uses mathematical algorithms to calculate the correlation between arterial blood pressure and intracranial pressure. Because many of you may be unfamiliar with PRx I’ve asked Ken and Jenny to write a “remembering the classics” to discuss this in much greater detail which we’ll publish in a future PAAD.
Finally, how low can you go was discussed at the recent SPA meeting in New Orleans by Dr. Faith Ross an anesthesiologist at Seattle Children’s Hospital and Dr. Shahab Noori, a neonatologist at Children’s Hospital of Los Angeles. Their lectures and slides are on line for registered attendees at the SPA website.
Original article
Czosnyka M, Santarius T, Donnelly J, van den Dool REC, Sperna Weiland NH. Pro-Con Debate: The Clinical (Ir)relevance of the Lower Limit of Cerebral Autoregulation for Anesthesiologists. Anesth Analg. 2022 Oct 1;135(4):734-743. PMID: 36108190
Editorial
Immink RV, Hollmann MW. The Clinical (Ir)relevance of the Lower Limit of Cerebral Autoregulation for Anesthesiologists. Anesth Analg. 2022 Oct 1;135(4):732-733. PMID: 36108189
Society for Pediatric Anesthesia Annual meeting New Orleans 2022
Faith J. Ross, MD Hypotension in Children: How Low Can You Go?
Shahab Noori, MD, MS CBTI. Management of Cardiovascular Insufficiency: Are Preterm Infants Just Small Babies?
In yesterday’s PAAD we discussed and reposted the PAAD first published on June 6, 2022 “How low is low”? Clearly, the consequences of hypoperfusion can be devastating. Preventing end organ damage, particularly the brain, heart, and kidneys are key goals driving our practice in the operating room and ICUs. Hypoperfusion of the brain, can be the direct result of reduced mean arterial pressure (MAP), or secondary to an increase in intracranial pressure (ICP). Csosnyka et al in today’s PAAD ask “Does a blood pressure below the lower limit of autoregulation (LLA) always justify correction? Or could it be that the brain does not demand such close monitoring? In other words, is the lower limit of autoregulation (LLA) clinically relevant to anesthesiologists?”[5] As pediatric anesthesiologists dealing with neonates in whom normal mean arterial blood pressures may be as low as 25-30, which is below the normal limit of autoregulation in older children and adults this question and when and how to treat “hypotension” is even more relevant.
This article is a wonderful review of the basic physiology on cerebral autoregulation and is a must read for virtually all of us. In essence, the idea is that cerebral blood flow (CBF) is kept relatively constant across a wide range of arterial blood pressures (or cerebral perfusion pressures: CPP= MAP-ICP) between a lower and upper limit. The upper and lower limits are set by maximal arterial vasoconstriction and -dilation respectively. Above or below these limits CBF will passively follow blood pressure (or CPP). Other parameters beyond arterial blood pressure and ICP affecting CBF are very much in our control in the OR and ICUs like Paco2, Pao2, and cerebral metabolic rate of oxygen [CMRO2] which will affect CBF independently of the autoregulation (Lassen) curve.
There are several methods of detecting and monitoring the lower limit of autoregulation: transcranial Doppler velocimetry (TCD), continuous direct ICP measurement, near-infrared spectroscopy (NIRS) oxygenation, and the pressure reactivity index (PRx). In PRx, waves of ICP (vasocycling, period from 20 seconds to 3 minutes) are derived from synchronous fluctuations of cerebral blood volume (CBV) in response to fluctuations of MAP. Positive PRx denotes passive MAP-ICP transmission (ie, failing vascular reactivity), while negative values indicate that reactivity is working well.
Monitoring of autoregulation using NIRS methodology with COx index as been quickly adopted to the management of MAP during cardiac surgery.[4, 6] Should we be using this technology routinely in our youngest patients for ALL surgery? “Preterm infants are particularly susceptible to brain injury as the brain undergoes rapid development during the last trimester of pregnancy. During this period, the brain does not only increase in volume but also undergoes increasing gyri- and sulcification and myelination and improves connectivity. Throughout this process, the brain is using substantial amounts of oxygen. Cerebral pathology can present as white matter injury, such as periventricular leukomalacia, or as periventricular–intraventricular hemorrhage (PIVH). Inadequate or fluctuating cerebral perfusion and oxygenation can result in brain injury (8). Hyperoxia, hypoxia, and fluctuations in cerebral oxygenation, indicative of poor cerebral autoregulation, can adversely affect brain development.”[7]
On the other hand, in anesthetized patients in the OR, the lower limit of autoregulation may not be of clinical importance because all anesthetic agents, barbiturates, volatile anesthetics, propofol, ketamine, and etomidate (YES, ketamine!) lower the cerebral metabolic rate of oxygen which may be neuroprotective despite a falling CBF. Further, allowing the CO2 to be normal or slightly elevated will help increase CBF despite a low arterial blood pressure. In the neonate wide swings in CO2 can be devastating and cause the delicate cerebrovascular blood supply to rupture and cause an intraventricular hemorrhage.
Finally, how should we measure blood pressure? Dr. Faith Ross in her SPA lecture pointed out that arterial pressure derived by the oscillometric devices, particularly in the upper arm, show acceptable levels of agreement with direct intra-arterial measurement if blood pressures were normal. However, during hypotension or shock, a clinically relevant overestimation of arterial pressure occurrs in young children and infants when measured by an upper arm cuff.[8, 9]
Thus, in the anesthetized older patient, keeping the MAP above 50 should be safe. In our youngest and most vulnerable patients or in patients in shock, the need for measures of adequate measurement of blood pressure and perfusion are available and I think should be used routinely. What do you think? Send me your comments and I’ll publish in a future PAAD.
References
1. Rhee, C.J., et al., Neonatal cerebrovascular autoregulation. Pediatr Res, 2018. 84(5): p. 602-610.
2. Lee, J.K., et al., Cerebrovascular reactivity measured by near-infrared spectroscopy. Stroke., 2009. 40(5): p. 1820-1826.
3. Brady, K.M., et al., Continuous monitoring of cerebrovascular pressure reactivity after traumatic brain injury in children. Pediatrics., 2009. 124(6): p. e1205-e1212.
4. Brady, K.M., et al., The lower limit of cerebral blood flow autoregulation is increased with elevated intracranial pressure. Anesth Analg., 2009. 108(4): p. 1278-1283.
5. Czosnyka, M., et al., Pro-Con Debate: The Clinical (Ir)relevance of the Lower Limit of Cerebral Autoregulation for Anesthesiologists. Anesth Analg, 2022. 135(4): p. 734-743.
6. Joshi, B., et al., Predicting the limits of cerebral autoregulation during cardiopulmonary bypass. Anesth Analg, 2012. 114(3): p. 503-10.
7. Dix, L.M., F. van Bel, and P.M. Lemmers, Monitoring Cerebral Oxygenation in Neonates: An Update. Front Pediatr, 2017. 5: p. 46.
8. Meidert, A.S., et al., Accuracy of oscillometric noninvasive blood pressure compared with intra-arterial blood pressure in infants and small children during neurosurgical procedures: An observational study. Eur J Anaesthesiol, 2019. 36(6): p. 400-405.
9. Meidert, A.S., et al., Oscillometric versus invasive blood pressure measurement in patients with shock: a prospective observational study in the emergency department. J Clin Monit Comput, 2021. 35(2): p. 387-393.