Positive end-expiratory pressure oscillation facilitates brain vascular reactivity monitoring

Abstract

Brady KM, Easley RB, Kibler K, Kaczka DW, Andropoulos D, Fraser CD 3rd, Smielewski P, Czosnyka M, Adams GJ, Rhee CJ, Rusin CG. Positive end-expiratory pressure oscillation facilitates brain vascular reactivity monitoring. J Appl Physiol 113: 1362-1368, 2012. First published September 13, 2012 doi:10.1152/japplphysiol.00853.2012.-The pressure reactivity index (PRx) identifies optimal cerebral perfusion pressure after traumatic brain injury. We describe a method to improve PRx precision by induced variations in arterial blood pressure (ABP) using positive end-expiratory pressure (PEEP) modulation (iPRx). Neonatal swine (n ϭ 10) were ventilated with static PEEP and then with PEEP oscillated between 5 and 10 cmH2O at a frequency of 1/min. PRx was recorded as a moving correlation coefficient between ABP and intracranial pressure (ICP) from spontaneous ABP activity (0.05-0.003 Hz) during static PEEP. iPRx was similarly recorded with PEEP oscillation-induced ABP waves. The lower limit of autoregulation (LLA) was delineated with continuous cortical laser Doppler flux monitoring. PEEP oscillation increased autoregulation-monitoring precision. The ratios of median absolute deviations to range of possible values for the PRx and iPRx were 9.5% (8.3-13.7%) and 6.2% (4.2-8.7%), respectively (P ϭ 0.006; median, interquartile range). The phase-angle difference between ABP and ICP above LLA was 161°(150°-166°) and below LLA, Ϫ31°(Ϫ42°to 12°, P Ͻ 0.0001). iPRx above LLA was Ϫ0.42 (Ϫ0.67 to Ϫ0.29) and below LLA, 0.32 (0.22-0.43, P ϭ 0.0004). A positive iPRx was 97% specific and 91% sensitive for perfusion pressure below LLA. PEEP oscillation caused stable, low-frequency ABP oscillations that reduced noise in the PRx. Safe translation of these findings to clinical settings is expected to yield more accurate and rapid delineation of individualized optimal perfusion-pressure goals for patients. cerebrovascular autoregulation; pressure reactivity; positive end-expiratory pressure; neonatal CEREBROVASCULAR PRESSURE AUTOREGULATION is a vital, homeostatic mechanism in the mammalian brain that constrains cerebral blood flow (CBF) during changes in arterial blood pressure (ABP). Dynamic cerebrovascular resistance (CVR), also known as pressure reactivity, mediates autoregulation.The pressure reactivity index (PRx) was first described in 1997 as a means of monitoring cerebrovascular reactivity by moving correlation between slow waves (0.05-0.008 Hz) of ABP and intracranial pressure (ICP) (9). When autoregulation is intact, vascular constriction occurs during increased ABP, and vascular dilatation occurs during decreased ABP. Dilatation and constriction of the cerebral vasculature due to autoregulation can be trended by ICP monitoring: ICP increases during vasodilation and decreases during vasoconstriction (13). Therefore, positive ABP-ICP correlation (positive PRx) indicates pressure-passive cerebral vasculature, a state seen when cerebral perfusion pressure (CPP) is either excessive or inadequate for cerebrovascular autoregulation. Convers...

Blood pressure regulation IX: cerebral autoregulation under blood pressure challenges

“…Studies employing empirical (Czosnyka et al 2001), piecewise (Brady et al 2012), and projection pursuit regression (Tan 2012) under various experimental settings have all demonstrated pressure–flow curves resembling Lassen’s autoregulatory curve in that pressure-passive regions shoulder an intervening plateau (Fig. 1).…”

“…1). However, studies are inconsistent in that the plateau regions are either relatively wide (>40 mmHg) (Brady et al 2012) or demonstrably narrow (i.e. 10–15 mmHg) (Tan 2012).…”

The frequency response of cerebral autoregulation

“…For time-sensitive clinical scenarios, it is desirable to have information about pressure autoregulation more rapidly. We have reported a method to increase PRx precision using induced, regular, low-amplitude sinusoidal oscillations of ABP, generated by positive endexpiratory pressure (PEEP) modulation in mechanically ventilated subjects (2).…”

Static cerebrovascular pressure autoregulation remains intact during deep hypothermia

“…Twenty male piglets weighing between 5 and 11 kg were randomly assigned to two experimental groups: normothermic and hypothermic. Initial preparation: All piglets were anesthetized and surgically prepared with previously described protocols used to characterize static autoregulation . In brief, anesthesia was induced with isoflurane inhalation, and maintained with 0.8% inspired isoflurane and 50% inspired nitrous oxide, fentanyl infusion at 30 mcg/kg/h, and vecuronium infusion at 1 mg/kg/h.…”