Transcranial Doppler assessment of the lower cerebral autoregulatory threshold

Effect of acute hypoxemia on cerebral blood flow velocity control during lower body negative pressure

The ability to maintain adequate cerebral blood flow and oxygenation determines tolerance to central hypovolemia. We tested the hypothesis that acute hypoxemia during simulated blood loss in humans would cause impairments in cerebral blood flow control. Ten healthy subjects (32 ± 6 years, BMI 27 ± 2 kg·m-2 ) were exposed to stepwise lower body negative pressure (LBNP, 5 min at 0, -15, -30, and -45 mmHg) during both normoxia and hypoxia (Fi O2  = 0.12-0.15 O2 titrated to an SaO2 of ~85%). Physiological responses during both protocols were expressed as absolute changes from baseline, one subject was excluded from analysis due to presyncope during the first stage of LBNP during hypoxia. LBNP induced greater reductions in mean arterial pressure during hypoxia versus normoxia (MAP, at -45 mmHg: -20 ± 3 vs. -5 ± 3 mmHg, P < 0.01). Despite differences in MAP, middle cerebral artery velocity responses (MCAv) were similar between protocols (P = 0.41) due to increased cerebrovascular conductance index (CVCi) during hypoxia (main effect, P = 0.04). Low frequency MAP (at -45 mmHg: 17 ± 5 vs. 0 ± 5 mmHg2 , P = 0.01) and MCAv (at -45 mmHg: 4 ± 2 vs. -1 ± 1 cm·s-2 , P = 0.04) spectral power density, as well as low frequency MAP-mean MCAv transfer function gain (at -30 mmHg: 0.09 ± 0.06 vs. -0.07 ± 0.06 cm·s-1 ·mmHg-1 , P = 0.04) increased more during hypoxia versus normoxia. Contrary to our hypothesis, these findings support the notion that cerebral blood flow control is not impaired during exposure to acute hypoxia and progressive central hypovolemia despite lower MAP as a result of compensated increases in cerebral conductance and flow variability.

Arterial pressure and cerebral blood flow variability: friend or foe? A review

Variability in arterial pressure and cerebral blood flow has traditionally been interpreted as a marker of cardiovascular decompensation, and has been associated with negative clinical outcomes across varying time scales, from impending orthostatic syncope to an increased risk of stroke. Emerging evidence, however, suggests that increased hemodynamic variability may, in fact, be protective in the face of acute challenges to perfusion, including significant central hypovolemia and hypotension (including hemorrhage), and during cardiac bypass surgery. This review presents the dichotomous views on the role of hemodynamic variability on clinical outcome, including the physiological mechanisms underlying these patterns, and the potential impact of increased and decreased variability on cerebral perfusion and oxygenation. We suggest that reconciliation of these two apparently discrepant views may lie in the time scale of hemodynamic variability; short time scale variability appears to be cerebroprotective, while mid to longer term fluctuations are associated with primary and secondary end-organ dysfunction.

The cerebrovascular response to graded Valsalva maneuvers while standing

The Valsalva maneuver (VM) produces large and abrupt increases in mean arterial pressure (MAP) at the onset of strain (Phase I), however, hypotension, sufficient to induce syncope, occurs upon VM release (phase III). We examined the effect of VM intensity and duration on middle cerebral artery blood velocity (MCAv) responses. Healthy men (n =10; mean ± SD: 26 ± 4 years) completed 30%, 60%, and 90% of their maximal VM mouth pressure, for 5 and 10 sec (order randomized) while standing. Beat‐to‐beat MCAv and MAP during phase I (peak), at nadir (phase III), and recovery are reported as the change from standing baseline. During phase I, MCAv rose 15 ± 6 cm·s⁻¹ (P <0.001), which was not reliably different between intensities (P =0.11), despite graded increases in MAP (P <0.001; e.g., +12 ± 9 mmHg vs. +35 ± 14 for 5 sec 30% and 90% VM, respectively). During Phase III, the MCAv response was duration‐ (P = 0.045) and intensity dependent (P < 0.001), with the largest decrease observed following the 90% VM (e.g., −19 ± 13 and −15 ± 11 cm·s⁻¹ for 5 and 10 sec VM, respectively) with a concomitant decrease in MAP (P <0.001, −23 ± 11 and −23 ± 9 mmHg). This asymmetric response may be attributable to the differential modulators of MCAv throughout the VM. The mechanical effects of the elevated intrathoracic pressure during phase I may restrain increases in cerebral perfusion via related increases in intracranial pressure; however, during phase III the decrease in MCAv arises from an abrupt hypotension, the extent of which is dependent upon both the duration and intensity of the VM.

More intense Valsalva maneuvers when standing are associated with an increase blood pressure response during Phase I of the maneuver although this is not accompanied by changes in cerebral blood flow. However, following the maneuver (phase III) more intense straining is associated with a greater decrease in both blood pressure and cerebral blood flow and in some instances is sufficient to induce syncope.

Slow breathing as a means to improve orthostatic tolerance: a randomized sham-controlled trial

Endogenous oscillations in blood pressure (BP) and cerebral blood flow have been associated with improved orthostatic tolerance. Although slow breathing induces such responses, it has not been tested as a therapeutic strategy to improve orthostatic tolerance. With the use of a randomized, crossover sham-controlled design, we tested the hypothesis that breathing at six breaths/min (vs. spontaneous breathing) would improve orthostatic tolerance via inducing oscillations in mean arterial BP (MAP) and cerebral blood flow. Sixteen healthy participants (aged 25 ± 4 yr; mean ± SD) had continuous beat-to-beat measurements of middle cerebral artery blood velocity (MCAv), BP (finometer), heart rate (ECG), and end-tidal carbon dioxide partial pressure during an incremental orthostatic stress test to presyncope by combining head-up tilt with incremental lower-body negative pressure. Tolerance time to presyncope was improved (+15%) with slow breathing compared with spontaneous breathing (29.2 ± 5.4 vs. 33.7 ± 6.0 min; P < 0.01). The improved tolerance was reflected in elevations in low-frequency (LF; 0.07-0.2 Hz) oscillations of MAP and mean MCAv, improved metrics of dynamic cerebrovascular control (increased LF phase and reduced LF gain), and a reduced rate of decline for MCAv (−0.60 ± 0.27 vs. −0.99 ± 0.51 cm·s−1·min−1; P < 0.01) and MAP (−0.50 ± 0.37 vs. −1.03 ± 0.80 mmHg/min; P = 0.01 vs. spontaneous breathing) across time from baseline to presyncope. Our findings show that orthostatic tolerance can be improved within healthy individuals with a simple, nonpharmacological breathing strategy. The mechanisms underlying this improvement are likely mediated via the generation of negative intrathoracic pressure during slow and deep breathing and the related beneficial impact on cerebrovascular and autonomic function.

Initial orthostatic hypotension and cerebral blood flow regulation: effect of α1-adrenoreceptor activity

We examined the hypothesis that α1-adrenergic blockade would lead to an inability to correct initial orthostatic hypotension (IOH) and cerebral hypoperfusion, leading to symptoms of presyncope. Twelve normotensive humans (aged 25 ± 1 yr; means ± SE) attempted to complete a 3-min upright stand, 90 min after the administration of either α1-blockade (prazosin, 1 mg/20 kg body wt) or placebo. Continuous beat-to-beat measurements of middle cerebral artery velocity (MCAv; Doppler), blood pressure (finometer), heart rate, and end-tidal Pco2were obtained. Compared with placebo, the α1-blockade reduced resting mean arterial blood pressure (MAP) (−15%; P < 0.01); MCAv remained unaltered ( P ≥ 0.28). Upon standing, although the absolute level of MAP was lower following α1-blockade (39 ± 10 mmHg vs. 51 ± 14 mmHg), the relative difference in IOH was negligible in both trials (mean difference in MAP: 2 ± 2 mmHg; P = 0.50). Compared with the placebo trial, the declines in MCAv and PetCO2during IOH were greater in the α1-blockade trial by 12 ± 4 cm/s and 4.4 ± 1.3 mmHg, respectively ( P ≤ 0.01). Standing tolerance was markedly reduced in the α1-blockade trial (75 ± 17 s vs. 180 ± 0 s; P < 0.001). In summary, while IOH was little affected by α1-blockade, the associated decline in MCAv was greater in the blockade condition. Unlike in the placebo trial, the extent of IOH and cerebral hypoperfusion failed to recover toward baseline in the α1-blockade trial leading to presyncope. Although the development of IOH is not influenced by the α1-adrenergic receptor pathway, this pathway is critical in the recovery from IOH to prevent cerebral hypoperfusion and ultimately syncope.

Cushing's mechanism maintains cerebral perfusion pressure in experimental subarachnoid haemorrhage

Mortality following subarachnoid haemorrhage (SAH) is high, especially within the first 48 h. Poor outcome is predicted by high intracranial pressure which causes diminished cerebral perfusion pressure unless a compensatory increase in mean arterial blood pressure occurs. Therefore blood pressure elevation can be protective following subarachnoid haemorrhage despite the potential for rebleeding. This study investigated blood pressure responses to SAH and the impact on cerebral perfusion pressure and outcome, as demonstrated by two experimental models. Various blood pressure responses were demonstrated, both at the ictus and within the following 5h. Elevated MABP at the ictus and at 2h following experimental SAH was associated with maintenance of CPP in the presence of raised ICP. Poor outcome (arrest of the cerebral circulation) was predicted by failure of MABP to increase significantly above sham levels within 2h of SAH. Rat SAH provides relatively inexpensive models to investigate physiological mechanisms that maintain cerebral perfusion in the presence of intracranial hypertension.

Monitoring cerebral autoregulation after head injury. Which component of transcranial Doppler flow velocity is optimal?

BACKGROUND

Cerebral autoregulation assessed using transcranial Doppler (TCD) mean flow velocity (FV) in response to various physiological challenges is predictive of outcome after traumatic brain injury (TBI). Systolic and diastolic FV have been explored in other diseases. This study aims to evaluate the systolic, mean and diastolic FV for monitoring autoregulation and predicting outcome after TBI.

METHODS

300 head-injured patients with blood pressure (ABP), intracranial pressure (ICP), cerebral perfusion pressure (CPP), and FV recordings were studied. Autoregulation was calculated as a correlation of slow changes in diastolic, mean and systolic components of FV with CPP (Dx, Mx, Sx, respectively) and ABP (Dxa, Mxa, Sxa, respectively) from 30 consecutive 10 s averaged values. The relationship with age, severity of injury, and dichotomized 6 months outcome was examined.

RESULTS

Association with outcome was significant for Mx and Sx. For favorable/unfavorable and death/survival outcomes Sx showed the strongest association (F = 20.11; P = 0.00001 and F = 13.10; P = 0.0003, respectively). Similarly, indices derived from ABP demonstrated the highest discriminatory value when systolic FV was used (F = 12.49; P = 0.0005 and F = 5.32; P = 0.02, respectively). Indices derived from diastolic FV demonstrated significant differences (when calculated using CPP) only when comparing between fatal and non-fatal outcome.

CONCLUSIONS

Systolic flow indices (Sx and Sxa) demonstrated a stronger association with outcome than the mean flow indices (Mx and Mxa), irrespective of whether CPP or ABP was used for calculation.

Tolerance to central hypovolemia: the influence of oscillations in arterial pressure and cerebral blood velocity

Higher oscillations of cerebral blood velocity and arterial pressure (AP) induced by breathing with inspiratory resistance are associated with delayed onset of symptoms and increased tolerance to central hypovolemia. We tested the hypothesis that subjects with high tolerance (HT) to central hypovolemia would display higher endogenous oscillations of cerebral blood velocity and AP at presyncope compared with subjects with low tolerance (LT). One-hundred thirty-five subjects were exposed to progressive lower body negative pressure (LBNP) until the presence of presyncopal symptoms. Subjects were classified as HT if they completed at least the −60-mmHg level of LBNP (93 subjects; LBNP time, 1,880 ± 259 s) and LT if they did not complete this level (42 subjects; LBNP time, 1,277 ± 199 s). Middle cerebral artery velocity (MCAv) was measured by transcranial Doppler, and AP was measured at the finger by photoplethysmography. Mean MCAv and mean arterial pressure (MAP) decreased progressively from baseline to presyncope for both LT and HT subjects ( P < 0.001). However, low frequency (0.04–0.15 Hz) oscillations of mean MCAv and MAP were higher at presyncope in HT subjects compared with LT subjects (MCAv: HT, 7.2 ± 0.7 vs. LT, 5.3 ± 0.6 (cm/s)2, P = 0.075; MAP: HT, 15.3 ± 1.4 vs. 7.9 ± 1.2 mmHg2, P < 0.001). Consistent with our previous findings using inspiratory resistance, high oscillations of mean MCAv and MAP are associated with HT to central hypovolemia.

Cerebrovascular reactivity and dynamic autoregulation in tetraplegia

Humans with spinal cord injury have impaired cardiovascular function proportional to the level and completeness of the lesion. The effect on cerebrovascular function is unclear, especially for high-level lesions. The purpose of this study was to evaluate the integrity of dynamic cerebral autoregulation (CA) and the cerebrovascular reactivity in chronic tetraplegia (Tetra). After baseline, steady-state hypercapnia (5% CO(2)) and hypocapnia (controlled hyperventilation) were used to assess cerebrovascular reactivity in 6 men with Tetra (C5-C7 lesion) and 14 men without [able-bodied (AB)]. Middle cerebral artery blood flow velocity (MCAv), cerebral oxygenation, arterial blood pressure (BP), heart rate (HR), cardiac output (Q; model flow), partial pressure of end-tidal CO(2) (Pet(CO(2))), and plasma catecholamines were measured. Dynamic CA was assessed by transfer function analysis of spontaneous fluctuations in BP and MCAv. MCAv pulsatility index (MCAv PI) was calculated as (MCAv(systolic) - MCAv(diastolic))/MCAv(mean) and standardized by dividing by mean arterial pressure (MAP). Resting BP, total peripheral resistance, and catecholamines were lower in Tetra (P < 0.05), and standardized MCAv PI was approximately 36% higher in Tetra (P = 0.003). Resting MCAv, cerebral oxygenation, HR, and Pet(CO(2)) were similar between groups (P > 0.05). Although phase and transfer function gain relationships in dynamic CA were maintained with Tetra (P > 0.05), coherence in the very low-frequency range (0.02-0.07 Hz) was approximately 21% lower in Tetra (P = 0.006). Full (hypo- and hypercapnic) cerebrovascular reactivity to CO(2) was unchanged with Tetra (P > 0.05). During hypercapnia, standardized MCAv PI reactivity was enhanced by approximately 78% in Tetra (P = 0.016). Despite impaired cardiovascular function, chronic Tetra involves subtle changes in dynamic CA and cerebrovascular reactivity to CO(2). Changes are evident in coherence at baseline and MCAv PI during baseline and hypercapnic states in chronic Tetra, which may be indicative of cerebrovascular adaptation.

Influence of changes in blood pressure on cerebral perfusion and oxygenation

Cerebral autoregulation (CA) is a critical process for the maintenance of cerebral blood flow and oxygenation. Assessment of CA is frequently used for experimental research and in the diagnosis, monitoring, or prognosis of cerebrovascular disease; however, despite the extensive use and reference to static CA, a valid quantification of "normal" CA has not been clearly identified. While controlling for the influence of arterial Pco(2), we provide the first clear examination of static CA in healthy humans over a wide range of blood pressure. In 11 healthy humans, beat-to-beat blood pressure (radial arterial), middle cerebral artery blood velocity (MCAv; transcranial Doppler ultrasound), end-tidal Pco(2), and cerebral oxygenation (near infrared spectroscopy) were recorded continuously during pharmacological-induced changes in mean blood pressure. In a randomized order, steady-state decreases and increases in mean blood pressure (8 to 14 levels; range: approximately 40 to approximately 125 mm Hg) were achieved using intravenous infusions of sodium nitroprusside or phenylephrine, respectively. MCAv(mean) was altered by 0.82+/-0.35% per millimeter of mercury change in mean blood pressure (R(2)=0.82). Changes in cortical oxygenation index were inversely related to changes in mean blood pressure (slope=-0.18%/mm Hg; R(2)=0.60) and MCAv(mean) (slope=-0.26%/cm . s(-1); R(2)=0.54). There was a progressive increase in MCAv pulsatility with hypotension. These findings indicate that cerebral blood flow closely follows pharmacological-induced changes in blood pressure in otherwise healthy humans. Thus, a finite slope of the plateau region does not necessarily imply a defective CA. Moreover, with progressive hypotension and hypertension there are differential changes in cerebral oxygenation and MCAv(mean).

Initial orthostatic hypotension is unrelated to orthostatic tolerance in healthy young subjects

The physiological challenge of standing upright is evidenced by temporary symptoms of light-headedness, dizziness, and nausea. It is not known, however, if initial orthostatic hypotension (IOH) and related symptoms associated with standing are related to the occurrence of syncope. Since IOH reflects immediate and temporary adjustments compared with the sustained adjustments during orthostatic stress, we anticipated that the severity of IOH would be unrelated to syncope. Following a standardized period of supine rest, healthy volunteers [ n = 46; 25 ± 5 yr old (mean ± SD)] were instructed to stand upright for 3 min, followed by 60° head-up tilt with lower-body negative pressure in 5-min increments of −10 mmHg, until presyncope. Beat-to-beat blood pressure (radial arterial or Finometer), middle cerebral artery blood velocity (MCAv), end-tidal Pco2, and cerebral oxygenation (near-infrared spectroscopy) were recorded continuously. At presyncope, although the reductions in mean arterial pressure, MCAv, and cerebral oxygenation were similar to those during IOH (40 ± 11 vs. 43 ± 12%; 36 ± 18 vs. 35 ± 13%; and 6 ± 5 vs. 4 ± 2%, respectively), the reduction in end-tidal CO2 was greater (−7 ± 6 vs. −4 ± 3 mmHg) and was related to the decline in MCAv ( R2 = 0.4; P < 0.05). While MCAv pulsatility was elevated with IOH, it was reduced at presyncope ( P < 0.05). The cardiorespiratory and cerebrovascular changes during IOH were unrelated to those at presyncope, and interestingly, there was no relationship between the hemodynamic changes and the incidence of subjective symptoms in either scenario. During IOH, the transient nature of physiological changes can be well tolerated; however, potentially mediated by a reduced MCAv pulsatility and greater degree of hypocapnic-induced cerebral vasoconstriction, when comparable changes are sustained, the development of syncope is imminent.

Brain injury: the pathophysiology of the first hours.'Talk and Die revisited'

In the 25 years since the 'Talk and Die' paper there have been substantial advances in the management of patients with severe closed head injury. This paper discusses developments in understanding of primary and secondary injury. Current management focuses on preventing secondary brain injury. That this has been successful is illustrated by a fall in mortality in recent decades. Evidence based guidelines have set standards of management but they do not take into account variations between individuals, between regions of the brain and variations with time from injury. Various monitoring techniques such as transcranial doppler, jugular venous oxygen saturation and ICP waveform analysis attempt to set individual therapeutic endpoints and to target therapy appropriately. Primary injury is no longer seen as a single irreversible event occurring at the time of impact, but rather as a process initiated by the impact and evolving over subsequent hours and days. Experimental studies have identified agents which reduce the evolution of brain injury and improve outcome. An experimental model of brain injury developed by the Adelaide He ad Injury Group identifies diffuse axonal injury as a target for therapeutic manipulation. Magnesium has been shown in other studies to improve outcome after diffuse brain injury. This has now been linked with upregulation of beta amyloid precursor prote in. Although this and several other experimental therapies have shown great promise, they have not so far produced benefit in large clinical studies. Avoiding secondary insults will remain the goal of management for the foreseeable future. Halting the evolution of the primary injury remains a highly sought after goal. Although elusive so far, it is likely to be the next major advance in clinical care.