Spectral analysis of arterial blood pressure and cerebral blood flow velocity during supine rest and orthostasis

Causality analysis reveals the link between cerebrovascular control and acute kidney dysfunction after coronary artery bypass grafting

BACKGROUND

Patients undergoing coronary artery bypass graft (CABG) surgery might experience postoperative complications and some of them, such as acute kidney dysfunction (AKD), are the likely consequence of hypoperfusion. We hypothesized that an impaired cerebrovascular control is a hallmark of a vascular damage that might favor AKD after CABG.

OBJECTIVE

Our aim is to characterize cerebrovascular control in CABG patients through the assessment of the relationship between mean arterial pressure (MAP) and mean cerebral blood flow velocity (MCBFV) and to check whether markers describing MCBFV-MAP dynamical interactions could identify subjects at risk to develop postoperative AKD.

APPROACH

MAP and MCBFV beat-to-beat series were extracted from invasive arterial pressure and transcranial Doppler recordings acquired simultaneously in 23 patients just before CABG after the induction of propofol general anesthesia. Subjects were divided into AKD group (n  =  9, age: 68  ±  9, 8 males) and noAKD group (n  =  14, age: 65  ±  8, 12 males) according to whether they developed postoperative AKD or not after CABG. We computed MAP and MCBFV time-domain and spectral markers as well as MCBFV-MAP cross-spectral indexes in very-low-frequency (VLF, 0.02-0.07 Hz), low-frequency (LF, 0.07-0.15 Hz) and high-frequency (HF, 0.15-0.30 Hz) bands. We also calculated model-based transfer entropy (TE) to quantify the degree of MCBFV dependence on MAP and vice versa. The null hypothesis of MCBFV-MAP uncoupling was tested via a surrogate approach associating MAP and MCBFV in different patients.

MAIN RESULTS

Time, spectral and cross-spectral markers had a limited power in separating AKD from noAKD individuals. Conversely, TE from MAP to MCBFV was significantly above the level set by surrogates only in AKD groups and significantly larger than that computed in noAKD.

SIGNIFICANCE

The reduced cerebrovascular autoregulation in AKD patients suggest a vascular impairment likely making them more at risk of hypoperfusion during CABG and AKD after CABG.

Effect of body position on bilateral EEG alterations and their relationship with autonomic nervous modulation in normal subjects

This study investigated the effect of body position on the electroencephalogram (EEG) and autonomic nervous modulation, and the relationship between them using spectral analysis of EEG and heart rate variability (HRV). All healthy volunteers recruited had their electrocardiogram and EEG recorded for power spectral analysis. We found that when changing position from supine to upright, the EEG spectral components below the α band, such as δ and θ bands, were significantly decreased while the EEG spectral components above the α band, such as β, γ and ω bands, were significantly increased in both scalps. Correlation analysis showed that the θ rhythm of both scalps might be associated with the control of HR, the α and β rhythms of right scalp might be associated with vagal modulation, and the γ rhythm of left scalp might be associated with sympathetic modulation of the subject. Thus, some EEG components might be associated with the autonomic nervous modulation of the subject during positional change. There might be a mechanism located in the brain-stem which jointly controls both autonomic influences on heart rate and EEG activation.

Dynamic cerebral autoregulation during brain activation paradigms

Dynamic cerebral autoregulation (CA) describes the transient response of cerebral blood flow (CBF) to rapid changes in arterial blood pressure (ABP). We tested the hypothesis that the efficiency of dynamic CA is increased by brain activation paradigms designed to induce hemispheric lateralization. CBF velocity [CBFV; bilateral, middle cerebral artery (MCA)], ABP, ECG, and end-tidal Pco2 were continuously recorded in 14 right-handed healthy subjects (21–43 yr of age), in the seated position, at rest and during 10 repeated presentations (30 s on-off) of a word generation test and a constructional puzzle. Nonstationarities were not found during rest or activation. Transfer function analysis of the ABP-CBFV (i.e., input-output) relation was performed for the 10 separate 51.2-s segments of data during activation and compared with baseline data. During activation, the coherence function below 0.05 Hz was significantly increased for the right MCA recordings for the puzzle tasks compared with baseline values (0.36 ± 0.16 vs. 0.26 ± 0.13, P < 0.05) and for the left MCA recordings for the word paradigm (0.48 ± 0.23 vs. 0.29 ± 0.16, P < 0.05). In the same frequency range, significant increases in gain were observed during the puzzle paradigm for the right (0.69 ± 0.37 vs. 0.46 ± 0.32 cm·s−1·mmHg−1, P < 0.05) and left (0.61 ± 0.29 vs. 0.45 ± 0.24 cm·s−1·mmHg−1, P < 0.05) hemispheres and during the word tasks for the left hemisphere (0.66 ± 0.31 vs. 0.39 ± 0.15 cm·s−1·mmHg−1, P < 0.01). Significant reductions in phase were observed during activation with the puzzle task for the right (−0.04 ± 1.01 vs. 0.80 ± 0.86 rad, P < 0.01) and left (0.11 ± 0.81 vs. 0.57 ± 0.51 rad, P < 0.05) hemispheres and with the word paradigm for the right hemisphere (0.05 ± 0.87 vs. 0.64 ± 0.59 rad, P < 0.05). Brain activation also led to changes in the temporal pattern of the CBFV step response. We conclude that transfer function analysis suggests important changes in dynamic CA during mental activation tasks.

Orthostatic modification of ventilatory dynamic response to carbon dioxide perturbations

In order to determine whether changes in ventilatory control contribute to the observed decrease in arterial partial pressure of carbon dioxide (PaCO(2)) during head up tilt, we assessed ventilatory dynamic sensitivity to changes in PaCO(2) during supine and 70 degrees passive head up tilt. In 24 adult normals, we stimulated the ventilatory control system by switching inspired CO(2) between room air and room air+5% CO(2) in a pseudo random binary sequence. A Box-Jenkins model was used to compute ventilatory response to CO(2). Airflow, CO(2), non-invasive beat by beat blood pressure, ECG and cerebral blood flow velocity (Doppler) were recorded. During tilt, sensitivity of the ventilatory controller to CO(2) disturbance increased (from 0.45 to 0.72 L/min/mm Hg, p<0.005); minute ventilation increased (7.63 to 8.47 L/min, p<0.01), end tidal CO(2) (ETCO(2)), cerebral blood flow velocity (CBF) and baroreflex sensitivity decreased (46.9 to 42.9 mm Hg, p<0.001; 84.9 to 72.9 cm/s, p<0.001; and 17.6 to 5.5 ms/mm Hg, p<0.001). The primary observation from our study was that the sensitivity of ventilatory control system to perturbations in ETCO(2) increased during tilt. Taken together with decrease in mean levels of ETCO(2) and an increase in minute ventilation, these results suggest that during tilt, a change in the regulated level or 'set point' of PaCO(2) may occur.

Postural syncope after marijuana: a transcranial Doppler study of the hemodynamics

Twenty-nine volunteers participated in a randomized, double-blind, placebo-controlled study. Cerebral blood velocity (CBV), pulse rate, blood pressure (BP), skin perfusion (SP) on forehead and plasma delta9 tetrahydrocannabinol (THC) levels were quantified during reclining and standing for 10 min before and after THC infusions and marijuana smoking. Both THC and marijuana induced postural dizziness, with 28% reporting severe symptoms. Intoxication and dizziness peaked immediately after drug. The severe dizziness group showed the most marked postural drop in CBV and BP and showed a drop in pulse rate after an initial increase during standing. Postural dizziness was unrelated to plasma levels of THC and other indices.

Dynamic modulation of cerebrovascular resistance as an index of autoregulation under tilt and controlled PET(CO(2))

Transfer function analysis of the arterial blood pressure (BP)-mean flow velocity (MFV) relationship describes an aspect of cerebrovascular autoregulation. We hypothesized that the transfer function relating BP to cerebrovascular resistance (CVRi) would be sensitive to low-frequency changes in autoregulation induced by head-up tilt (HUT) and altered arterial PCO(2). Nine subjects were studied in supine and HUT positions with end-tidal PCO(2) (PET(CO(2))) kept constant at normal levels: +5 and -5 mmHg. The BP-MFV relationship had low coherence at low frequencies, and there were significant effects of HUT on gain only at high frequencies and of PCO(2) on phase only at low frequencies. BP --> CVRi had coherence >0.5 from very low to low frequencies. There was a significant reduction of gain with increased PCO(2) in the very low and low frequencies and with HUT at the low frequency. Phase was affected by PCO(2) in the very low frequencies. Transfer function analysis of BP --> CVRi provides direct evidence of altered cerebrovascular autoregulation under HUT and higher levels of PCO(2).

Assessment of cerebral autoregulation using time-domain cross-correlation analysis

Time domain cross-correlation analysis of pre-filtered mean arterial blood pressure (MABP) and mean cerebral blood flow velocity (MCBFV) was applied to assess the cerebral autoregulation (CA). Beat-to-beat time series of spontaneous arterial blood pressure and cerebral blood flow velocity were obtained from 13 young normal volunteers with the Finapres device and the transcranial Doppler for periods of approximately 5 min in the supine position. Cross-correlation functions (CCFs) were estimated using a 64 beat wide moving window. Mean CCF patterns were obtained for each subject and for the entire population. The MABP and MCBFV signals were bandpass filtered in the very low-frequency range (VLF, 0.015-0.07 Hz), low-frequency range (LF, 0.07-0.15 Hz) and high-frequency range (HF, 0.15-0.40 Hz) before applying CCF for the purpose of studying the effect of different bandwidths on the resulting mean CCFs. Results revealed that the corresponding time lags of the peak values of the MABP-MCBFV CCFs increased significantly between the LF and HF frequency ranges (LF: -1.20+/-0.91 s, HF: -0.07+/-0.42 s,p < 0.001; paired sign test). The left-shift (negative lag) of the CCF peak between MABP and MCBFV is a result of the phase-lead property. The increasing time lag of the CCF peak indicated evidence of the autoregulatory disturbance. The CCF of pre-filtered spontaneous MABP and MCBFV could be a useful tool to estimate the CA dynamic response.

Increased arterial wave reflection may predispose syncopal attacks

BACKGROUND

The incidence of syncope increases with age, while aging is also associated with increased arterial wave reflection.

HYPOTHESIS

The study was undertaken to determine whether increased arterial wave reflection is a predisposing factor of syncope.

METHODS

We recruited 38 patients (28 men and 10 women, mean age 57.2 +/- 20.3 years, range 17-87 years) with a history of syncope within 6 months of entry. The etiology of syncope was documented for each patient by a complete assessment of vasomotor function and cerebral flow. All patients received a comprehensive echocardiographic evaluation of cardiac structure and function. Carotid augmentation index (AI) was estimated noninvasively with the tonometry technique. The results were compared with those from 54 age- and gender-matched controls.

RESULTS

The most frequent diagnoses of syncope were postural hypotension (13 patients) and cerebrovascular dysautoregulation (10 patients), and the cause could not be determined in 9 patients. Compared with the control group, the syncope group had a greater AI (20 +/- 21 vs. 10 +/- 15%, p = 0.013). Subgroup analysis of 20 patients aged > 50 years and with the aforementioned diagnoses showed even more striking results: AI, 29 +/- 10 vs. 11 +/- 15%, p < 0.001. The enhanced augmentation in the patients remained when age, systolic blood pressure, height, and heart rate were accounted for. Analysis of the carotid pulse wave suggested that both the timing and intensity of wave reflection were enhanced in patients with a history of syncope compared with controls.

CONCLUSIONS

Our results support the hypothesis that enhanced arterial wave reflection is associated with the occurrence of syncope, especially in the elderly.