Biomedical subjects

A A Birch

Publications and source records attributed to A A Birch.

At least 19 recordsLinked to original sources

PubMed2003

Dynamic cerebral autoregulation assessment using an ARX model: comparative study using step response and phase shift analysis.

Middle cerebral arterial blood velocity (MCAv) response to spontaneous and manipulated changes of arterial blood pressure (ABP) was studied in eight subjects using a linear autoregressive with exogenous input (ARX) model. ABP and MCAv were measured non-invasively by photoplethysmograph and transcranial Doppler ultrasound, respectively. Data were recorded at rest (spontaneous changes in ABP) and during thigh cuff (step-wise changes) and lower body negative pressure (sinusoidal changes of 1/12 Hz) tests in both normocapnia and hypercapnia (5% CO2). Since autoregulation is modulated by CO2, respiratory CO2 was simultaneously monitored to allow comparison of cerebral autoregulation status with different CO2 levels. ABP and MCAv were fitted by ARX models and dynamic cerebral autoregulation was estimated by analysing both the step responses and phase shift at the 1/12 Hz of the corresponding ARX models. The ARX model consistently modelled the phase lead of MCAv to ABP and it showed that the phase shift at 1/12 Hz of ARX model is consistent with the real phase shift of the data (p=0.59). Strong linear relationships between pCO2 and gradient of the step response (r=-0.58, p<0.0001) and between pCO2 and phase shift (r=-0.76, p<0.0001) were observed, which suggests that cerebral autoregulation can be assessed by step response or phase shift analysis of the ARX model fitted to ABP and MCAv data with spontaneous changes.

Adaptation, Physiological↗

PubMed2002

The repeatability of cerebral autoregulation assessment using sinusoidal lower body negative pressure.

A forced periodic variation in blood pressure produces a similar variation in cerebral blood velocity. The amplitudes and phases of the pressure and velocity waveforms are indicative of the dynamic response of the cerebral autoregulation. The phase of the velocity leads the pressure; the greater the phase difference the faster the autoregulation response. Various techniques have been employed to oscillate arterial blood pressure but measurement reproducibility has been poor. The purpose of this study was to assess the reproducibility of phase measurements when sinusoidal lower body negative pressure is used to vary blood pressure. Five healthy volunteers were assessed at two vacuum levels on each of eight visits. For each measurement a 12 s sinusoidal cycle was maintained for 5 min. The Fourier components of blood pressure and the middle cerebral artery velocity were determined at the oscillation frequency. The phase of velocity consistently led the pressure. The mean phase difference was 42+/-13 degrees for the stronger vacuum and 36+/-42 degrees for the weaker vacuum. The variation given is the within-subjects standard deviation estimated from a one-way analysis of variance. Sinusoidal lower body negative pressure is a useful stimulus for investigating autoregulation; it has advantages over other methods. High vacuums show good reproducibility but are too uncomfortable for patient use.

Adult↗

PubMed2001

Intra-aortic balloon counterpulsation: augmentation of cerebral blood flow after aneurysmal subarachnoid haemorrhage.

OBJECTIVE: To measure cerebral blood flow before and after intra-aortic balloon counterpulsation (IABC) in patients at high risk of developing delayed cerebral ischaemia after aneurysm surgery following subarachnoid haemorrhage. METHODS: Six prospectively selected patients at high risk of developing delayed ischaemia had elective IABC after clipping of their cerebral aneurysm(s). The IAB inflates in early diastole and deflates at the end of diastole to increase cardiac perfusion and decrease afterload. This results in enhanced cardiac efficiency. It also augments cerebral blood flow (CBF). RESULTS: We demonstrated a significant increase in the mean hemispheric CBF from the preoperative (preIABC) value of 35.6 mls/100 g/min to 50.9 +/- 12.3 mls/100 g/min (p = 0.0042) as a result of balloon augmentation. Each patient developed a neurological deficit as a result of delayed cerebral ischaemia. These were reversed in 5 patients with increased CBF. There were minimal balloon related complications. CONCLUSION: IABC consistently enhanced CBF in these patients and resulted in stable cardiovascular parameters. This represents a possible new technique in the management of cerebral ischaemia following subarachnoid haemorrhage and needs further assessment to ascertainlits role.

Adult↗

PubMed2001

A new mathematical model of dynamic cerebral autoregulation based on a flow dependent feedback mechanism.

A new mathematical model representing dynamic cerebral autoregulation as a flow dependent feedback mechanism is presented. Two modelling parameters are introduced, lambda, the rate of restoration, and tau, a time delay. Velocity profiles are found for a general arterial blood pressure, allowing the model to be applied to any experiment that uses changes in arterial blood pressure to assess dynamic cerebral autoregulation. Two such techniques, thigh cuffs and a lower body negative pressure box, which produce step changes and oscillatory variations in arterial blood pressure respectively, are investigated. Results derived using the mathematical model are compared with data from the two experiments. The comparisons yield similar estimates for lambda and tau, suggesting these parameters are independent of the pressure change stimulus and depend only on the main features of the dynamic cerebral autoregulation process. The modelling also indicates that for imposed oscillatory variations in arterial blood pressure a small phase difference between pressure and velocity waveforms does not necessarily imply impaired autoregulation. It is shown that the ratio between the variation in maximum velocity and pressure variation can be used, along with the phase difference, to indicate the nature of the autoregulatory response.

Blood Flow Velocity↗

PubMed1995

The ultrasonic localization of retrobulbar needles during retrobulbar block.

PURPOSE: The tips of retrobulbar needles were visualized during retrobulbar block using ultrasound. The purpose of this study is to see how often the needle tip was located 5 mm behind the hind surface of the globe. METHODS: Twenty-five patients ranging in age from 22 to 84 years were studied after written informed consent was obtained. An ultrasonic photograph was taken before needle placement, with the needle in the proper position, and after local anesthetic had been injected. RESULTS: In none of the 25 patients studied was the needle tip at the 5-mm mark behind the eye. Placement ranged from 0.2 to 3.3 mm behind the globe. In 14 to 25 placements, the needle shaft was seen actually to indent the globe. The optic nerve was seen to be at least 9 mm from the needle tip in 6 of 25 patients. CONCLUSION: This study demonstrates that retrobulbar needle tips are closer to the globe than thought previously. A false sense of security may occur when performing retrobulbar blocks using only anatomic landmarks.

Adult↗

PubMed1995

Assessment of autoregulation by means of periodic changes in blood pressure.

BACKGROUND AND PURPOSE: The aim of this study was to test the hypothesis that the phase difference that occurs between an induced oscillation in blood pressure and the resultant oscillation in middle cerebral artery (MCA) flow velocity could reflect the competence of cerebral autoregulation. METHODS: Fourteen volunteers performed 19 cycles of 10 seconds of squatting followed by 10 seconds of standing. Peak MCA velocity was measured with transcranial Doppler ultrasound, and blood pressure was measured with a servo-controlled finger plethysmograph held level with the head. Waveforms from each cycle were added to obtain averaged waveforms of arterial blood pressure and MCA velocity. These results were processed by Fourier analysis to extract the phase difference between the fundamental components of velocity and pressure. Each volunteer performed the exercise three times: first breathing normally, secondly hyperventilating (hypocapnia), and finally while breathing air containing 5% carbon dioxide (hypercapnia). Under these conditions the volunteers were expected to have normal, enhanced, and impaired auto-regulation, respectively. RESULTS: The measurements made with normal breathing showed a phase lead of velocity ahead of pressure of 46 +/- 14 degrees (mean +/- SD). We noted a highly significant reduction in phase lead with hypercapnia (P < .00015) (Wilcoxon signed rank test, two-tailed) and a highly significant increase in phase lead with hypocapnia (P < .002). CONCLUSIONS: The results support our hypothesis and may lead to a technique for assessing the competence of cerebral autoregulation.

Adolescent↗

PubMed1993