A vascular subtraction method for improving the variability of evoked tympanic membrane displacement measurements.

OBJECTIVE: Evoked tympanic membrane displacement (TMD) measurements show a correlation with intracranial pressure (ICP). Attempts to use these measurements for non-invasive monitoring of ICP in patients have been limited by high measurement variability. Pulsing of the tympanic membrane at the cardiac frequency has been shown to be a significant source of the variability. In this study we describe a post processing method to remove the cardiac pulse waveform and assess the impact of this on the measurement and its repeatability. APPROACH: Three-hundred and sixteen healthy volunteers were recruited for evoked TMD measurements. The measurements were quantified by V m, defined as the mean displacement between the point of maximum inward displacement and the end of the stimulus. A sample of spontaneously pulsing TMDs was measured immediately before the evoked measurements. Simultaneous recording of the ECG allowed a heartbeat template to be extracted from the spontaneous data and subtracted from the evoked data. Intra-subject repeatability of V m was assessed from 20 repeats of the evoked measurement. Results with and without subtraction of the heartbeat template were compared. The difference was tested for significance using the Wilcoxon sign rank test. MAIN RESULTS: In left and right ears, both sitting and supine, application of the pulse correction significantly reduced the intra-subject variability of V m (p value range 4.0 × 10-27 to 2.0 × 10-31). The average improvement was from 98 ± 6 nl to 56 ± 4 nl. SIGNIFICANCE: The pulse subtraction technique substantially improves the repeatability of evoked TMD measurements. This justifies further investigations to assess the use of TMD measurements in clinical applications where non-invasive tracking of changes in ICP would be useful.

Erroneous intracranial pressure measurements from simultaneous pressure monitoring and ventricular drainage catheters.

The objective of this report is to highlight the potential for false pressure measurements from systems that combine intracranial pressure (ICP) measurement and ventricular drainage. If the ports of the drain become blocked to the extent that they present a high resistance to cerebrospinal fluid flow, then a significant pressure gradient between the inside and outside of the catheter may be established. Thus, any intracatheter transducer will faithfully record a pressure much lower than true ICP. This holds true for catheter-tip transducers when the transducer lies inside the catheter. In the absence of flow, however, pressures will equalize; therefore, accurate measurements may be taken if the drain is temporarily closed. We model this situation and provide simulations of expected measurements in such situations; these compare well to observed clinical readings.

Comparison of caffeine-induced changes in cerebral blood flow and middle cerebral artery blood velocity shows that caffeine reduces middle cerebral artery diameter.

Changes in cerebral blood flow (CBF) can be assessed directly with xenon clearance (XeC) or indirectly by measuring changes in middle cerebral artery blood velocity (Vmca) with transcranial Doppler (TCD). The aim of this study was to compare the changes in CBF and Vmca following caffeine ingestion. Nineteen patients (age 48-86, recovering from an acute stroke) and ten controls (age 52-85) were each studied twice. Bilateral measurements of CBF and Vmca were made before and after ingestion of 250 mg caffeine or matched placebo. The percentage change in CBF and Vmca after caffeine was calculated. Full results (CBF and Vmca) were obtained from 14 patients and 9 controls. There was no significant difference between patients and controls, so results were combined. Caffeine reduced CBF by 22% (95% confidence interval (CI) = 17% to 28%) and reduced Vmca by 13% (95% CI = 10% to 17%). The fall in Vmca was significantly less than that in CBF (p = 0.0016), showing that caffeine reduces mca diameter. Analysis based on Poiseuille flow in the arterioles suggests that caffeine reduced arteriole diameter by 5.9% (95% CI = 4.6% to 7.3%) and mca diameter by 4.3% (95% CI = 2.0% to 6.6%). TCD is being used as an alternative to XeC for assessing the effect of vasoconstrictors and vasodilators on CBF. This study has demonstrated that in mca diameter can be changed by the vasoactive agents, and that changes in Vmca do not necessarily reflect changes in CBF.

Do the Finapres and Colin radial artery tonometer measure the same blood pressure changes following deflation of thigh cuffs?

The objective of this work was to determine if systematic differences exist between blood pressure time series measured by two non-invasive techniques. Cerebral blood flow autoregulation is often measured while a change in blood pressure is induced by deflation of thigh cuffs. To interpret the result a continuous measurement of arterial blood pressure is required. The Finapres is a non-invasive blood pressure monitor that is often used when assessing autoregulation but there is uncertainty about its reliability. A more recent device, the Colin tonometer, uses radial artery tonometry, which may prove to be a more reliable non-invasive method of obtaining a blood pressure waveform. Twenty healthy volunteers were recruited; blood pressure trends following cuff deflation were measured simultaneously with a Finapres and a Colin tonometer. A significant difference was found between the blood pressure waveforms measured with the two devices. The most striking difference was a slower recovery time measured with the Finapres (Wilcoxon signed rank test P < 0.001). Peripheral vasoaction may be distorting the measurement of blood pressure by the Finapres. This would account for the difference that exists between the techniques. Comparison with a direct arterial line would confirm which non-invasive measurement is more accurate.

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.

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.

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.

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.

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.

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.

Changing renal blood flow following sodium nitroprusside in patients undergoing nephrolithotomy.

During nephrolithotomy in 23 adult patients under sodium thiopental-N2O-O2-fentanyl anesthesia, renal blood flow to one kidney was studied. Muscle relaxation was obtained with pancuronium, and renal blood flow (RBF) was measured with an electromagnetic flow probe. Percutaneous arterial pressures were also recorded. Administration of sodium nitroprusside caused a decrease in RBF in all patients when mean arterial pressure (MAP) was reduced approximately 44 percent below baseline. Patients whose initial RBF was below 300 ml/min showed greater falls in RBF with hypotension. Changes in renal vascular resistance (MAP/RBF) was found to be markedly higher in the "low-flow" group. As expected, the low-flow group started with a higher renal vascular resistance. From these observations, it is felt that more concern should be exercised when planning deliberate hypotension with nitroprusside, especially in patients with decreased renal function.

Hypertension and decreased renal blood flow following methylene blue injection.

Renal blood flow (RBF) and arterial blood pressure (BP) were monitored in 12 patients undergoing nephrolithotomy in the lateral flexed position. All patients were preoxygenated and were anesthetized with sodium thiopental, N2O, O2, and fentanyl. Maintenance relaxation was obtained with pancuronium bromide. Arterial pressure was monitored by percutaneous arterial catheter. Following exposure of the kidney and renal pedicle, an electromagnetic flow probe was attached to the renal artery and baseline flows recorded. Following baseline measurements, 20 ml of 1 percent methylene blue was given intravenously. All patients studied showed an immediate rise in BP, and 11/12 showed a simultaneous decrease in RBF. The average fall in RBF was 35 percent at one minute. Both parameters returned to normal values on the average in 177 seconds. The decreased RBF appeared to be part of a generalized vasoconstrition caused either by sympathetic reflexes or by the direct action of methylene blue.

Anesthesia considerations during nephrolithotomy with slush.

A retrospective study was made of 113 patients who underwent nephrolithotomy between 1962 and 1973. Multiple parameters from a surgical and anesthesia viewpoint were tabulated. The main findings were a high incidence of pulmonary complications (37 per cent), a general lowering of body temperature during anesthesia and operation and initial decreases in blood pressure, apparently related to the use of d-tubocurarine. Currently, prospective studies are underway to more clearly delineate these problems and perhaps find ways to eliminate them.