Intracranial pressure monitoring: modeling cerebrovascular pressure transmission.

OBJECTIVES: To examine changes in cerebrovascular pressure transmission derived from arterial blood pressure (ABP) and intracranial pressure (ICP) recordings by autoregressive moving average modeling technique. METHODS: Digitized ICP and ABP recordings were obtained from patients with brain injury. Two groups were defined: Group A with 4 patients who demonstrated plateau waves, and Group B with 4 intracranial hypertensive, hypoperfused patients. For each 16.5 s interval, mean values of ICP, ABP, cerebral perfusion pressure (CPP), and corresponding highest modal frequency (HMF) of cerebrovascular pressure transmission were computed. RESULTS: Mean values of CPP and HMF of 56.2 mmHg and 2.0 Hz for Group A were significantly higher (p < 0.005) than corresponding mean values of 31.9 mmHg and 0.744 Hz for Group B. The mean value of the slope of the regression line between HMF and CPP for group A of -0.034 Hz/mmHg was significantly different (p < 0.025) than the mean value of 0.0077 Hz/mmHg for Group B. Computations of HMF, pressure reactivity, and correlation pressure reactivity index on continuous pressure recordings are illustrated. CONCLUSIONS: Values of HMF of cerebrovascular pressure transmission are inversely related to CPP when pressure regulation is thought to be intact, and directly related when regulation is likely lost.

Plateau waves: changes of cerebrovascular pressure transmission.

OBJECTIVE: To test the validity of the hypothesis that active vasodilatation and vasoconstriction underlie the occurrence of intracranial pressure (ICP) plateau waves by evaluating corresponding changes of cerebrovascular pressure transmission of arterial blood pressure (ABP) to ICP. METHODS: Digitized recordings of ICP and ABP sampled at 30 Hz were obtained from nine patients with traumatic brain injury. For each 16.5 s recording interval mean values of ICP, ABP, cerebral perfusion pressure (CPP), and the corresponding highest modal frequency (HMF) of cerebrovascular pressure transmission were calculated. RESULTS: Mean ICP and HMF significantly increased (P < 0.003) and mean CPP decreased significantly (P < 0.00036) at onset of the wave. Conversely at termination, mean ICP and HMF significantly decreased (P < 0.026) and mean CPP significantly increased (P < 0.028). In addition, the strong negative correlations between mean ICP and mean CPP (r = -0.87) and mean HMF and CPP (r = -0.87) were demonstrated. CONCLUSION: The findings that HMF increased at onset and decreased at the termination of plateau wave support the validity of the vasodilatatory/constriction cascade model that postulates active vasodilation at the onset and active vasoconstriction of the cerebrovascular bed at the termination of a plateau wave.

Cyclic variation of cerebral pial arteriolar diameter synchronized with positive-pressure inhalation.

Laboratory observations have noted that during normocapnia and intact vascular tone, changes in the intracranial pressure (ICP) and arterial blood pressure (ABP) recordings are not similar. The purpose of the present study was to determine whether: 1) the diameter of cerebral pial arterioles synchronously increases during positive pressure inhalation and decreases during the expiration phase of ventilation; and 2) the variation in arteriolar diameter is greater when vascular tone is intact than when arterioles are maximally dilated. Severe hypercapnia was induced by prolonged ventilation with gas mixture of 20% O2, 10% CO2, and 70% N2 in 8 piglets. S-VHS recordings of 3600X magnification of nine pial arterioles were obtained during conditions of normal vascular tone and severe hypercapnia and digitally analyzed by an image-averaging method. Variation of the amplitude of arteriolar diameter over the ventilation cycle was computed and found to synchronously increase during positive pressure inhalation and to decrease to a steady state baseline during passive expiration. Mean amplitude of variation in diameter (+/- S.D.) during normal tone and hypercapnia was computed as 1.63 (+/- 0.693) microns and 1.012 (+/- 0.869) microns respectively and found to be significantly greater during the condition of normal vascular tone than during hypercapnia (p < .006). These results suggest that a cyclic variation of arteriolar blood volume is one causal factor related to the low frequency variation of the baseline of ICP associated with positive pressure ventilation.

Spectral characteristics of B-waves and other low-frequency activity.

Low frequency oscillations of intracranial pressure (ICP) between 0.5 to 2.0 cycles/min have been termed B-waves. While such low frequency activity may be generated by cerebral vasomotor activity, activity in the B-wave band may also be related to other causes. The objective of this study is to: 1) describe the low frequency characteristics of ICP and arterial blood pressure (ABP) recordings with a mathematical model based on ventilator-induced intrathoracic pressure (ITP) modulation of arterial blood pressure; and 2) use the model to eliminate low frequency activity within the B-wave range unrelated to vasomotor activity. The model describes the frequency locations of spectra associated with ventilation about both the principal and first harmonic cardiac frequency to within 1% error. Spectra within the B-wave range harmonically related to the ventilator frequency were classified as caused by patient-ventilator interaction. Of 11 patients with severe head-injury, 64% (7/11) demonstrated B-wave activity, and 4 patients demonstrated the activity solely in the ICP recording. Variation of heart rate did not correspond to the occurrence of B-waves. The proposed model describes the low frequency spectra found within ABP and ICP recordings and can be used to eliminate spectra with the B-wave range related to ventilation.

Variation of proposed correlation indices of cerebrovascular reactivity with change of arteriolar diameter.

The purpose of this study was to examine the relationship between proposed correlation indices of cerebrovascular reserve and corresponding changes of cerebrovascular reserve as measured by changes of pial arteriolar diameter. Mild and severe physiologic challenge was produced in piglets by appropriate ventilation with foreign gas mixture and altered alveolar ventilation. Intracranial pressure (ICP), arterial blood pressure (ABP), and video micrometer recordings of pial arteriolar diameter were made. Serial values of ICP and ABP were used to compute the Correlation Coefficient index (Corrx) and the Pressure Reactivity Index (PrX). For the 10 mild physiologic challenge experiments, correlations between percent change index (% delta Corrx and % delta PrX) and percent change of pial arteriolar diameter (% delta dia.) induced by mild challenges (n = 40) were 0.51 (p < .005) and 0.097 (p n.s.) respectively. For 8 asphyxia experiments, serial values with respect to time of the correlation indices were correlated with % delta dia. obtained before, during induction and recovery. The grand mean (n = 8) correlation values (+/- S.D.) of the Corrx and PrX were 0.76 (+/- 0.18, p < .025) and 0.21 (+/- 0.38, p n.s.) respectively. In contrast to the PrX index, changes of the Corrx index significantly correlated with changes of pial arteriolar diameter. However, Corrx and PrX were simultaneously high only during a state of maximum dilation. These findings suggest that sudden salient increases of the Corrx and PrX indices induced by physiologic challenge are indicative of vigorous dilatory response of the cerebral arterioles and loss of cerebrovascular reserve.

Use of resistance-area product derived from Doppler MCA velocity to estimate the range of active cerebrovascular regulation.

The purpose of the present study was to explore the development of a method to estimate the range of active cerebrovascular regulation using resistance-area product (RAP) derived from transcranial Doppler ultrasound recordings of middle cerebral artery blood velocity (MCAV) obtained before and after physiologic challenge. Intracranial pressure (ICP), arterial blood pressure (ABP), and MCAV were monitored on 9 patients before and after challenges with either Noradrenlin or CO2 inhalation. One patient was monitored at 24, 48, 72, and 96 hrs. For each cardiac cycle a value of RAP was computed from the inverse of the slope of the regression line of paired values of aligned MCAV and ABP recordings. Mean values of percent change of difference of RAP (% delta RAP) versus change of cerebral perfusion pressure (CPP) were computed. Theoretical values of % delta RAP were derived from reported lab values of percent change diameter of arterioles versus ABP. Of the nine patients, 22% (2/9) demonstrated either impaired dilation and/or constriction. Furthermore, the correlation between theoretical and experimental values of % delta RAP and CPP (n = 34) was strong (r = .86, p < .005). The serially-monitored patient failed to dilate during all CO2 challenges and to fully constrict to the Noradrenlin challenge at 72 hrs. % delta RAP associated with corresponding changes of CPP may have value as a clinical method to estimate the range of active cerebrovascular regulation.

An estimated compliance index derived from intracranial pressure recording.

The purpose of this study is to test the validity of a proposed compliance index, percent change of compliance per mmHg of intracranial pressure (%CC/mmHg), by comparison of values of the index with corresponding experimentally derived values of compliance. The derivation of %CC/mmHg is based on two assumptions: 1) the pressure-volume characteristic of the craniospinal sac can be linearized for small perturbations about an equilibrium; and 2) during a brief interval in which these perturbations occur the pathophysiologic state of the sac does not significantly change. By rapid infusion of 0.2 ml of mock cerebrospinal fluid (CSF) experimental values of compliance (n = 44) were obtained from 10 piglets during monitoring of intracranial pressure (ICP). A strong correlation (r = 0.89, p < .001, n = 44) was obtained between values of %CC/mmHg and corresponding values of experimentally derived compliance (ml/mmHg). The value of the proposed index of compliance, %CC/mmHg, needs to be evaluated in the clinical setting. However, from the view of validation of ICP instrumentation, high values of both mean ICP and mean %CC/mmHg are contradictory and indicative of the occurrence of instrumentation error due to electronic direct current drift.

NIRS: dose dependency of local changes of cerebral HbO2 and Hb with pCO2 in parietal cortex.

Regional cerebral oxygenated hemoglobin and total hemoglobin increased systematically with increasing depth of hypercapnia, but the concentration of deoxygenated hemoglobin remained relatively constant. Relative mean changes of oxygenated and total hemoglobin increased nearly linearly, corresponding to the characteristic increase of the cerebral vascular dilation with increasing depth of hypercapnia.

Correlation coefficient between intracranial and arterial pressures: a gauge of cerebral vascular dilation.

With the use of a laboratory model, arterial and intracranial pressure signals were obtained under conditions of varying depths of hypercapnia ranging from normocapnia to deep hypercapnia. Also, with the use of a closed cranial window, measures of cerebral arteriolar diameter and estimates of cerebral venous flow were obtained. The correlation of the intracranial and arterial pressure signals, arteriolar diameter, and estimates of venous flow exhibit a dose-dependent characteristic by increasing monotonically with increasing progressive states of increasing hypercapnia. These results indicate that the correlation between intracranial and arterial pressure signals provides an estimate of the gauge of the cerebral vasculature.

Factor V deficiency and menstruation: a gynecologic challenge.

BACKGROUND: Menorrhagia is a well-known complication of factor V deficiency. CASE: A 13-year-old girl with factor V deficiency presented with acute hemorrhage on day 4 of menses. Laboratory assessment revealed a hemoglobin of 36 g/L. The ultrasound appearance of a large loculated cystic mass was consistent with substantial intraperitoneal bleeding. Stabilization with blood products was followed by GnRH agonist therapy. Medical management was instituted effectively. Continued compliance with medication is essential to prevent recurrences. CONCLUSION: Factor V deficiency is rare. Coagulation disorders of this nature pose a challenge to gynecologists involved in patient management at the time of menses.

Detection of loss of cerebral vascular tone by correlation of arterial and intracranial pressure signals.

With the use of a laboratory model, arterial and intracranial pressure signals were obtained under conditions of intact regulation of cerebral blood flow and massive dilation. During elevated intracranial pressure and intact regulation, positive pressure inhalation appears to briefly occlude venous flow into the cranial sinuses during inspiration. As a result, the intracranial pressure and arterial pressure signals are not similar. In contrast, when maximal dilation causes failure of regulation of cerebral blood flow, the intracranial pressure signal is approximately proportional to the arterial pressure signal. Comparison of the cross-correlation function derived from the intracranial and arterial pressure signals to the autocorrelation function of the arterial signal reveals that the two correlation functions are: 1) different during intact regulation and 2) nearly identical during dilation induced failure of regulation of cerebral blood flow.

A model for the apparent decrease in optical transmittance of the diabetic eye.

This paper compares observed changes of ocular transmittance at short and long wavelengths in diabetic patients with values predicted by a model based on the Rayleigh light scattering properties of albumin. Selective chromatic adaptation was used to obtain critical flicker fusion (CFF) frequency thresholds from 21 subjects and 18 patients with insulin-dependent diabetes. The Ferry-Porter characteristic of each color-sensitive mechanism of each patient was compared to age-specific control values. For those eyes without an indication of neural injury, changes in optical density associated with the red- and blue-sensitive mechanisms were calculated and adjusted to reflect accelerated yellowing of the lens produced by increased duration of diabetes. The range of concentration of glycosylated albumin required to fit the model to the adjusted short-wavelength changes in optical density was determined and used to calculate the theoretical long-wavelength changes in optical density. The experimentally derived long-wavelength changes in optical density fell within the 95% confidence level of the values described by the model. These results support the premise that the apparent decrease in optical transmittance observed in patients with diabetes mellitus is caused by light scattering produced by dilute increase of plasma proteins within the retina.

Early loss of blue-sensitive color vision in patients with type I diabetes.

Existing methods for early detection of ocular injury from diabetes have serious limitations. We describe a new method, measuring visual flicker discrimination of the blue-sensitive mechanism of vision. This method is noninvasive, quantitative, and capable of distinguishing two types of impairment. Blue-flicker discrimination was measured in 10 adults with type I (insulin-dependent) diabetes for less than 5 yr. Although no evidence of diabetic changes was detected by careful ophthalmic examination by an experienced ophthalmologist, 12 of 19 eyes (63%) had flicker discrimination scores considered abnormal in comparison with those of a control group, and 8 of 10 subjects (80%) had at least 1 eye with abnormal performance. In all but 2 abnormal eyes the deficit of blue-flicker discrimination was of the "absorptive" type, suggesting increased absorbance or scattering of blue light in the optical media. These data show that a functional impairment of vision can be measured very early in the course of type I diabetes, before visible retinopathy is present, and suggest this test procedure may have both investigative and clinical applications.

Impaired visual flicker discrimination with hypothalamic pituitary disease.

In a group of 19 patients with pituitary dysfunction, we assessed peripheral visual fields and flicker discrimination. Two subgroups of patients were defined: (1) nonsurgical (n = 6), and (2) surgical (n = 13). For the nonsurgical group, 16.7% (2/12) of the eyes tested had impaired flicker discrimination, whereas 33% (4/12) had a deficit in visual field. For the surgical group, none of the patients had a deficit of visual field, whereas 60% (15/25) of the eyes tested had an alteration of flicker discrimination. For this latter group, abnormalities of the flicker fusion threshold were present several months to several years after surgery.

Fluctuation of intracranial pressure associated with the cardiac cycle.

Within the intensive care setting, a portable microcomputer system was used to extract three parameters from the intracranial pressure fluctuation associated with the cardiac cycle. One parameter, the mean of sampled intracranial pressure, was defined as the average value of pressure for a 1.08-second interval following the R wave of the electrocardiogram. Another parameter, the amplitude of intracranial pressure, was defined as the difference between the mean and the peak value of the sampled intracranial pressure for the interval considered. The third parameter, a latent interval, was defined as the time period between the occurrence of the R wave and the occurrence of the peak value of the subsequent intracranial pressure fluctuation. Six adults and one pediatric patient were monitored. Both the amplitude and the mean of sampled pressure tended to vary inversely with the latent interval. For the adult patients, the latent interval varied between 503 and 804 ms; the mean pressure ranged between 2.4 and 19.0 mm Hg and the amplitude pressure ranged between 0.6 and 7.2 mm Hg. The latent interval for the child was much shorter (ranging between 269 and 325 ms), and both the mean and the amplitude pressures were much higher (ranging between 38.4 and 57.3 mm Hg and 14.2 and 16.5 mm Hg, respectively). Statistical correlation between hourly pulse rates and the latent interval among the adult cases revealed little association (r = -0.20). For all patients considered, the correlation between the amplitude and the mean of sampled intracranial pressure was quite high, with an r value of +0.91. These reported observations support a conceptual model in which blood volume changes associated with the cardiac cycle occurring within the semirigid craniospinal sac are assumed to underlie the fluctuation of intracranial pressure.

Flicker fusion thresholds in multiple sclerosis. A functional measure of neurological damage.

Patients with multiple sclerosis (MS) have been shown to have a measurable deterioration in the ability to perceive temporal variations of light. Compared to the traditional critical flicker fusion (CFF) measure, a modified flicker fusion test showed an improved sensitivity for denoting neurological deficit in temporal vision. One hundred twenty-two patients with MS were examined. The traditional CFF test exhibited abnormalities in 48%; the modified test exhibited abnormalities in 78% of the same patients. These results appear to be independent of whether or not the demyelinating disease clinically involved the visual system. Also, the percentage of abnormalities demonstrated by combining both techniques was unexpectedly high (60% to 80%) in patients with minimal evidence of neurological involvement.