The effect of continuous positive airway pressure on total cerebral blood flow in healthy awake volunteers

Impact of Altered Airway Pressure on Intracranial Pressure, Perfusion, and Oxygenation: A Narrative Review

OBJECTIVES

A narrative review of the pathophysiology linking altered airway pressure and intracranial pressure and cerebral oxygenation.

DATA SOURCES

Online search of PubMed and manual review of articles (laboratory and patient studies) of the altered airway pressure on intracranial pressure, cerebral perfusion, or cerebral oxygenation.

STUDY SELECTION

Randomized trials, observational and physiologic studies.

DATA EXTRACTION

Our group determined by consensus which resources would best inform this review.

DATA SYNTHESIS

In the normal brain, positive-pressure ventilation does not significantly alter intracranial pressure, cerebral oxygenation, or perfusion. In injured brains, the impact of airway pressure on intracranial pressure is variable and determined by several factors; a cerebral venous Starling resistor explains much of the variability. Negative-pressure ventilation can improve cerebral perfusion and oxygenation and reduce intracranial pressure in experimental models, but data are limited, and mechanisms and clinical benefit remain uncertain.

CONCLUSIONS

The effects of airway pressure and ventilation on cerebral perfusion and oxygenation are increasingly understood, especially in the setting of brain injury. In the face of competing mechanisms and priorities, multimodal monitoring and individualized titration will increasingly be required to optimize care.

Combined lung and brain ultrasonography for an individualized "brain-protective ventilation strategy" in neurocritical care patients with challenging ventilation needs

When intracranial hypertension and severe lung damage coexist in the same clinical scenario, their management poses a difficult challenge, especially as concerns mechanical ventilation management. The needs of combined lung and brain protection from secondary damage may conflict, as ventilation strategies commonly used in patients with ARDS are potentially associated with an increased risk of intracranial hypertension. In particular, the use of positive end-expiratory pressure, recruitment maneuvers, prone positioning, and protective lung ventilation can have undesirable effects on cerebral physiology: they may positively or negatively affect intracranial pressure, based on the final repercussions on PaO₂ and cerebral perfusion pressure (through changes in cardiac output, mean arterial pressure, venous return, PaO₂ and PaCO₂), also according to the baseline conditions of cerebral autoregulation. Lung ultrasound (LUS) and brain ultrasound (BUS, as a combination of optic nerve sheath diameter assessment and cerebrovascular Doppler ultrasound) have independently proven their potential in respectively monitoring lung aeration and brain physiology at the bedside. In this narrative review, we describe how the combined use of LUS and BUS on neurocritical patients with demanding mechanical ventilation needs can contribute to ventilation management, with the aim of a tailored "brain-protective ventilation strategy."

Effects of Prone Position and Positive End-Expiratory Pressure on Noninvasive Estimators of ICP: A Pilot Study

BACKGROUND

Prone positioning and positive end-expiratory pressure can improve pulmonary gas exchange and respiratory mechanics. However, they may be associated with the development of intracranial hypertension. Intracranial pressure (ICP) can be noninvasively estimated from the sonographic measurement of the optic nerve sheath diameter (ONSD) and from the transcranial Doppler analysis of the pulsatility (ICPPI) and the diastolic component (ICPFVd) of the velocity waveform.

METHODS

The effect of the prone positioning and positive end-expiratory pressure on ONSD, ICPFVd, and ICPPI was assessed in a prospective study of 30 patients undergoing spine surgery. One-way repeated measures analysis of variance, fixed-effect multivariate regression models, and receiver operating characteristic analyses were used to analyze numerical data.

RESULTS

The mean values of ONSD, ICPFVd, and ICPPI significantly increased after change from supine to prone position. Receiver operating characteristic analyses demonstrated that, among the noninvasive methods, the mean ONSD measure had the greatest area under the curve signifying it is the most effective in distinguishing a hypothetical change in ICP between supine and prone positioning (0.86±0.034 [0.79 to 0.92]). A cutoff of 0.43 cm was found to be a best separator of ONSD value between supine and prone with a specificity of 75.0 and a sensitivity of 86.7.

CONCLUSIONS

Noninvasive ICP estimation may be useful in patients at risk of developing intracranial hypertension who require prone positioning.

Cerebral hemodynamic changes in obstructive sleep apnea syndrome after continuous positive airway pressure treatment

BACKGROUND

Patients with obstructive sleep apnea syndrome (OSAS) are at increased risk for cerebrovascular diseases. The underlying mechanisms remain obscure. It may occur through a reduction in cerebral vascular reactivity. Continuous positive airway pressure (CPAP) is effective in reducing the occurrence of apneas. We hypothesized that treatment with CPAP improves cerebral vascular reactivity.

METHODS

This is a prospective study with OSAS patients. The apnea test (ApT) was calculated as an increase of mean artery velocity during apnea: [Artery velocity in apnea minus Resting artery velocity]/Resting artery velocity expressed as percentage. After 2 years of CPAP treatment, the test was repeated.

RESULTS

Seventy-six patients represented the study pool. After 2 years of treatment with CPAP, we were able to conduct a reassessment in 65 patients. Of the 65 patients who finished the clinical study, 56 were men, and 9 were women, with an average age of 48.1 ± 10.4 years. There was an improvement in the ApT after CPAP treatment (30.8 ± 12.1 vs 39.8 ± 15.1; p:0.000). The values of cerebral blood flow velocities, diastolic blood pressure in apnea, and basal heart rate decreased.

CONCLUSIONS

Cerebral vascular reactivity in OSAS patients measured by ApT improved after 2 years of CPAP.

The relationship between cephalometric carotid artery calcification and Framingham Risk Score profile in patients with obstructive sleep apnea

PURPOSE

The morbidity rate of arteriosclerosis becomes clinically manifested as acute cardiovascular events. In the progress of atherosclerosis, the carotid artery calcifies and sometimes appears as a calcified mass on a cephalometric radiograph. This study was designed to evaluate cardiovascular risks according to the Framingham Risk Score (FRS) between subjects with and without visible carotid artery calcification on a cephalogram.

METHODS

Subjects diagnosed with obstructive sleep apnea (OSA) were divided into two groups according to whether or not calcification was visible on a cephalometric radiograph in the carotid artery area, and the characteristic differences between the two groups were analyzed. The evaluated variables included age, BMI, apnea-hypopnea index (AHI), SpO2, ESS, blood pressure, medication history, diabetes mellitus (DM), drinking, smoking, and lipid-related measurements. FRSs for stroke, general cardiovascular disease (GCD), and coronary heart disease (CHD) were calculated. Statistical analyses were performed (SPSS 18.0) with significance defined as a two-tailed p value less than 0.05.

RESULTS

A total of 811 subjects completed the data collection (727 males, age 53.0 ± 12.5 years, AHI 31.7 ± 22.6, times/h). From FRSs, probabilities of a GCD, stroke, and CHD within 10 years were 16.0 ± 9.7, 9.8 ± 6.7, and 11.9 ± 8.3 %, respectively. Some 84 subjects exhibited calcification in the carotid arterial area. Calcification subjects were higher GCD risk and older than subjects who had no identified calcification (20.3 ± 10.1 vs 15.6 ± 20.3 %, p = 0.013, 58.8 ± 11.4 vs. 52.3 ± 12.5 years, p < 0.001). Although there is no significant difference in OSA-related variables and FRSs, subjects with visible calcifications have higher prevalence of high blood pressure medication and DM (p < 0.01).

CONCLUSION

While the presence of a calcified mass on a cephalometric radiograph is not diagnostic of atherosclerosis, this information indicates some cardiovascular risk.