Physiological impact of continuous positive airway pressure (CPAP) on total cerebral blood flow in healthy awake volunteers (ultrasound study)

Figure 1. Transverse section through the spine. The abdominal pressure (black arrows) is transmitted along the veins (white arrows) and is reflected in the CSF which is represented as clear, unshaded region around the cord (figure and caption reprinted from Lockey et al.) (Lockey, Poots et al. 1975).
Figure 1. Anatomy of the dural venous sinuses.


The problem

Continuous positive airway pressure (CPAP) is used for the treatment of sleep apnea patients, acting as a pneumatic splint to prevent collapse of the pharyngeal airway by increasing intrathoracic pressure (Bowie, O'Connor et al. 2001). Communication between intrathoracic and intracranial pressure (ICP) through the venous system has been established (Williams 1981; Williams 1981; Heiss, Patronas et al. 1999; Sansur, Heiss et al. 2003) (Figure 1), which are linked with changes in intracranial compliance (Feldman, Robertson et al. 1997) and cardiac output (Feldman, Robertson et al. 1997; Zaramella, Freato et al. 2006). However, the influence CPAP has on cerebral blood flow (CBF) dynamics is not well understood.

Some studies have quantified CBF velocity using transcranial Doppler ultrasound in the cerebral middle cerebral artery during CPAP (Werner, Kochs et al. 1990; Haring, Hormann et al. 1994; Bowie, O'Connor et al. 2001; Scala, Turkington et al. 2003). However, TCD measurements do not take into account changes in artery diameter which can occur, however minute, resulting from CBF autoregulation. Thus, the precise blood flow rate (ml/s) in the vessels of interest was not obtained, and total CBF was not measured. The proposed study investigates the influence of CPAP on total CBF by measuring velocity, rate, and lumen diameter in the internal carotid and vertebral arteries, and velocity in the middle cerebral arteries using duplex Doppler ultrasound with and without CPAP.

Hypothesis and research objectives

We hypothesize that an increase in CPAP will decrease CBF rate in healthy volunteers. The goal of this research is thus to improve understanding of the influence of CPAP on CBF dynamics in healthy individuals.

Methods and study outline

Figure 3. Flow velocity (a) and vessel diameter (b) measurement at the right internal carotid artery. Flow velocity (c) and vessel diameter (d) measurement at the left vertebral artery.

In vivo Ultrasound measurements. Our approach is to obtain CBF rate measurements before and during CPAP using duplex color Doppler ultrasound on 20 age-matched healthy non smoking volunteers (20-30 yrs.). Each subject will have blood velocity and lumen diameter in the internal carotid and vertebral arteries, and velocity in the middle cerebral arteries, measured with duplex Doppler ultrasound (Soustiel, Glenn et al. 2005). Velocity and diameter measurements will be conducted at least three times at each measurement location. These measurements will be obtained with subjects in the supine position with the head at 30 degrees at atmospheric pressure and after CPAP has been administered for 15 minutes at +15 cm H2O. Transcutaneous carbon dioxide tension PCO2 and oxygen saturation SO2 will be monitored during the entire testing period for each subject. Heart rate and blood pressure will be recorded before and after each test. Pressure test order will be randomized for each subject. Measurement reproducibility will be examined by performing the same measurements three times on three subjects.

Expected results and potential impact

The proposed work will improve understanding of how CBF in healthy volunteers is influenced by CPAP. The test could be further explored to investigate how CPAP influences CBF in patients with sleep apnea and various types of cerebrovascular and craniospinal disorders such as stroke and hydrocephalus.

Preliminary results

File:Data cpap-us.xls

Figure 4. Total cerebral blood flow measured without CPAP (CPAP0) and with CPAP administered at 15 cm H2O (CPAP15). Results from two trials conducted on the same healthy volunteer on two different days.

Measurement scheduling


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