Cerebral Blood Flow and Cerebral Tissue Oxygen Responses to Inspiratory Resistance Breathing During Hypoxia

Abstract

Increasing resistance to inspiration further decreases intrathoracic and intracranial pressures, leading to increases in venous return, stroke volume, and consequently, arterial pressure and cerebral perfusion pressure. Accordingly, inspiratory resistance breathing may be of therapeutic interest for clinical conditions associated with decreased delivery of blood and oxygen to the brain. Previous studies in healthy humans have investigated the effect of inspiratory resistance breathing on the cerebral circulation, by studying blood velocity within intracranial vessels (via transcranial Doppler (TCD) ultrasound), with equivocal results. The aim of this study was to investigate the effect of inspiratory resistance breathing on blood flow through the internal carotid artery (ICA), and cerebral tissue oxygenation during hypoxia. We hypothesized that inspiratory resistance breathing would increase ICA blood flow, and improve cerebral tissue oxygenation during a mild hypoxic stimulus.MethodsSix healthy human subjects (5M/1F) completed two 10‐min protocols in randomized order: 1) breathing a hypoxic gas mix (16% O2, balance N2), and 2) breathing a hypoxic gas mix (16% O2, balance N2) with inspiratory resistance of −7 cmH2O. ICA blood flow was derived from simultaneous measurements of blood velocity and diameter measured by duplex ultrasound, middle cerebral artery velocity (MCAv) was recorded continuously by TCD ultrasound, and cerebral tissue oxygenation (ScO2) was recorded by near‐infrared spectroscopy. Arterial oxygen saturation (SpO2) was measured by pulse oximetry, and end‐tidal O2 (etO2) was measured by a gas analyzer. Data were analyzed from the last 1‐min of baseline and each protocol with a linear mixed model.ResultsThe hypoxic stimulus decreased SpO2 and etO2 (P<0.0001), and inspiratory resistance breathing did not protect against these responses (P≥0.21). ICA diameter increased with hypoxia under both conditions (P=0.002), regardless of resistance breathing (P=0.56). ICA velocity (23.6 ± 1.6 cm/s vs. 25.7 ± 1.1 cm/s) and flow (298.8 ± 36.6 ml/min vs. 328.8 ± 39.5 ml/min) were lower with hypoxia and inspiratory resistance breathing compared with hypoxia alone (P≤0.07), while mean MCAv was not affected by either hypoxia or resistance breathing (P≥0.15). ScO2 decreased under both conditions (P=0.0009; hypoxia: −2.7 ± 0.9 %; hypoxia + resistance breathing: −3.6 ± 0.8 %), with no effect of inspiratory resistance breathing (P=0.90).ConclusionThese preliminary data demonstrate that, contrary to our hypothesis, inspiratory resistance breathing did not increase ICA blood flow, nor improve cerebral tissue oxygenation during a mild hypoxic stimulus. Rather, ICA blood flow decreased with inspiratory resistance breathing during hypoxia, primarily due to a reduction in ICA blood velocity. Further studies are required to elucidate whether inspiratory resistance breathing may be of therapeutic interest for clinical conditions affecting cerebral blood flow and tissue oxygenation.