Assessing central and peripheral respiratory chemoreceptor interaction in humans

NEW FINDINGS

What is the central question of this study? We investigated the interaction between central and peripheral respiratory chemoreceptors in healthy, awake human participants by (a) using a background of step increases in steady-state normoxic fraction of inspired carbon dioxide to alter central chemoreceptor activation and (b) using the transient hypoxia test to target the peripheral chemoreceptors. What is the main finding and its importance? Our data suggests that the central-peripheral respiratory chemoreceptor interaction is additive in minute ventilation and respiratory rate, but hypoadditive in tidal volume. Our study adds important new data in reconciling chemoreceptor interaction in awake healthy humans, and is consistent with previous reports of simple addition in intact rodents and humans.

ABSTRACT

Arterial blood gas levels are maintained through respiratory chemoreflexes, mediated by central (CCR) in the CNS and peripheral (PCR) chemoreceptors located in the carotid bodies. The interaction between central and peripheral chemoreceptors is controversial, and few studies have investigated this interaction in awake healthy humans, in part due to methodological challenges. We investigated the interaction between the CCRs and PCRs in healthy humans using a transient hypoxia test (three consecutive breaths of 100% N₂ ; TT-HVR), which targets the stimulus and temporal domain specificity of the PCRs. TT-HVRs were superimposed upon three randomized background levels of steady-state inspired fraction of normoxic CO₂ (F_I CO₂ ; 0, 0.02 and 0.04). Chemostimuli (calculated oxygen saturation; ScO₂ ) and respiratory variable responses (respiratory rate, inspired tidal volume and ventilation; R_R , V_TI , V̇_I ), were averaged from all three TT-HVR trials at each F_I CO₂ level. Responses were assessed as (a) a change from BL (delta; ∆) and (b) indexed against ∆ScO₂ . Aside from a significantly lower ∆V_TI response in 0.04 F_I CO₂ (P = 0.01), the hypoxic rate responses (∆R_R or ∆R_R /∆ScO₂ ; P = 0.46, P = 0.81), hypoxic tidal volume response (∆V_TI /∆ScO₂ ; P = 0.08) and the hypoxic ventilatory responses (∆V̇_I and (∆V̇_I /∆ScO₂ ; P = 0.09 and P = 0.31) were not significantly different across F_I CO₂ trials. Our data suggests simple addition between central and peripheral chemoreceptors in V̇_I , which is mediated through simple addition in R_R responses, but hypo-addition in V_TI responses. Our study adds important new data in reconciling chemoreceptor interaction in awake healthy humans, and is consistent with previous reports of simple addition in intact rodents and humans. This article is protected by copyright. All rights reserved.

Cerebrovascular and blood pressure responses during voluntary apneas are larger than rebreathing

Both voluntary rebreathing (RB) of expired air and voluntary apneas (VA) elicit changes in arterial carbon dioxide and oxygen (CO₂ and O₂) chemostimuli. These chemostimuli elicit synergistic increases in cerebral blood flow (CBF) and sympathetic nervous system activation, with the latter increasing systemic blood pressure. The extent that simultaneous and inverse changes in arterial CO₂ and O₂ and associated increases in blood pressure affect the CBF responses during RB versus VAs are unclear. We instrumented 21 healthy participants with a finometer (beat-by-beat mean arterial blood pressure; MAP), transcranial Doppler ultrasound (middle and posterior cerebral artery velocity; MCAv, PCAv) and a mouthpiece with sample line attached to a dual gas analyzer to assess pressure of end-tidal (P_ET)CO₂ and P_ETO₂. Participants performed two protocols: RB and a maximal end-inspiratory VA. A second-by-second stimulus index (SI) was calculated as P_ETCO₂/P_ETO₂ during RB. For VA, where P_ETCO₂ and P_ETO₂ could not be measured throughout, SI values were calculated using interpolated end-tidal gas values before and at the end of the apneas. MAP reactivity (MAPR) was calculated as the slope of the MAP/SI, and cerebrovascular reactivity (CVR) was calculated as the slope of MCAv or PCAv/SI. We found that compared to RB, VA elicited ~ fourfold increases in MAPR slope (P < 0.001), translating to larger anterior and posterior CVR (P ≤ 0.01). However, cerebrovascular conductance (MCAv or PCAv/MAP) was unchanged between interventions (P ≥ 0.2). MAP responses during VAs are larger than those during RB across similar chemostimuli, and differential CVR may be driven by increases in perfusion pressure.

Prior oxygenation, but not chemoreflex responsiveness, determines breath-hold duration during voluntary apnea

Central and peripheral respiratory chemoreceptors are stimulated during voluntary breath holding due to chemostimuli (i.e., hypoxia and hypercapnia) accumulating at the metabolic rate. We hypothesized that voluntary breath-hold duration (BHD) would be (a) positively related to the initial pressure of inspired oxygen prior to breath holding, and (b) negatively correlated with respiratory chemoreflex responsiveness. In 16 healthy participants, voluntary breath holds were performed under three conditions: hyperoxia (following five normal tidal breaths of 100% O2 ), normoxia (breathing room air), and hypoxia (following ~30-min of 13.5%-14% inspired O2 ). In addition, the hypoxic ventilatory response (HVR) was tested and steady-state chemoreflex drive (SS-CD) was calculated in room air and during steady-state hypoxia. We found that (a) voluntary BHD was positively related to initial oxygen status in a dose-dependent fashion, (b) the HVR was not correlated with BHD in any oxygen condition, and (c) SS-CD magnitude was not correlated with BHD in normoxia or hypoxia. Although chemoreceptors are likely stimulated during breath holding, they appear to contribute less to BHD compared to other factors such as volitional drive or lung volume.

Peripheral chemoreceptor deactivation attenuates the sympathetic response to glucose ingestion

Acute increases in blood glucose are associated with heightened muscle sympathetic nerve activity (MSNA). Animal studies have implicated a role for peripheral chemoreceptors in this response, but this has not been examined in humans. Heart rate, cardiac output (CO), mean arterial pressure, total peripheral conductance, and blood glucose concentrations were collected in 11 participants. MSNA was recorded in a subset of 5 participants via microneurography. Participants came to the lab on 2 separate days (i.e., 1 control and 1 experimental day). On both days, participants ingested 75 g of glucose following baseline measurements. On the experimental day, participants breathed 100% oxygen for 3 min at baseline and again at 20, 40, and 60 min after glucose ingestion to deactivate peripheral chemoreceptors. Supplemental oxygen was not given to participants on the control day. There was a main effect of time on blood glucose (P < 0.001), heart rate (P < 0.001), CO (P < 0.001), sympathetic burst frequency (P < 0.001), burst incidence (P = 0.01), and total MSNA (P = 0.001) for both days. Blood glucose concentrations and burst frequency were positively correlated on the control day (r = 0.42; P = 0.03) and experimental day (r = 0.62; P = 0.003). There was a time × condition interaction (i.e., normoxia vs. hyperoxia) on burst frequency, in which hyperoxia significantly blunted burst frequency at 20 and 60 min after glucose ingestion only. Given that hyperoxia blunted burst frequency only during hyperglycemia, our results suggest that the peripheral chemoreceptors are involved in activating MSNA after glucose ingestion.

Assessing chemoreflexes and oxygenation in the context of acute hypoxia: Implications for field studies

Carotid chemoreceptors detect changes in PO₂ and elicit a peripheral respiratory chemoreflex (PCR). The PCR can be tested through a transient hypoxic ventilatory response test (TT-HVR), which may not be safe nor feasible at altitude. We characterized a transient hyperoxic ventilatory withdrawal test in the setting of steady-state normobaric hypoxia (13.5-14% F_IO₂) and compared it to a TT-HVR and a steady-state poikilocapnic hypoxia test, within-individuals. No PCR test magnitude was correlated with any other test, nor was any test magnitude correlated with oxygenation while in steady-state hypoxia. Due to the heterogeneity between the different PCR test procedures and magnitudes, and the confounding effects of alterations in CO₂ acting on both central and peripheral chemoreceptors, we developed a novel method to assess prevailing steady-state chemoreflex drive in the context of hypoxia. Quantifying peak hypoxic/hyperoxic responses at low altitude may have minimal utility in predicting oxygenation during ascent to altitude, and here we advance a novel index of chemoreflex drive.

Extreme respiratory sinus arrhythmia in response to superimposed head-down tilt and deep breathing

BACKGROUND

Respiratory sinus arrhythmia (RSA) is characterized by normal fluctuations in heart rate in phase with the respiratory cycle. There are many proposed mechanisms underlying the RSA phenomenon, including respiratory-induced cardiac loading (i.e., Bainbridge reflex), arterial baroreflex activation, vagal feedback from pulmonary stretch receptors, and central neural mechanisms. It is currently unclear to what extent these mechanisms are responsible for eliciting RSA in humans, particularly in response to stressors.

CASE REPORT

Here we present a case report of a healthy 26-yr-old woman (BMI 22.95 kg · m(-2)) who developed extreme RSA when exposed to the simultaneous cardiac loading stressors of 45° head-down tilt (HDT) and increased tidal volume during CO2 rebreathing. During baseline breathing in both supine and 45° HDT position, RSA magnitude was similar (mean ∼10-14 bpm). RSA was tidal volume-dependent, whereby in the supine position the RSA magnitude doubled with an approximate doubling in tidal volume during rebreathing (mean ∼20 bpm). However, when HDT and rebreathing were superimposed, extreme RSA was elicited (mean ∼45 bpm; range ∼38-110 bpm), approximately 450% over baseline breathing in the supine position. ECG analysis and follow up medical assessment revealed no underlying cardiac pathology.

DISCUSSION

The existence of extreme RSA when HDT and increased inspired volumes were superimposed suggests that the dual cardiac loading stimuli acted synergistically, increasing RSA magnitude over either stimulus alone. This case report may be relevant to situations where orthostatic stress and augmented tidal volumes are superimposed, or more generally when conflicting sympathetic and parasympathetic activation is simultaneous.