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 using a background of step increases in steady-state normoxic fraction of inspired carbon dioxide to alter central chemoreceptor activation and by using the transient hypoxia test to target the peripheral chemoreceptors. What is the main finding and its importance? Our data suggest that the interaction between central and peripheral respiratory chemoreceptors is additive in minute ventilation and respiratory rate, but hypo-additive 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 chemoreceptors in the CNS and peripheral 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, owing, in part, to methodological challenges. We investigated the interaction between the central and peripheral chemoreceptors in healthy humans using a transient hypoxia test (three consecutive breaths of 100% N2 ; TT-HVR), which targets the temporal domain and stimulus specificity of the peripheral chemoreceptors. The TT-HVRs were superimposed upon three randomized background levels of steady-state inspired fraction of normoxic CO2 ( F I , C O 2 ${F}_{{\rm{I,C}}{{\rm{O}}}_{\rm{2}}}$ ; 0, 0.02 and 0.04). Chemostimuli [calculated oxygen saturation ( S cO 2 ${S}_{{\rm{cO}}_{\rm{2}}}$ )] and respiratory variable responses [respiratory rate (RR ), inspired tidal volume (VTI ) and ventilation ( V ̇ I ${{{\dot{V}}}_{\rm{I}}}$ )] were averaged from all three TT-HVR trials at each F I , C O 2 ${F}_{{\rm{I,C}}{{\rm{O}}}_{\rm{2}}}$ level. Responses were assessed as: (1) a change (∆) from baseline; and (2) indexed against Δ S cO 2 $\Delta {S}_{{\rm{cO}}_{\rm{2}}}$ . Aside from a significantly lower ∆VTI response in 0.04 F I , C O 2 ${F}_{{\rm{I,C}}{{\rm{O}}}_{\rm{2}}}$ (P = 0.01), the hypoxic rate responses (∆RR or ∆RR / Δ S cO 2 $\Delta {S}_{{\rm{cO}}_{\rm{2}}}$ ; P = 0.46 and P = 0.81), hypoxic tidal volume response ( Δ V TI / Δ V TI Δ S cO 2 Δ S cO 2 ${{\Delta {V}_{{\rm{TI}}}} \mathord{/ {\vphantom {{\Delta {V}_{{\rm{TI}}}} {\Delta {S}_{{\rm{cO}}_{\rm{2}}}}}} \kern-\nulldelimiterspace} {\Delta {S}_{{\rm{cO}}_{\rm{2}}}}}$ ; P = 0.08) and the hypoxic ventilatory responses ( Δ V ̇ I ${{\Delta {{\dot{V}}}_{\rm{I}}}}$ and Δ V ̇ I / Δ V ̇ I Δ S cO 2 Δ S cO 2 ${{\Delta {{\dot{V}}}_{\rm{I}}} \mathord{/ {\vphantom {{\Delta {{\dot{V}}}_{\rm{I}}} {\Delta {S}_{{\rm{cO}}_{\rm{2}}}}}} \kern-\nulldelimiterspace} {\Delta {S}_{{\rm{cO}}_{\rm{2}}}}}$ ; P = 0.09 and P = 0.31) were not significantly different across F I , C O 2 ${F}_{{\rm{I,C}}{{\rm{O}}}_{\rm{2}}}$ trials. Our data suggest simple addition between central and peripheral chemoreceptors in V ̇ I ${{{\dot{V}}}_{\rm{I}}}$ , which is mediated through simple addition in RR responses, but hypo-addition in VTI 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.

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 (PET)CO2 and PETO2. Participants performed two protocols: RB and a maximal end-inspiratory VA. A second-by-second stimulus index (SI) was calculated as PETCO2/PETO2 during RB. For VA, where PETCO2 and PETO2 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.

What Is the Point of the Peak? Assessing Steady-State Respiratory Chemoreflex Drive in High Altitude Field Studies.

Measurements of central and peripheral respiratory chemoreflexes are important in the context of high altitude as indices of ventilatory acclimatization. However, respiratory chemoreflex tests have many caveats in the field, including considerations of safety, portability and consistency. This overview will (a) outline commonly utilized tests of the hypoxic ventilatory response (HVR) in humans, (b) outline the caveats associated with a variety of peak response HVR tests in the laboratory and in high altitude fieldwork contexts, and (c) advance a novel index of steady-state chemoreflex drive (SS-CD) that addresses the many limitations of other chemoreflex tests. The SS-CD takes into account the contribution of central and peripheral respiratory chemoreceptors, and eliminates the need for complex equipment and transient respiratory gas perturbation tests. To quantify the SS-CD, steady-state measurements of the pressure of end-tidal (PET)CO2 (Torr) and peripheral oxygen saturation (SpO2; %) are used to quantify a stimulus index (SI; PETCO2/SpO2). The SS-CD is then calculated by indexing resting ventilation (L/min) against the SI. SS-CD data are subsequently reported from 13 participants during incremental ascent to high altitude (5160 m) in the Nepal Himalaya. The mean SS-CD magnitude increased approximately 96% over 10 days of incremental exposure to hypobaric hypoxia, suggesting that the SS-CD tracks ventilatory acclimatization. This novel SS-CD may have future utility in fieldwork studies assessing ventilatory acclimatization during incremental or prolonged stays at altitude, and may replace the use of complex and potentially confounded transient peak response tests of the HVR in humans.

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% FIO2) 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.

Intra-individual variability in cerebrovascular and respiratory chemosensitivity: Can we characterize a chemoreflex "reactivity profile"?

Intra-individual variability in the magnitude of human cerebrovascular and respiratory chemoreflex responses is largely unexplored. By comparing response magnitudes of cerebrovascular CO2 reactivity (CVR; middle and posterior cerebral arteries; MCA, PCA), central (CCR; CO2) and peripheral respiratory chemoreflexes (PCR; CO2 and O2), we tested the hypothesis that a within-individual reactivity magnitude profile could be characterized. The magnitudes of CVR and CCR were tested with hyperoxic rebreathing and PCR magnitudes were tested through transient respiratory tests (TT-CO2, hypercapnia; TT-N2, hypoxia). No significant intra-individual relationships were found between CCR vs. CVR (MCA and PCA), CCR vs. PCR (TT-N2 or TT-CO2) (r<0.2, P>0.3) response magnitudes. Statistically significant relationships were found between MCA vs. PCA reactivity (r=0.45, P<0.01) and PCR TT-N2 vs. PCR TT-CO2 (r=0.79, P<0.001) responses. Using qualitative and quantitative comparisons, we conclude that an intra-individual chemoreflex reactivity magnitude profile cannot be characterized. These data highlight the considerable between- and within-individual variability that exists in human cerebrovascular and respiratory chemoreflexes.

Quantifying cerebrovascular reactivity in anterior and posterior cerebral circulations during voluntary breath holding.

NEW FINDINGS: What is the central question of this study? We developed and validated a 'stimulus index' (SI; ratio of end-tidal partial pressures of CO2 and O2 ) method to quantify cerebrovascular reactivity (CVR) in anterior and posterior cerebral circulations during breath holding. We aimed to determine whether the magnitude of CVR is correlated with breath-hold duration. What is the main finding and its importance? Using the SI method and transcranial Doppler ultrasound, we found that the magnitude of CVR of the anterior and posterior cerebral circulations is not positively correlated with physiological or psychological break-point during end-inspiratory breath holding. Our study expands the ability to quantify CVR during breath holding and elucidates factors that affect break-point. The central respiratory chemoreflex contributes to blood gas homeostasis, particularly in response to accumulation of brainstem CO2 . Cerebrovascular reactivity (CVR) affects chemoreceptor stimulation inversely through CO2 washout from brainstem tissue. Voluntary breath holding imposes alterations in blood gases, eliciting respiratory chemoreflexes, potentially contributing to breath-hold duration (i.e. break-point). However, the effects of cerebrovascular reactivity on break-point have yet to be determined. We tested the hypothesis that the magnitude of CVR contributes directly to breath-hold duration in 23 healthy human participants. We developed and validated a cerebrovascular stimulus index methodology [SI; ratio of end-tidal partial pressures of CO2 and O2 (P ET ,CO2/P ET ,O2)] to quantify CVR by correlating measured and interpolated values of P ET ,CO2 (r = 0.95, P < 0.0001), P ET ,O2 (r = 0.98, P < 0.0001) and SI (r = 0.94, P < 0.0001) during rebreathing. Using transcranial Doppler ultrasound, we then quantified the CVR of the middle (MCAv) and posterior (PCAv) cerebral arteries by plotting cerebral blood velocity against interpolated SI during a maximal end-inspiratory breath hold. The MCAv CVR magnitude was larger than PCAv (P = 0.001; +70%) during breath holding. We then correlated MCAv and PCAv CVR with the physiological (involuntary diaphragmatic contractions) and psychological (end-point) break-point, within individuals. There were significant inverse but modest relationships between both MCAv and PCAv CVR and both physiological and psychological break-points (r < -0.53, P < 0.03). However, these relationships were absent when MCAv and PCAv cerebrovascular conductance reactivity was correlated with both physiological and psychological break-points (r > -0.42; P > 0.06). Although central chemoreceptor activation is likely to be contributing to break-point, our data suggest that CVR-mediated CO2 washout from central chemoreceptors plays no role in determining break-point, probably because of a reduced arterial-to-tissue CO2 gradient during breath holding.

Comparing and characterizing transient and steady-state tests of the peripheral chemoreflex in humans.

NEW FINDINGS: What is the central question of this study? We aimed to characterize the cardiorespiratory and cerebrovascular responses to transient and steady-state tests of the peripheral chemoreflex and to compare the hypoxic ventilatory responses (HVRs) between these tests. What is the main finding and its importance? The cardiovascular and cerebrovascular responses to transient tests were small in magnitude and short in duration. The steady-state isocapnic hypoxia test elicited a larger HVR than the transient 100% N(2) test, but the response magnitudes were correlated within individuals. The transient test of the HVR elicits fewer systemic effects than steady-state techniques and may have greater experimental utility than previously appreciated. Carotid chemoreceptors detect changes in arterial PO(2) and PCO(2), eliciting a peripheral chemoreflex (PCR). Steady-state (SS) hypoxia tests using dynamic end-tidal forcing (DEF) have been used to assess the hypoxic ventilatory response (HVR) but may be confounded by concomitant systemic effects. Transient tests of the PCR have also been developed but are not widely used, nor have the cardiovascular and cerebrovascular responses been characterized. We characterized the cardiorespiratory and cerebrovascular responses to transient tests of the PCR and compared the HVR between transient and SS-DEF tests. We hypothesized that the cardiovascular and cerebrovascular responses to the transient tests would be minimal and that the respiratory responses elicited from the transient and SS-DEF tests would be different in magnitude and not well correlated within individuals. Participants underwent five consecutive trials of two transient tests [three-breath 100% N(2) (TT-N(2)) and a single-breath 13% CO(2), in air] and two 10 min SS-DEF tests [isocapnic (SS-ISO) and poikilocapnic (SS-POI) hypoxia]. In response to the transient tests, heart rate, mean arterial pressure and the middle and posterior cerebral artery blood velocity increased (all P < 0.01), but responses were small (all <10%) and transient. Although the TT-N(2) and SS-POI tests elicited similar HVR magnitudes, they were not well correlated within individuals (r = 0.064, P = 0.79). The TT-N(2) test elicited a smaller HVR than the SS-ISO test, but they were correlated within individuals (r = 0.57, P = 0.008). Given that the transient tests exploit the temporal domain of the peripheral chemoreceptors and have minimal cardiovascular and cerebrovascular confounders, we suggest that they may have broader utility than previously appreciated.

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.