Acute intermittent hypercapnic-hypoxia elicits central neural respiratory motor plasticity in humans

Abstract

Acute intermittent hypoxia (AIH) elicits long-term facilitation (LTF) of respiration. Although LTF is observed when CO₂ is elevated during AIH in awake humans, the influence of CO₂ on corticospinal respiratory motor plasticity is unknown. Thus, we tested the hypotheses that acute intermittent hypercapnic-hypoxia (AIHH): (1) enhances cortico-phrenic neurotransmission (reflecting volitional respiratory control); and (2) elicits ventilatory LTF (reflecting automatic respiratory control). Eighteen healthy adults completed four study visits. Day 1 consisted of anthropometry and pulmonary function testing. On Days 2, 3 and 4, in a balanced alternating sequence, participants received: AIHH, poikilocapnic AIH, and normocapnic-normoxia (Sham). Protocols consisted of 15, 60 s exposures with 90 s normoxic intervals. Transcranial (TMS) and cervical (CMS) magnetic stimulation were used to induce diaphragmatic motor-evoked potentials and compound muscle action potentials, respectively. Respiratory drive was assessed via mouth occlusion pressure (P_0.1 ), and minute ventilation measured at rest. Dependent variables were assessed at baseline and 30-60 min after exposures. Increases in TMS-evoked diaphragm potential amplitudes were observed following AIHH vs. Sham (+28 ± 41%, P = 0.003), but not after AIH. No changes were observed in CMS-evoked diaphragm potential amplitudes. Mouth occlusion pressure also increased after AIHH (+21 ± 34%, P = 0.033), but not after AIH. Ventilatory LTF was not observed after any treatment. We demonstrate that AIHH elicits central neural mechanisms of respiratory motor plasticity and increases resting respiratory drive in awake humans. These findings may have important implications for neurorehabilitation after spinal cord injury and other neuromuscular disorders compromising breathing. KEY POINTS: The occurrence of respiratory long-term facilitation following acute exposure to intermittent hypoxia is believed to be dependent upon CO₂ regulation - mechanisms governing the critical role of CO₂ have seldom been explored. We tested the hypothesis that acute intermittent hypercapnic-hypoxia (AIHH) enhances cortico-phrenic neurotransmission in awake healthy humans. The amplitude of diaphragmatic motor-evoked potentials induced by transcranial magnetic stimulation was increased after AIHH, but not the amplitude of compound muscle action potentials evoked by cervical magnetic stimulation. Mouth occlusion pressure (P_0.1 , an indicator of neural respiratory drive) was also increased after AIHH, but not tidal volume or minute ventilation. Thus, moderate AIHH elicits central neural mechanisms of respiratory motor plasticity, without measurable ventilatory long-term facilitation in awake humans.

Keywords: acute intermittent hypoxia; asphyxia; diaphragm; hypercapnia; long-term facilitation; magnetic stimulation; motor-evoked potential; neuroplasticity; phrenic.

Figure 1.. Example of respiratory traces during acute intermittent hypercapnic-hypoxia

Raw traces from one representative participant. Abbreviations: AIHH, acute intermittent hypercapnic-hypoxia; $P_{{\text{CO}}_2}$, partial pressure of ${\text{CO}}_2$; $P_{\text{m}}$, mouth pressure; $P_{{\text{O}}_2}$, partial pressure of ${\text{O}}_2$.

Figure 2.. Respiratory responses during exposures

Abbreviations: AIH, poikilocapnic acute intermittent hypoxia; AIHH, acute intermittent hypercapnic-hypoxia; B, baseline; $f_{\text{b}}$, breathing frequency; ${\mathrm{PTP}}_{\text{m}}$, inspiratory muscle pressure-time product; R, recovery (first 5 min following exposure); ${\dot{V}}_{\text{I}}$, minute ventilation; $V_{\text{T}}$, tidal volume. Statistics: two-way (condition and time-point) repeated-measures ANOVA with Dunnett’s post hoc test. *Main effect of condition ($V_{\text{T}}:P<0.001$; $f_{\text{b}}:P=0.586$; ${\dot{V}}_{\text{I}}:P<0.001$; $\text{PT}{\text{P}}_{\text{m}}:P=0.002$); ^§main effect of time ($V_{\text{T}}:P<0.001$; $f_{\text{b}}:P=0.014$; ${\dot{V}}_{\text{I}}:P<0.001$; ${\text{PTP}}_{\text{m}}:P<0.001$); ^†significant difference within condition vs. baseline; ^‡significant difference within time-point vs. Sham.

Figure 3.. End-tidal gases during exposures

Abbreviations: AIH, poikilocapnic acute intermittent hypoxia; AIHH, acute intermittent hypercapnic-hypoxia; B, baseline; $P_{{\text{ETCO}}_2}$, end-tidal partial pressure of ${\text{CO}}_2$; $P_{{\text{ETO}}_2}$, end-tidal partial pressure of ${\text{O}}_2$; R, recovery (first 5 min following exposure). Statistics: two-way (condition and time-point) repeated-measures ANOVA with Dunnett’s post hoc test. *Main effect of condition ($P_{{\text{ETO}}_2}$: P < 0.001; $P_{{\text{ETCO}}_2}$: P < 0.001); ^§main effect of time ($P_{{\text{ETO}}_2}$: P < 0.001; $P_{{\text{ETCO}}_2}$: P < 0.001); ^†significant difference within condition vs. baseline; ^‡significant difference within time-point vs. Sham.

Figure 4.. Cardiovascular responses during exposures

Abbreviations: AIH, poikilocapnic acute intermittent hypoxia; AIHH, acute intermittent hypercapnic-hypoxia; B, baseline; HR, heart rate; MAP, mean arterial pressure; R, recovery (first 5 min following exposure); $S_{{\text{pO}}_2}$, peripheral ${\text{O}}_2$ saturation. Statistics: two-way (condition and time-point) repeated-measures ANOVA with Dunnett’s post hoc test. *Main effect of condition (HR: P = 0.788; MAP: P = 0.407; $S_{{\text{pO}}_2}$: P < 0.001); ^§main effect of time (HR: P = 0.107; MAP: P = 0.339; $S_{{\text{pO}}_2}$: P < 0.001); ^†significant difference within condition vs. baseline; ^‡significant difference within time-point vs. Sham.

Figure 5.. Breathing discomfort during exposures

Abbreviations: AIH, poikilocapnic acute intermittent hypoxia; AIHH, acute intermittent hypercapnic-hypoxia; B, baseline; R, recovery (first 5 min following exposure). Statistics: two-way (condition and time-point) repeated-measures ANOVA with Dunnett’s post hoc test. *Main effect of condition (P < 0.001); ^§main effect of time (P < 0.001); ^†significant difference within condition vs. baseline; ^‡significant difference within time-point vs. Sham.

Figure 6.. Raw evoked diaphragm potentials

Traces are composite averages of all participants at baseline (PRE, thin black lines) and 60 min following exposures (POST, thick black lines). Abbreviations: AIH, poikilocapnic acute intermittent hypoxia; AIHH, acute intermittent hypercapnic-hypoxia; CMS, cervical magnetic stimulation; TMS, transcranial magnetic stimulation.

Figure 7.. Absolute and relative changes in evoked diaphragm potential amplitudes

Abbreviations: AIH, poikilocapnic acute intermittent hypoxia; AIHH, acute intermittent hypercapnic-hypoxia; CMAP, compound muscle action potential; MEP, motor-evoked potential; n.s., not statistically significant; POST, post-exposure; PRE, pre-exposure. Statistics: one-way (condition) repeated-measures ANOVA with Dunnett’s post hoc test. *Significant difference vs. Sham (AIH: P = 0.728; AIHH: P = 0.003).

Figure 8.. Relationship between the magnitude of plasticity and chemoreflex sensitivity

Abbreviations: MEP, motor-evoked potential. Statistics: Pearson’s correlation.

Figure 9.. Mouth occlusion pressure

Abbreviations: AIH, poikilocapnic acute intermittent hypoxia; AIHH, acute intermittent hypercapnic-hypoxia; n.s., not statistically significant; POST, post-exposure; PRE, pre-exposure; $P_{0.1}$, mouth occlusion pressure. Statistics: one-way (condition) repeated-measures ANOVA with Dunnett’s post hoc test. *Significant difference vs. Sham (AIH: P = 0.987; AIHH: P = 0.033).

Figure 10.. Ventilation

Abbreviations: AIH, poikilocapnic acute intermittent hypoxia; AIHH, acute intermittent hypercapnic-hypoxia; B, baseline; $f_{\text{b}}$, breathing frequency; $P_{{\text{ETCO}}_2}$, end-tidal partial pressure of ${\text{CO}}_2$; ${\dot{V}}_{{\text{CO}}_2}$, rate of ${\text{CO}}_2$ production; ${\dot{V}}_{\text{I}}$, minute ventilation; $V_{\text{T}}$, tidal volume. Statistics: two-way (condition and time-point) repeated-measures ANOVA with Dunnett’s post hoc test. *Main effect of condition ($V_{\text{T}}$: P = 0.290; $f_{\text{b}}$: P = 0.142; ${\dot{V}}_{\text{I}}$: P = 0.134; $P_{{\text{ETCO}}_2}$: P = 0.551; ${\dot{V}}_{{\text{CO}}_2}$: P = 0.466; ${\dot{V}}_{\text{I}}/{\dot{V}}_{{\text{CO}}_2}$: P = 0.381); ^§main effect of time ($V_{\text{T}}$: P = 0.003; $f_{\text{b}}$: P = 0.022; ${\dot{V}}_{\text{I}}$: P = 0.345; $P_{{\text{ETCO}}_2}$: P = 0.147; ${\dot{V}}_{{\text{CO}}_2}$: P = 0.537; ${\dot{V}}_{\text{I}}/{\dot{V}}_{{\text{CO}}_2}$: P = 0.144); ^†significant difference within condition vs. baseline; ^‡significant difference within time-point vs. Sham.