Postural change alters autonomic responses to breath-holding

Abstract

Objective: We used breath-holding during inspiration as a model to study the effect of pulmonary stretch on sympathetic nerve activity.

Methods: Twelve healthy subjects (7 females, 5 males; 19-27 years) were tested while they performed an inspiratory breath-hold, both supine and during a 60 degrees head-up tilt (HUT 60). Heart rate (HR), mean arterial blood pressure (MAP), respiration, muscle sympathetic nerve activity (MSNA), oxygen saturation (SaO(2)) and end tidal carbon dioxide (ETCO(2)) were recorded. Cardiac output (CO) and total peripheral resistance (TPR) were calculated.

Results: While breath-holding, ETCO(2) increased significantly from 41 +/- 2 to 60 +/- 2 Torr during supine (p < 0.05) and 38 +/- 2 Torr to 58 +/- 2 during HUT60 (p < 0.05); SaO(2) decreased from 98 +/- 1.5% to 95 +/- 1.4% supine, and from 97 +/- 1.5% to 94 +/- 1.7% during HUT60 (p = NS). MSNA showed three distinctive phases, a quiescent phase due to pulmonary stretch associated with decreased MAP, HR, CO, and TPR; a second phase of baroreflex-mediated elevated MSNA which was associated with recovery of MAP and HR only during HUT60; CO and peripheral resistance returned to baseline while supine and HUT60; a third phase of further increased MSNA activity related to hypercapnia and associated with increased TPR.

Interpretation: Breath-holding results in initial reductions of MSNA, MAP, and HR by the pulmonary stretch reflex followed by increased sympathetic activity related to the arterial baroreflex and chemoreflex.

Figure 1

Representative tracing of breath-holding during inspiration. Mean Arterial Pressure (MAP), Heart Rate (HR), Muscle Sympathetic Nerve Activity (MSNA), Baseline (Bsl), Inspiration (Insp), Early Recovery (ER), Late Recovery (LR), Phase (Ph). Within the same breath-hold, there is decrease in MAP followed by an increase in MAP, HR, and MSNA. The phases are as follows: Phase 0-Baseline; the 5 min of resting baseline followed by Phase1-Inspiration; the time for deep inspiration. MAP begins to decrease shortly after inspiration is complete defining the start of Phase2-Fall in MAP. Blood pressure recovery defines the end of Phase 2 and the start of Phase 3-Rise in MAP which initiates a period of blood pressure stability. An increase in MAP begins near the end of the breath-hold defines the start of Phase 4-early recovery (ER), and the beginning of the Phase 5-late recovery (LR) phase is defined by the return of MAP, HR, MSNA and ETCO2 to baseline values.

Figure 2

Breath-holding time during inspiration in supine and HUT60 positions. The HUT60 position increases Breath-holding time. Left panel (Figure 2A) shows combined males and females data; Right panel (Figure 2B) shows gender differences in breath-holding time. Males have a higher breath-holding time than females. Data is expressed as Means±SE. *, p<0.05, supine vs. HUT60; ^†, males vs. females.

Figure 3

Posture-related gender differences in response to breath-holding. Left upper panel shows that breath-holding increases muscle sympathetic nerve activity (MSNA) in supine and HUT60. Right upper panel shows that breath-holding induced increase in MSNA frequency during HUT60 is much higher in males than in females. Lower panel shows males have higher total peripheral resistance than females during supine and HUT 60 breath-holding. Muscle sympathetic nerve activity (MSNA), head uptilt at 60° (HUT60), Breath-holding (BH), Phase (Ph); Baseline (Bsl); Inspiration (Inspi) ; MAP fall (MAPfa); MAP rise (MAPris); Early Recovery (ER); LR, Late Recovery (LR) *, p<0.05 supine vs. HUT60; #, p<0.05 supine-baseline vs. supine-breath-hold; ^†, p<0.05, males vs. females.

Figure 4

Comparison of supine and HUT60 breath-holding in inspiration. Values are expressed as Mean±SE for mean arterial pressure (MAP); cardiac output (CO); total peripheral resistance (TPR); total muscle sympathetic nerve (MSNA) activity (bursts/min*mean burst area); heart rate (HR); and end tidal carbon dioxide (ETCO₂); Phase (Ph); Baseline (Bsl); Inspiration (Inspi) ; MAP fall (MAPfa); MAP rise (MAPris); Early Recovery (ER); LR, Late Recovery (LR) *,p<0.05 vs. Baseline; ^†, p<0.05 Supine vs. HUT60.