Negative pressure breathing: the response of human respiration and circulation to different levels of rarefaction during inspiration

Abstract

Introduction: Negative pressure breathing is breathing with decreased pressure in the respiratory tract without lowering pressure acting on the torso. We lowered pressure only during inspiration (NPBin). NPBin is used to increase venous return to the heart and is considered as a countermeasure against redistribution of body fluids toward the head during spaceflight. Aims of our study were: to obtain quantitative information on NPBin-induced changes in parameters of circulation and respiration in healthy human at various rarefactions; to identify main processes involved in cardiorespiratory response to NPBin.

Methods: Cardiorespiratory response to 25 min of NPBin were studied, rarefaction ranged from -10 to -25 cmH₂O. Parameters of systemic, cerebral, and peripheral hemodynamics, as well as respiratory and gas exchange parameters, were measured with non-invasive methods (Finometer, impedance cardiography, rheoencephalography, transcranial Doppler ultrasonography, laser Doppler flowmetry, capillaroscopy). Concentrations of endothelin-1, atrial and brain natriuretic peptides precursors in venous blood, O₂ and CO₂ tensions in arterialized capillary blood were measured.

Results: Tidal volume increased, respiratory rate decreased under NPBin with no significant changes in minute ventilation. Group averaged, respiratory rate and tidal volume changed approximately twice relative to their values observed under normal breathing. Despite the decrease in respiratory rate (up to 2-3 breaths/min), the results indicate CO₂ wash-out. Changes in respiratory and gas exchange parameters were virtually independent of rarefaction level. Synchronous with breathing oscillations of circulatory parameters increased in amplitude under NPBin, while values of the parameters averaged over NPBin period changed little. Amplitude of oscillations in parameters associated with arteries virtually did not change with increasing rarefaction. Inspiration under NPBin reduced left ventricle stroke volume and arterial blood pressure, increased heart rate. Head blood filling decreased during inspiration under NPBin, the decrease increased almost linearly with increasing rarefaction. Parameters returned to their initial values after the end of inspiration. Peak-to-peak amplitude of oscillations under NPBin ranged: stroke volume 17-25 mL, mean arterial pressure 7-9 mmHg, heart rate 14-18 bpm. Peripheral hemodynamics responded to NPBin little.

Conclusion: Changes in stroke volume and central venous pressure during inspiration under NPBin appear to be the major phenomena mediating the effects of NPBin on the cardiorespiratory system.

Keywords: cerebral circulation; circulation; gas exchange; heart; negative pressure breathing; respiration.

FIGURE 1

Rarefaction in the mouthpiece (Pm) at “NPBin” stage of experimental series. The rarefaction increases when moving up the vertical axis. (A) represents the average Pm values for the entire “NPBin” stage (simple average over the entire recording–without distinguishing inspiration and expiration). (B) shows changes in Pm during inspiration under NPBin (curves are averaged over the group of volunteers and over inspirations; at about 2.5 of time normalized to individual duration of inspiration, the next breath began in some volunteers). Colors correspond to series: blue–NPBin_0 (control series, no additional rarefaction), cyan–NPBin_10 (rarefaction during inspiration set to −10 cmH₂O), green–NPBin_15 (rarefaction during inspiration set to −15 cmH₂O), magenta–NPBin_20 (rarefaction during inspiration set to −20 cmH₂O), red–NPBin_25 (rarefaction during inspiration set to −25 cmH₂O). Solid curve–average Pm changes at “NPBin” stage of each series; dashed curves of the same color shows the boundaries of ±SD interval. Vertical lines mark the edges of inspiration.

FIGURE 2

Respiratory oscillations of hemodynamic parameters. SAP–systolic arterial pressure (A), DAP–diastolic arterial pressure (B), MAP–mean arterial pressure (C), HR–heart rate (D), SV–left ventricle stroke volume (E), CO–cardiac output (F), TPR–total peripheral resistance (G), RCG A—systolic maximum of RCG signal (H), LDP–laser Doppler perfusion (I). Colors correspond to series: blue–NPBin_0 (control series, no additional rarefaction), cyan–NPBin_10 (rarefaction during inspiration set to −10 cmH₂O), green–NPBin_15 (rarefaction during inspiration set to −15 cmH₂O), magenta–NPBin_20 (rarefaction during inspiration set to −20 cmH₂O), red–NPBin_25 (rarefaction during inspiration set to −25 cmH₂O). Solid curve–changes of parameters at “NPBin” stage of each series averaged over breathing cycles and over the group of volunteers; dashed curves of the same color shows the boundaries of ±SD interval. Vertical lines mark the edges of inspiration. Prefix “d” denotes that the average value of the parameter was subtracted before the calculating of the oscillation (i.e., only the variable component was analyzed).

FIGURE 3

Respiratory oscillations of blood flow velocity parameters in the right common carotid artery (CCA) and in the right middle cerebral artery (MCA). PSV–peak systolic velocity (A and D); EDV–end-diastolic velocity (B and E); TAM–velocity averaged over the current cardiocycle (C and F). Colors correspond to series: blue–NPBin_0 (control series, no additional rarefaction), cyan–NPBin_10 (rarefaction during inspiration set to −10 cmH₂O), green–NPBin_15 (rarefaction during inspiration set to −15 cmH₂O), magenta–NPBin_20 (rarefaction during inspiration set to −20 cmH₂O), red–NPBin_25 (rarefaction during inspiration set to −25 cmH₂O). Solid curve–changes of parameters at “NPBin” stage of each series averaged over breathing cycles and over the group of volunteers; dashed curves of the same color shows the boundaries of ±SD interval. Vertical lines mark the edges of inspiration. Prefix “d” denotes that the average value of the parameter was subtracted before the calculating of the oscillation (i.e., only the variable component was analyzed).

FIGURE 4

Respiratory oscillations of head electrical impedance. REG BI–basic impedance of REG. Colors correspond to series: blue–NPBin_0 (control series, no additional rarefaction), cyan–NPBin_10 (rarefaction during inspiration set to −10 cmH₂O), green–NPBin_15 (rarefaction during inspiration set to −15 cmH₂O), magenta–NPBin_20 (rarefaction during inspiration set to −20 cmH₂O), red–NPBin_25 (rarefaction during inspiration set to −25 cmH₂O). Solid curve–changes of parameters at “NPBin” stage of each series averaged over breathing cycles and over the group of volunteers; dashed curves of the same color shows the boundaries of ±SD interval. Vertical lines mark the edges of inspiration. Prefix “d” denotes that the average value of the parameter was subtracted before the calculating of the oscillation (i.e., only the variable component was analyzed).

FIGURE 5

Illustrative ultrasonograms of common carotid artery and internal jugular vein under NPBin of −20 cm of water column. Vessels are marked with arrows; in each of the images, the common carotid artery is on the right. The sonogram on the left (A) was obtained during a pause before inspiration, on the right (B) – during inspiration under NPBin. The volunteer was lying on his back horizontally. Sonograms were recorded with SonoSite 180 plus (SonoSite, Inc., United States).