Effects of vagotomy on cardiovascular and heart rate variability alterations following chronic normobaric hypoxia in adult rabbits

Figures

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  Fig. 1. Experimental chamber and environmental measures in the chamber during hypoxic conditioning. a The chamber could contain two rabbits caged independently. Chamber O2 level (FIO2) was monitored continuously with an O2 sensor (OS). If FIO2 values were > 10% or < 8.9%, then the O2 sensor triggered a relay controller to activate a solenoid valve to admit either N2 (S1) or air (S2), respectively, and another (S3) to exhaust chamber gas (arrows on the gas lines indicate direction of gas flow). A safety release valve (R) opened if the chamber pressure increased to 15 mmHg ≥ atmospheric pressure. Chamber gas was mixed continually by fans (F, n = 4). b Every time the FIO2 reached about 8.7% (upper panel) the system injected compressed air, increasing FIO2 up to around 9.8% (upper panel) and the pressure in the chamber near 15 mmHg (lower panel). After the initial pressure increase, the pressure fell (inset) with an initial fast exponential decay due to the exhaust valve and a second slower decay due to pressure leak through the lid of the chamber. c Temperature recordings of 48 h periods in different experiments (n = 7). No temporal pattern of temperature variation was observed in the chamber

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  Fig. 2. Heart rate before and after bilateral vagotomy in control and CNH rabbits. a Mean heart rate was significantly increased in control (n = 6) but not in CNH rabbits (n = 6) after bilateral vagotomy (VgTx). b Scatter plot of mean heart rate after bilateral vagotomy as a function of basal mean heart rate. Dotted line: line of identity. c In control rabbits, changes in mean heart rate were maximal in the midrange of basal heart rate; in CNH rabbits, changes were inversely and linearly related to basal mean heart rate. Control rabbits: empty circles, continuous line. CNH rabbits: filled circles, segmented line. Bars: SEM. *p < 0.05

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  Fig. 3. Effect of chronic normobaric hypoxia on time-domain analysis of heart rate variability. a Relationship between the duration of a cardiac cycle (RR interval, n; (R–R)n) and that of the next one (RR interval, n + 1; (R–R)n+1) and in a control rabbit. b Relationship between the duration of nth RR interval, (R–R)n, and the following RR interval, (R–R)n+1, in a CNH rabbit. c Mean SD1 was similar in control and CNH rabbits, while mean SD2 was significantly lower in CNH than in control rabbits. Empty bars: control rabbits (n = 11). Filled bars: CNH rabbits (n = 8). *p < 0.05. Dispersion bars: SEM

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  Fig. 4. Effect of vagotomy (VgTx) in the time-domain analysis of heart rate variability. a Relationship between the duration of the following cardiac cycle ((R–R)n+1) and the previous one ((R–R)n) in a control animal, before and after vagotomy. b Relationship between the duration of (R–R)n+1 and (R–R)n in a CNH rabbit, before and after vagotomy. c Mean SD1 (left) and SD2 (right) from control (n = 6) and CNH (n = 6) rabbits before and after vagotomy (VgTx). Mean SD1 and SD2 were statistically unaffected by vagotomy. Before vagotomy: filled circles. After vagotomy: empty circles. Empty bars: control rabbits. Dotted bars: control rabbits after vagotomy. Filled bars: CNH rabbits. Lined bars: CNH rabbits after vagotomy. Dispersion bars: SEM

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  Fig. 5. Power spectra of the R–R intervals from ten consecutive minutes of ECG recording. a Power spectra of control rabbits (n = 11) had components with the greatest power at the very-low-frequency (< 0.2 Hz) and high frequency (0.5–2.1 Hz) bands, with additional components with lower power at low-frequency (0.2–0.5 Hz) band. b Power spectra of CNH rabbits (n = 8) had reduced power at very-low-frequency band, almost no components at the high-frequency band but an increase in the power of components between in the low-frequency band. PSD power spectral density

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  Fig. 6. Mean power spectra of the R–R intervals from ten consecutive minutes of ECG recording. The mean power spectrum of CNH rabbits (dotted line, filled circles; n = 8) showed a large reduction of the components at the very low (< 0.2 Hz) and high (> 0.8 Hz) frequency bands, with respect to the mean power spectrum of control rabbits (continuous line, empty circles; n = 11). The low-frequency band between 0.2 and 0.5 Hz was increased in CNH rabbits with respect to the mean power spectrum of control rabbits. Inset: mean values, in standardized units (S.U.), of low frequency (LF), high frequency (HF) and LF/HF ratio in control (empty bars) and CNH rabbits (filled bars). *Significantly different from control rabbits; p < 0.05. Dispersion bars: SEM. PSD power spectral density

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  Fig. 7. Power spectra of the R–R intervals of control rabbits (n = 6) before and after bilateral supra-nodose vagotomy. a Before vagotomy, the prominent components of the power spectral density (PSD) occurred in the very-low- (< 0.2 Hz) and high- (0.8–1.7 Hz) frequency bands. b After vagotomy, the power of the very-low-frequency components (< 0.2 Hz) increased, while the high-frequency band decreased. c The mean power spectrum of vagotomized rabbits (filled circles, continuous line) increased in the very-low frequency band compared to that before vagotomy (empty circles), while the power in the high frequency band decreased, nearly absent between 0.8 and 1.7 Hz compared to that before vagotomy. Left inset: mean power spectra of the high-frequency band (0.8–2.0 Hz). Right inset: mean values, in standardized units (S.U.), of low frequency (LF), high frequency (HF) and LF/HF ratio in control conditions (empty bars) and after bilateral vagotomy (filled bars). *Significantly larger than intact condition; p < 0.05. Dispersion bars: SEM. PSD power spectral density

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  Fig. 8. Power spectra of the R–R intervals of CNH rabbits (n = 6) before and after bilateral supra-nodose vagotomy. a The principal components of the power spectra of CNH rabbits before vagotomy occurred in very-low- and low- frequency bands (< 0.5 Hz) and almost no components at high frequency band (0.5–2.1 Hz). b After vagotomy, the power very-low- and low- frequency bands (< 0.6 Hz) increased. c The mean power spectrum of vagotomized animals (filled circles) showed a large increase in the power of the lower frequency bands compared to those before vagotomy (continuous line, empty circles), while the components in the 0.8 and 1.7 Hz range remained weak and appeared unaffected. Upper inset: mean power spectra of the 0.8–2.0 Hz frequency range. Lower inset: mean values, in standardized units (S.U.), of low frequency (LF), high frequency (HF) and LF/HF ratio in control conditions (empty bars) and after bilateral vagotomy (filled bars). *Significantly different from than intact condition; p < 0.05. Dispersion bars: SEM. PSD: power spectral density