The vasovagal response of the rat: its relation to the vestibulosympathetic reflex and to Mayer waves

Abstract

Vasovagal responses (VVRs) are characterized by transient drops in blood pressure (BP) and heart rate (HR) and increased amplitude of low-frequency oscillations in the Mayer wave frequency range. Typical VVRs were induced in anesthetized, male, Long-Evans rats by sinusoidal galvanic vestibular stimulation (sGVS). VVRs were also produced by single sinusoids that transiently increased BP and HR, by 70-90° nose-up tilts, and by 60° tilts of the gravitoinertial acceleration vector using translation while rotating (TWR). The average power of the BP signal in the Mayer wave range increased substantially when tilts were >70° (0.91 g), i.e., when linear accelerations in the x-z plane were ≥0.9-1.0 g. The standard deviations of the wavelet-filtered BP signals during tilt and TWR overlaid when they were normalized to 1 g. Thus, the amplitudes of the Mayer waves coded the magnitude of the linear acceleration ≥1 g acting on the head and body, and the average power in this frequency range was associated with the generation of VVRs. These data show that VVRs are a natural outcome of stimulation of the vestibulosympathetic reflex and are not a disease. The results also demonstrate the usefulness of the rat as a small animal model for studying human VVRs.

Keywords: blood pressure; faints; heart rate; linear acceleration; otolith system; sinusoidal galvanic vestibular stimulation.

Figures

Figure 1.

Wavelet analyses of a VVR induced in rat R672 by sGVS at 0.025 Hz, 3 mA. A) Traces of BP (left panel) and HR (right panel). B) sGVS. C–G) Wavelet decomposition of the data from A into 5 frequency bands that comprise the transient response, which includes all low frequencies (band 12 and higher), labeled band 12 for simplicity (C), and the Mayer wave range: band 11 (D), band 10 (E), band 9 (F), and band 8 (G). Left panels: BP. Right panels: HR.

Figure 2.

Changes in systolic BP in response to 10 sine waves of 3 mA at 0.05 Hz. Consecutive responses are labeled with numbers on the right. Horizontal dashed lines in responses 2–5 are BP levels at the onset of each stimulus. The fifth sinusoid induced a brief increase in BP with superimposed large oscillations. This was followed by a profound decrease in BP that gradually recovered over several minutes. Subsequent stimulations over the next 25 min only induced increases in BP at a fixed latency, and there were no further drops in BP associated with these increases. Bottom traces: sGVS stimulus (mA) and time (s).

Figure 3.

A–C) HR of animals 644 (A), 668 (B), and 672 (C) during tilts at various angles. D) A tilt angle of 90° (red traces) induced VVR in all 3 animals, tilt of 70° induced VVR in rat 644, and tilts <70° did not induce VVR in any of the 3 animals.

Figure 4.

Wavelet analyses of the VVR induced in rat R644 by 90° nose-up tilt. A) Traces of BP (left panel) and HR (right panel). B) Tilt angle. C–G) Wavelet decomposition of the data from A into 5 frequency bands that comprise the transient response, which includes all low frequencies (band 12 and higher), labeled band 12 for simplicity (C), and the Mayer wave range: band 11 (D), band 10 (E), band 9 (F), and band 8 (G). Left panels: BP. Right panels: HR.

Figure 5.

Wavelet analyses of the VVR induced in rat R644 by 90° nose-up tilt. At 200 s after the animal was tilted from prone to upright, it was brought back to the prone position, terminating the induced VVR. A) Traces of BP (left panel) and HR (right panel). B) Tilt angle. C–G) Wavelet decomposition of the data from A into 5 frequency bands that comprise the transient response, which includes all low frequencies (band 12 and higher), labeled band 12 for simplicity (C), and the Mayer wave range: band 11 (D), band 10 (E), band 9 (F), and band 8 (G). Left panels: BP. Right panels: HR.

Figure 6.

Average power for the signal representing the summation of four detail bands ranging in frequencies associated with Mayer waves of BP after onset of tilt for animals R664 (A), R668 (B), and R672 (C). This was done by summating 4 detail bands with frequencies just above the approximation signal (transient response), comprising frequency bands 11, 10, 9, and 8, with corresponding frequency ranges of 0.015–0.03, 0.03–0.06, 0.06–0.12, and 0.12–0.24 Hz, respectively. Average power was computed over 10 s (squares), 40 s (circles), and 200 s (triangles) s. In all 3 rats, the maximum average power occurred when the averaging was 10 s (shortest time) and for tilts >70°. In all instances, the blood pressure was estimated from the PPG.

Figure 7.

Wavelet analyses of the VVR induced in rat R676 by translation while rotating (300°/s, ±1.8 g). A) Traces of BP (left panel) and HR (right panel). B) Rotational velocity. C) Translation along the long axis of the body. D–H) Wavelet decomposition of the data from A into 5 frequency bands that comprise the transient response, which includes all low frequencies (band 12 and higher), labeled band 12 for simplicity (D), and the Mayer wave range: band 11 (E), band 10 (F), band 9 (G), and band 8 (H). Left panels: BP. Right panels: HR. TWR induces BP and HR traces in the Mayer wave range closely resembling those of tilt and sGVS.

Figure 8.

A) Standard deviations of the responses (ordinate) for each frequency band (number on abscissa) for data obtained during static tilt (blue) and TWR (red). Data peaked in the Mayer wave range. B) Graph similar to A, but TWR data are normalized to 1 g. Data obtained during 0.025 Hz, 3 mA sGVS are shown in black. Data indicate that when TWR is normalized to 1 g, the standard deviations of BP associated with tilt, TWR, and sGVS overlaid.