Vagal tone

[1] In this context, tone specifically refers to the continual nature of baseline parasympathetic action that the vagus nerve exerts.

Invasive procedures are in the minority and include vagus nerve stimulation by specific manual, breathing or electrical techniques.

In a healthy heart, the main pacemaker is a collection of cells on the border of the atria and vena cava called the sinoatrial node.

[1] In absence of external stimuli, sinoatrial pacing generally, while awake, maintains the heart rate in the range of 60–100 beats per minute (bpm).

The vagus nerve acts on the sinoatrial node, slowing its conduction and modulating vagal tone, via the neurotransmitter acetylcholine and downstream changes to ionic currents and calcium of heart cells.

Atrial pressure is also lowered as a result, causing increased blood flow to the heart, which in turn decreases baroreceptors firing response which diminishes vagal tone.

[1] During exhalation, the diaphragm relaxes, moving upward, and decreases the size of the chest cavity, causing an increase in intrathoracic pressure.

[1] Respiratory sinus arrhythmia (RSA) is frequently used as a noninvasive method for investigating vagal tone, in physiological, behavioral, and several clinical studies.

[21] It has been suggested that RSA may have evolved to save energy for both cardiac and respiratory systems by reducing the heart rate[22] and by suppressing ineffective ventilation during the ebb of perfusion (delivery of blood from arteries to capillaries for oxygenation and nutrition).

[26] Baseline vagal tone can be used either as a potential predictor of behavior or as a signal of mental health (particularly emotion regulation, anxiety, and internalizing and externalizing disorders).

Heart rate (HR) (first row), tidal volume (Vt) (second row), Vt and superimposed HR (third row). The HR modulation is clearly visible: HR increases with inspiration and decreases with expiration.
Heart rate (HR) (first row), ECG signal (ECG) (second row), and respiration (third row) for a newborn subject in a 15-seconds recording. HR expresses oscillations synchronous with respect to respiration.
RSA magnitude estimation based on a multivariate approach based on joint analysis of ECG and respiration. [ 15 ] The green line shows the heart rate variations averaged over several breathing cycles. This clearly shows the trends that are typical of RSA.