By Alan Macy, BIOPAC Systems, Inc.

 

Respiration affects the ECG signal in two ways. The first results from the mechanical relationship between the diaphragm and the heart. The second arises from respiration’s influence on the vagus nerve and is commonly known as respiratory sinus arrhythmia (RSA).

1. Because the pericardium is firmly attached to the diaphragm’s central tendon, the diaphragm’s position and movement determine the position of the heart. During a deep inhalation, the diaphragm contracts and flattens. As it moves, the heart descends, shifts posteriorly, rotates to the right, and becomes narrower and more elongated. The apex beat occurs lower and more medially. During a deep exhalation, the diaphragm relaxes, rises, and becomes more dome-shaped, and the heart moves in the opposite direction. Similar directional movement occurs during ordinary breathing, although less markedly than during diaphragmatic breathing and sometimes too subtly to notice. Because diaphragmatic breathing periodically changes the heart’s position, it affects the ECG signal.
2. Heart rate increases during inhalation and decreases during exhalation. It is normally regulated by respiratory centers in the medulla oblongata. The nucleus ambiguus, one of these centers, increases parasympathetic input to the heart through the vagus nerve. Vagal activity reduces the firing rate of the sinoatrial (SA) node—the heart’s pacemaker—and thereby lowers heart rate. During exhalation, cells in the nucleus ambiguus become active and heart rate decreases. Inhalation, by contrast, inhibits the nucleus ambiguus, leaving the vagus nerve unstimulated. In ECG recordings, this phenomenon appears as subtle respiration-synchronized changes in the R–R interval, the interval between successive R waves. The R–R interval shortens during inhalation and lengthens during exhalation.

*This post is translated and republished with permission from copyright holder Alan Macy. Copyright in this translation belongs to BRnC; reproduction without documented permission is prohibited.