ECG-Derived Respiration

Several techniques can be used to obtain a respiration signal from an ECG. A popular technique used in bedside patient monitors involves measuring transthoracic impedance using the ECG electrodes. Unlike those described below, this method obtains the respiration signal from the ECG electrodes rather than from the ECG signal. It requires special-purpose hardware and is not suited to recovering respiration from a recorded ECG, but it has the significant advantage (relative to the other methods mentioned here) that a continuous respiration-related waveform can be obtained.

Another well-known method is based on observing the beat-to-beat variations in RR intervals or their reciprocals, which are primarily due to respiratory sinus arrhythmia (RSA) in most individuals. This method works best in young, healthy subjects, in whom RSA is most pronounced.

The EDR (ECG-Derived Respiration) technique is a third approach. It is based on the observation that the positions of ECG electrodes on the chest surface move relative to the heart, and transthoracic impedance varies, as the lungs fill and empty. Thus the lead axes vary at different points in the respiratory cycle, and any sufficiently precise measurement of the mean cardiac electrical axis shows variations that are correlated with respiration. This method is particularly effective if two or more ECG signals are available, but a single lead is sufficient. The EDR can be obtained even in the context of congestive heart failure, in which RSA may be absent.

None of these methods supplies a calibrated respiration signal; for tidal volume and air flow measurements, other techniques (spirometry, measurements from nasal thermistors, and plethysmography) are best. Nevertheless, any of these methods can provide a basis for estimating respiration rate and for detecting apneas.

Here are:

• a short C program implementing the EDR, which can be used to derive a respiration-related signal from one or two ECG signals (precompiled binaries for Linux and MS-Windows are also available)
• the paper that introduced the technique
• a second paper that describes applications for the EDR

Another paper that describes an application of the EDR is:

Address for correspondence:

George B. Moody
MIT Room E25-505A
Cambridge, MA 02139 USA

e-mail: george@mit.edu