An optimized method for estimating the tidal volume from intracardiac or body surface electrocardiographic signals: implications for estimating minute ventilation

Abstract

The ability to accurately monitor tidal volume (TV) from electrocardiographic (ECG) signals holds significant promise for improving diagnosis treatment across a variety of clinical settings. The objective of this study was to develop a novel method for estimating the TV from ECG signals. In 10 mechanically ventilated swine, we collected intracardiac electrograms from catheters in the coronary sinus (CS), left ventricle (LV), and right ventricle (RV), as well as body surface electrograms, while TV was varied between 0 and 750 ml at respiratory rates of 7-14 breaths/min. We devised an algorithm to determine the optimized respirophasic modulation of the amplitude of the ECG-derived respiratory signal. Instantaneous measurement of respiratory modulation showed an absolute error of 72.55, 147.46, 85.68, 116.62, and 50.89 ml for body surface, CS, LV, RV, and RV-CS leads, respectively. Minute TV estimation demonstrated a more accurate estimation with an absolute error of 69.56, 153.39, 79.33, 122.16, and 48.41 ml for body surface, CS, LV, RV, and RV-CS leads, respectively. The RV-CS and body surface leads provided the most accurate estimations that were within 7 and 10% of the true TV, respectively. Finally, the absolute error of the bipolar RV-CS lead was significantly lower than any other lead configuration (P < 0.0001). In conclusion, we have demonstrated that ECG-derived respiratory modulation provides an accurate estimation of the TV using intracardiac or body surface signals, without the need for additional hardware.

Keywords: body surface; intracardiac electrograms; minute ventilation; percent modulation; tidal volume.

Fig. 1.

Normalized (to maximum value) QRS root-mean-squared amplitude signal from a body surface (ECG lead II) and intracardiac coronary sinus lead 4 (CS4), left ventricle lead 10 (LV10), right ventricle lead 8 (RV8), and RV2CS8 lead (A), and the estimated percent modulation (B) of a single swine in which the tidal volume (TV) of the ventilator was changed 2 min after the beginning of the recording (dashed line) from 750 to 250 ml at a fixed respiratory rate of 9 breaths/min. As the TV decreases, both the peak-to-peak amplitude of the respiratory modulation and the estimated percent modulation also decrease in all leads. Following the change of TV from 750 to 250 ml, the percent modulation in this illustration changes in 12 s from 49.01 ± 2.66 to 15.39 ± 3.67% in body surface, from 27.95 ± 1.93 to 4.83 ± 1.46% in CS, from 19.52 ± 1.31 to 7.00 ± 0.79% in LV, from 68.89 ± 2.82 to 15.09 ± 2.58% in RV, and from 48.51 ± 1.98 to 9.89 ± 2.75% in RV-CS.

Fig. 2.

The percent modulation (medians ± SE) at TVs of 0, 250, 500, and 750 ml, computed across all animals (n = 10), for body surface leads (A), unipolar leads (B), near-field bipolar leads (C), far-field bipolar leads (D), and RV-CS leads (E). For every lead configuration, as the TV increases, the percent modulation also increases. The data present significant variability among the different lead configurations.

Fig. 3.

The optimized instantaneous percent modulation (■) and optimized minute percent modulation (●) (medians ± SE) at TVs of 0, 250, 500, and 750 ml, computed across all animals (n = 10), for body surface (A), CS (B), LV (C), RV (D), and RV-CS leads (E). For every lead configuration, as the TV increases, the percent modulation also increases.

Fig. 4.

A and B: total validation error (true TV − estimated TV) using the instantaneous percent modulation at each of the TVs of 0, 250, 500, and 750 ml compiled over all 10-folds of the cross-validation for the body surface (SURF) and intracardiac CS, LV, RV, and RV-CS lead configurations. Overall, the error increases as the TV increases from 0 to 750 ml in all leads. Data are presented as median (horizontal solid line), 25–75th percentiles (box), and 10–90th percentiles (error bars). For each lead and each TV, the total number of cycles (N) used to estimate the error is shown in A (gray bars).

Fig. 5.

A and B: total validation error (true TV − estimated TV) using the minute percent modulation at each of the TVs of 0, 250, 500, and 750 ml compiled over all 10-folds of the cross-validation for the body surface, and intracardiac CS, LV, RV, and RV-CS lead configurations. Overall, the error increases as the TV increases from 0 to 750 ml in all leads. Data are presented as median (horizontal solid line), 25–75th percentiles (box), and 10–90th percentiles (error bars). For each lead and each TV, the total number of cycles (N) used to estimate the error is shown in A (gray bars).

Fig. 6.

Absolute error (medians ± SE) for each of the body surface and intracardiac leads at all TVs, computed across all animals (n = 10) for the instantaneous percent modulation (white bars) and the minute percent modulation (black bars). No statistical difference of the error was found between the instantaneous and the minute percent modulation for the body surface and intracardiac leads. However, the absolute error of the bipolar RV-CS lead is significantly lower than any other lead configuration including body surface and unipolar CS, LV, and RV leads (P < 0.0001).

Fig. 7.

The probability of having a TV estimation error less than X ml as a function of X, for the body surface and intracardiac leads, using the instantaneous percent modulation measurement, at TV 0 ml (A), 250 ml (B), 500 ml (C), and 750 ml (D). An arbitrary point (x, y) in these plots reflects that the probability of having an estimation error of less than x ml is equal to y.

Fig. 8.

The probability of having a TV estimation error less than X ml as a function of X, for the body surface and intracardiac leads, using the minute percent modulation measurement, at TV 0 ml (A), 250 ml (B), 500 ml (C), and 750 ml (D). An arbitrary point (x, y) in these plots reflects that the probability of having an estimation error of less than x ml is equal to y.

Fig. 9.

The TV estimation error as a function of premature ventricular complexes (PVCs) percentage within each respiration cycle for body surface leads (A), CS (B), LV (C), RV (D), and RV-CS (E) leads using the instantaneous percent modulation estimation. For every lead configuration, as the PVC percentage increases, the error between the true TV and the ECG-derived TV increases. However, the increase in unipolar intracardiac signals is more profound, compared to the changes in the body surface and particularly, the RV-CS triangular lead configuration. Data are presented as median (horizontal solid line), 25–75th percentiles (box), and 10–90th percentiles (error bars). For each lead, the number of cycles (N) used to estimate the error is also shown on the top (in log scale).

Fig. 10.

The TV estimation error as a function of PVC percentage within each respiration cycle for body surface leads (A), CS (B), LV (C), RV (D), and RV-CS (E) leads using the minute percent modulation estimation. For every lead configuration, as the PVC percentage increases, the error between the true TV and the ECG-derived TV increases. However, the body surface and, particularly, the RV-CS triangular lead configuration demonstrate a smaller error increase rate. Data are presented as median (horizontal solid line), 25–75th percentiles (box), and 10–90th percentiles (error bars). For each lead, the number of cycles (N) used to estimate the error is also shown at top (in log scale).