Force-Interval Relationships of the Heart Measured With Photoplethysmography During Atrial Fibrillation

Force-interval relationships (FIRs) of the heart represent the relationships between inter-beat intervals (IBIs) and strength of the ventricular contractions. These relationships are typically measured invasively and are altered from normal in heart failure (HF). An unobtrusive and continuous measurement of FIRs could be beneficial when HF and atrial fibrillation (AF) coexist in order to understand if AF causes progression of HF. We hypothesize that FIRs could be assessed during AF with IBIs and hemodynamic changes captured unobtrusively by photo-plethysmography (PPG) at the wrist. FIRs were assessed by using Spearman's rank correlation between the pulse onset change in the PPG waveform and either the preceding or pre-preceding IBIs $(r_{pre}\ and\ r_{pre-pre})$ in 5-minute segments. 32 patients (14 continuous AF, 18 no AF) were measured during the night with PPG and electrocardiography as a reference. The mean and standard deviation of $r_{pre}$ were $-0.25\pm 0.08$ and $0.05\pm 0.12(p < 0.0001)$, and of $r_{pre-pre} 0.60\pm 0.09$ and $0.16\pm 0.14 (p < 0.0001)$, during AF and sinus rhythm, respectively. Areas under the Receiver Operating Characteristics curve were 0.987 and 0.998, respectively. Thus, during AF the IBIs correlate with the beat-to-beat changes of blood volume measured with PPG, likely to indicate that FIRs can be measured unobtrusively with the PPG signal.