['We tested the hypothesis that cycle length-dependent cardiac contractility in atrial fibrillation is primarily governed by the negative interval-force relation in patients with normal and depressed systolic function.', 'We performed two-dimensional guided M-mode echocardiography in 41 patients (mean age, 69 +/- 4 years; range, 48 to 92 years; 19 men, 11 women). Twelve patients had objective evidence of left ventricular systolic dysfunction (CMP; mean ejection fraction, 37% +/- 7%) in the absence of coronary artery disease (CAD), 13 patients had documented CAD (mean ejection fraction, 43% +/- 6%), and 16 patients had normal resting left ventricular systolic function (mean ejection fraction, 58% +/- 7%). Simultaneous beat-to-beat blood pressure, end-systolic and end-diastolic dimension, circumferential velocity of fiber shortening (Vcf), and end-systolic wall stress (ESWS) were calculated for all patients. All three groups showed a significant linear relation between beat-to-beat Vcf and Vcf corrected for afterload (represented as the Vcf/ESWS ratio) and preceding cycle length. There was, however, no significant difference in the relation between either of these variables and cycle length among the three groups. There was also no difference in the rate of change in either Vcf or Vcf corrected for afterload (Vcf/ESWS ratio) from beat-to-beat among the three groups. Control patients with normal systolic function showed greater Vcf at any given cycle length compared with patients with CMP or CAD.'] Is interval-force relation unaffected by the presence of cardiomyopathy or coronary artery disease in patients with atrial fibrillation?