VALIDITY OF THE STRINGER FORMULA FOR EVALUATING THE PROGRESSION OF CARDIAC STROKE VOLUME DURING INCREMENTAL EXERCISE

Abstract

PURPOSE: The Stringer formula (ST; Stringer et al. 1997) aims to estimate cardiac output (Q̇) non-invasively during cardiopulmonary exercise testing (CPET). ST considers the arterio-venous oxygen difference as a function of the percentage oxygen consumption (V˙O2), where Q̇ST = V˙O2 / [5.72 + 0.105 * %V˙O2max]. If heart rate (HR) is known, stroke volume (SV) = Q̇/HR. It is unclear whether the progression of SV during incremental exercise (i.e., increase, plateau, or decrease) can be validly monitored via Q̇ST. Our aim was to compare the progression of SV obtained with ST with an established inert gas rebreathing method (RB). METHODS: Sixteen male participants (181 ± 8 cm, 78.0 ± 6.7 kg, 26 ± 4 years; V˙O2max 55 ± 10 mL/min/kg) completed an incremental test (6 steps, 3 min) ranging 40-90% V˙O2max with CPET and RB (Innocor, Innovision, DK) on a cycle ergometer. At the end of each stage, Q̇RB was measured via RB. V˙O2max had been determined before. The data set was analyzed graphically and statistically (regression, Bland-Altman plots, correlation). To assess individual progressions, SVST and SVRB were normalized (nor) to the corresponding maximum. Positive correlations of SVnorRB and SVnorST were used as indicators of progression agreement and also validated graphically. RESULTS: Ninety measurements >40% V˙O2max resulted in the following regressions: Q̇RB = 5.162*V˙O2 + 4.414 (r = 0.84) and Q̇ST = 4.658* V˙O2 + 8.511 (r = 0.91). The correlation of absolute Q̇ST vs. Q̇RB was r = 0.83 and SVST vs. SVRB was r = 0.71, respectively. Correlation of Q̇norST vs. Q̇norRB was r = 0.93 and SVnorST vs. SVnorRB was r = 0.21. The mean of the differences for Q̇RB-Q̇ST was -2.6 ± 2.9 L/min and for SVRB-SVST -20 ± 19 mL. In 12 participants, correlations between SVRB and SVST were positive (r ≥ 0.48 ≤ 0.86), indicating an agreement of the progression of SVST and SVRB. Four correlation coefficients were negative. Graphical analysis confirmed the interpretation of the correlation coefficients. CONCLUSIONS: Despite the strong correlation of Q̇RB vs. Q̇ST during incremental exercise, the progression of SVRB was not validly determined by ST in 25% of the participants. Hence, ST does not appear to be appropriate for this purpose.