External validation and ergometer interchangeability of equations for estimating heart rate at ventilatory thresholds
Authors: Milani J G P O , Milani M , Machado F V C , Ascenzi F D , Cavigli L , Keytsman C , Falter M , Bonnechere B , Meesen R , Wilhelm M , Marcin T , Verboven K , Cornelissen V , Cipriano Junior G , Hansen D 
Published: June 2024

Abstract

Background
Cardiopulmonary exercise testing (CPET) is the gold-standard for determining ventilatory thresholds (VTs) and prescribing exercise intensity for patients with cardiometabolic diseases (CMD). Multivariable equations to predict heart rate (HR) at the first and second VTs (VT1, VT2) offer a new alternative for indirect prescription when CPET is unavailable. However, there is still a need for external validation, and it is unknown if the equations are ergometer-specific.

Purpose
To externally validate and test the ergometer interchangeability of treadmill HR prediction equations for VTs in a large multicentric cycle-ergometer European database; to compare the agreement, accuracy, and reliability of the equations with the guideline-based recommendations.

Methods
Observational study of cycle-ergometer CPETs performed on CMD patients from eight European countries (Belgium, Denmark, France, Germany, Italy, Spain, the Netherlands, Switzerland). Inclusion criteria: sinus rhythm, identification of both VTs, and a respiratory exchange ratio ≥ 1.00. Measured HR at VT was compared to estimated values using the equations [HR at VT1 = 3.453 + (0.887 × HRpeak) – (0.555 × (HRpeak - HRrest)) + (1.044 × METpeak); HR at VT2 = - 8.256 + (0.979 × HRpeak) – (0.232 × (HRpeak - HRrest)) + (1.418 × METpeak)]. Accuracy, agreement, and reliability were assessed with Bland-Altman plots, mean absolute percentage error (MAPE), and intraclass correlation coefficient (ICC). Results were compared to European and Brazilian recommendations for moderate intensity according to the percentage of HRpeak and HR reserve (%HRpeak, %HRR).

Results
The analysis included 1,117 CPETs (81% males, median age: 69 years). Most had coronary artery disease (69%). The measured and estimated median HR differed between methods. Accuracy analysis showed a MAPE of 7.1% for VT1 and 5.6% for VT2. The results for %HRpeak recommendations revealed lower accuracy, with MAPE ranging from 10.8 to 26.3% for VT1 and 7.1 to 17.2% for VT2, while the results for %HRR were similar to the equation for VT1 (6.4 to 7.4%) and VT2 (5.1 to 5.7%). The %HRpeak method also showed the lowest values for ICC and the highest bias in Bland-Altman plots. The equations exhibited agreement and reliability similar to the %HRR methods (Figures 1 and 2). Thus, despite being developed for the treadmill, the prediction equations showed clinically acceptable accuracy, agreement, and reliability in a cycle ergometer sample.

Conclusion
The prediction equations for HR at VTs were externally validated and revealed ergometer interchangeability in a large multicentric European database. The %HRpeak method showed higher inaccuracy and lower reliability, raising concerns about its clinical applicability. The %HRR method had similar accuracy and reliability to the equations. Thus, prescribing exercise intensity using two variables (HRpeak and HRrest), by both %HRR or the equations, is an appropriate method when CPET is unavailable.