Smarttouch® SF (Johnson&Johnson, New Brunswick, NJ, USA):
The tip of the catheter is suspended by a machined precision spring. A magnetic transducer generates a small magnetic field in the distal part of the catheter. Three magnetic field sensors are located in the more proximal part of the catheter, distributed around the circumference. As forces are acting on the catheter tip, it moves slightly towards the sensors, changing the signal received by the sensor coils. By Hooke’s law, the acting force and direction can be calculated from the three sensors and the known characteristics of the spring.

Measurement protocol

All catheters were fixed 18-20mm proximal to the tip to get an adequately low bending of the distal part while not compromising the force sensor tip by the clamping mechanism. Prior to the measurements, the catheters were submerged in the heated saline bath for five minutes, allowing for warm-up. The catheter was zeroed after each change of the contact angle. Measurements were taken repeatedly adjusting the exerted force in-between measurements until a minimum of 100 measurements at one specific angle was reached. During acquisition, equal distribution of the measurements across the full measurement range of 0g to 60g was ensured. Subsequently, the catheter was fixed at a different angle and the same protocol was repeated for all angles of 0°, 30°, 45°, 60°, and 90°. The error made by the catheter was calculated by subtracting the weight displayed on the scale (real force) from the contact force displayed on the catheter readout (measured force). A resulting negative value means that the catheter underestimates the real force; a positive value means the catheter overestimates the real force. We evaluated three catheters for each model to verify reproducibility and to quantify inter-catheter variability. For the Stablepointcatheter, measured-force values above 50g are not displayed at angles >45°.

Statistical analysis

Continuous variables are presented as mean ± standard deviation or as median and interquartile range as appropriate. Linear regression analysis was used to determine the measurement error due to friction. Spearman correlation coefficients were used to assess the correlation between measured force and real force. A local smoothing function (locally estimated scatterplot smoothing (LOESS)) was used for the interpretation of the measurement data (loess function, span = 0.3). Statistical analyses were made by using R 4.0.2 (R Core Team, Vienna, Austria).