11. Cardiac Resynchronization Therapy: Different Issues

Mechanical left heart AV normalization is an important aspect for the success of cardiac resynchronization therapy (CRT). A normal right heart AV may cause a non-physiological left heart AV interval due to P wave sense offset (PSO) and interatrial electromechanical delay (IAEMD), leading to pacemaker syndrome. A short left heart AV, for example, may affect left atrial transport due to truncation of atrial flow, and thus diminish or abolish the clinical benefits of CRT. It was previously shown that the normal mechanical left AV is 85 ± 15 ms, that the IAEMD (the interval between the right heart P wave and the onset of the A wave of mitral flow) can be predicted from the P wave duration. In addition, and PSO can be easily calculated as the difference between the measured AV (from surface ECG) and programmed AV. Therefore, a computer simulation software was developed to demonstrate the hemodynamic consequences of AV programming in CRT devices based on the following equation: AV = 85 +IAEMD − PSO − VV delay , which normalizes left AV by compensating for above delays. The computer model was designed using actual Doppler findings from a total of 110 patients with and without pacemakers, and demonstrates the interaction between left atrial and left ventricular Doppler flows and pressures as the AV delay is changed. Inputs for the software are the value of IAEMD (predicted from P width), pacing mode (atrial sensing or atrial pacing), the measured value of PSO, and the programmed value of VV delay. The software simulates the time relationship between left atrial and left ventricular pressures and between atrial flow and the onset of mitral regurgitation, thus revealing AV delay conditions that may lead to pacemaker syndrome. Conclusion Computer simulation demonstrates that the use of above equation preserves a mechanical left AV within a normal range for pacing mode changes and different values of IAEMD and rate. A normalized left AV may reduce the risk of pacemaker syndrome.