Physiological Changes During Sleep Affecting Leadless Pacemaker Thresholds

Several important physiological changes occur during sleep that could influence pacing thresholds in patients with leadless pacemakers:

Autonomic Tone Shifts

During sleep, particularly during non-REM sleep stages, there's a notable shift toward parasympathetic (vagal) dominance. This causes:

• Decreased heart rate (sinus bradycardia)

• Reduced myocardial contractility

• Potentially altered myocardial excitability that may require higher energy for effective capture

Intrathoracic Pressure Changes

Sleep is associated with significant respiratory-related pressure changes:

• During inspiration, intrathoracic pressure becomes more negative (down to -7 to -10 cmH₂O)

• This negative pressure can alter the physical relationship between the leadless pacemaker and the endocardium

• Right-side sleeping specifically creates asymmetric pressure distribution within the thoracic cavity

• These pressure gradients might temporarily increase the functional distance between the device electrode and excitable myocardium

Cardiac Output and Hemodynamic Shifts

Sleep position impacts venous return and cardiac chamber filling:

• Right-side sleeping increases right ventricular filling due to gravitational effects

• RV volume expansion can change the position of the leadless pacemaker relative to the myocardium

• The resulting stretch of myocardial tissue might alter local electrical properties and capture thresholds

Positional Changes in Right Ventricular Geometry

• The right ventricle has a complex geometry that changes with body position

• Right-side sleeping may cause subtle shifts in RV dimension and shape

• Leadless pacemakers typically anchor to the RV apical septum, and position changes could affect the electrode-tissue interface

Fluid Shifts and Electrolyte Changes

• Recumbent position during sleep causes redistribution of body fluids

• This can alter local tissue hydration and electrolyte concentrations

• Both factors can influence myocardial excitability and conduction properties

These physiological changes acting together likely explain the observed variations in capture thresholds during sleep, particularly in right-side sleeping positions. The effect may be more pronounced with leadless pacemakers compared to traditional lead-based systems due to their smaller electrode surface area and unique fixation mechanisms.