Why ICDs Are Placed on the Left Side
ICDs are preferentially placed on the left side because left pectoral placement results in significantly lower defibrillation thresholds (8.9 J vs 10.6 J) and creates a more favorable shock vector through the cardiac mass, though right-sided placement remains a viable alternative when left-sided access is contraindicated. 1
Physiologic Rationale for Left-Sided Placement
The left pectoral position creates an optimal electrical field distribution:
The shock vector formed between the active generator can and the right ventricular lead system passes more directly through the left ventricular myocardium when the device is positioned on the left side. 1, 2
Left-sided implantation directs defibrillating current through the bulk of the cardiac mass more efficiently than right-sided placement, which directs current toward the right hemithorax. 2
The traditional approach since pectoral implantation became standard in 1995 has favored left-sided placement based on these electrical considerations. 3
Evidence Comparing Left vs Right Placement
Defibrillation Threshold Differences
Patients receiving right pectoral implants demonstrate significantly higher defibrillation thresholds (10.6 ± 3.8 J) compared to left pectoral implants (8.9 ± 4.2 J, p = 0.01), despite similar shock impedances. 1
Right-sided transvenous ICD systems show mean DFTs of 17.0 ± 4.9 J versus 11.3 ± 5.3 J for left-sided systems (p < 0.0001). 2
Clinical Efficacy Considerations
Despite higher thresholds, conversion efficacy remains comparable:
Spontaneous arrhythmia conversion rates are not significantly different between right-sided (100%) and left-sided (97%) implants when adequate safety margins are maintained. 1
Induced ventricular fibrillation conversion efficacy is similar (99% right-sided vs 98% left-sided, p = 0.18). 1
Critical Mortality Finding
A concerning finding is that all-cause mortality rates are nearly doubled (hazard ratio 1.93, p < 0.004) in patients with right-sided implants compared to left-sided placement. 1
This mortality difference must be carefully considered when device placement is being planned, though the mechanism remains incompletely understood.
When Right-Sided Placement Is Necessary
Right pectoral implantation becomes the appropriate choice when:
- Left-sided venous access is compromised or unavailable 1, 2
- A preexisting pacemaker occupies the left pectoral space 2
- Anatomic considerations preclude left-sided access 1
Optimizing Right-Sided Implants
When right-sided placement is clinically mandated:
Active can (hot can) devices should be used preferentially, as they result in lower DFTs (15 ± 4.1 J) compared to cold can systems (19 ± 4.8 J, p = 0.05) with right-sided implantation. 2
Defibrillation threshold testing should be strongly considered to ensure adequate safety margins, particularly with right-sided implants, single-coil leads, or epicardial systems. 4
Standard Lead Positioning
Regardless of generator side:
The right ventricular apex remains the traditional and most commonly used target site for ICD lead placement, with well-established long-term data. 4
Alternative RV positions including the septum and outflow tract are equally effective for sensing and defibrillation in most patients. 4
Important Clinical Pitfalls
Avoid excessive RV apical pacing (>40-50% of the time) in patients with reduced ejection fraction, as this is associated with increased heart failure hospitalizations and mortality. 4
For patients requiring cardiac resynchronization therapy with ICD backup, left ventricular lead placement via coronary sinus targeting the lateral or posterolateral wall is recommended, avoiding apical LV positions which significantly worsen outcomes. 4
When defibrillation threshold testing is performed with right-sided implants, ensure adequate safety margins given the inherently higher thresholds. 1, 2