Nothing happens without energy cost both in physics and biology. How much is the metabolic cost of an extra systole ? While an occasional VPD is benign and has a negligible effect, a high burden of VPDs can consume extra energy and drain its metabolic reserves.
Three ways it can happen
- Inefficiency: VPDs cause premature contractions before AV valves fill and hence there is a mechanically wasted work However the electrical cost is small.
- Dyssynchrony: dys or desynchrony due to the VPDs also hikes myocardial oxygen consumption (MVO₂) for identical workloads.
- Post-Extrasystolic Potentiation: We know, the beat following a VPD shows increased force due to calcium accumulation. This can elevates contractility and wall tension, increasing energy demands.
Molecular basis of energy depletion
At the cellular level, the variable timing of VPDs disrupts calcium homeostasis in phase 2 of action potential within the myocytes. The sarcoplasmic reticulum must continuously pump calcium ions against concentration gradients via SERCA2 channels, a process requiring ATP hydrolysis. Additionally, the chronic workload mismatch induces mitochondrial oxidative stress, which damages the electron transport chain, impairs ATP synthesis resulting in energy consumption mis mamtch.
What is the cut off number ?
A high VPD burden* (10% to 20% all heart beats in day ) is considered a chronic strain, leading to cardiomyopathy and heart failure.First step is a Holter monitor assessment. Yet, we are not clear about the exact number For example : Does bigeminy counted then 50 % of all beats )
*VPDS and elEctrical SCD is different topic , and is not discussed here.
Does suppression help ?
We think so. If and only if we can document porgressive LV dysfunction.
Final message
VPDs are usually benign unless linked to LV dysfunction, which complicates prognosis. The electrical energy cost of VPDs is negligible . But its impact on the mechanical behaviour on myocardial contractility is the issue. It’s crucial to determine if they are primarily electrical or secondary to associated LV dysfunction. Treatment is based on individual assessment.
Reference
Postamble
Difference between APD and VPDs in terms of energy consumption ?
Need to be discussed further.(We know frequent focal APDs or ATs, MAT can result in AF or atrial cardiomyopathy)
Dr.S.Venkatesan MD 