Cardiac Conduction and Pharmacokinetics

Understanding Cardiac Conduction

Effective cardiac function relies on a precisely timed electrical conduction system. This section explores the key structures, their roles, and the ionic mechanisms that regulate heart rate and rhythm.

1. The Atrioventricular (AV) Node – The Critical Delay

The AV node is the primary structure that delays the electrical impulse, allowing the atria to finish emptying before the ventricles contract. This delay, typically 120–200 ms, ensures optimal ventricular filling and prevents atrial‑ventricular overlap.

• Location: Situated in the lower part of the interatrial septum, near the tricuspid valve.
• Function: Provides the only physiologic delay in the conduction pathway.
• Clinical relevance: AV‑node block can lead to escape rhythms (see Section 4).

2. Sympathetic Stimulation and Heart Rate

During sympathetic activation, the most direct ionic change that increases heart rate is an increase in calcium (Ca²⁺) influx, which lowers the threshold for depolarization in nodal cells. This accelerates phase 4 depolarization, shortening the pacemaker interval.

• Mechanism: β‑adrenergic receptors stimulate cAMP → PKA → phosphorylation of L‑type Ca²⁺ channels.
• Result: Faster rise to threshold, higher firing rate.
• Contrast: Na⁺ influx, K⁺ efflux, or reduced Na⁺ permeability have less immediate impact on rate.

3. Propagation Speed in Purkinje Fibers

The rapid conduction of impulses through the Purkinje system is chiefly determined by the high density of gap junctions between cells. These connexin‑mediated channels allow ions to flow freely, creating a low‑resistance pathway.

• Key feature: Gap junctions provide electrical continuity, enabling velocities up to 4 m/s.
• Other factors: While sarcoplasmic reticulum and contractile proteins are important for contraction, they do not directly affect conduction speed.

4. Escape Rhythms When the AV Node Fails

If the AV node ceases to conduct, the ventricles can generate an ventricular escape rhythm at an approximate rate of 15–20 beats per minute (bpm). This intrinsic pacemaker originates from the distal Purkinje network and serves as a life‑saving backup.

• Typical rate: 15–20 bpm, slower than atrial or junctional escape rhythms.
• Clinical sign: Wide QRS complexes on ECG due to ventricular origin.
• Management: May require temporary pacing if hemodynamically unstable.

5. Ionic Movements During Nodal Repolarization

During the repolarization phase of nodal action potentials, the predominant ion movement is the efflux of potassium (K⁺) through slow channels. This outward current restores the resting membrane potential.

• Channels involved: Delayed‑rectifier K⁺ channels (IK).
• Outcome: Membrane potential becomes more negative, preparing the cell for the next depolarization.

Fundamentals of Pharmacokinetics

Pharmacokinetics describes how the body absorbs, distributes, metabolizes, and eliminates drugs. Mastery of these concepts is essential for safe and effective medication use.

1. Bioavailability of Intravenous (IV) Drugs

When a drug is administered intravenously, its bioavailability is 100 % because it directly enters the systemic circulation, bypassing any absorption barriers or first‑pass metabolism.

• Definition: Bioavailability (F) = (AUC oral / AUC IV) × (Dose IV / Dose oral).
• Implication: Dosing calculations for IV drugs assume complete systemic exposure.

2. Plasma Protein Binding and Free Drug Concentration

A drug that is highly bound to plasma albumin primarily decreases the free (unbound) drug concentration. Only the unbound fraction is pharmacologically active and available for tissue distribution.

• Effect on Vd: High binding usually reduces the apparent volume of distribution.
• Clinical relevance: Changes in albumin levels (e.g., in liver disease) can alter drug efficacy and toxicity.

3. Sublingual Administration and First‑Pass Metabolism

Sublingual drugs avoid first‑pass metabolism because the sub‑lingual vessels drain directly into the systemic (jugular) circulation, bypassing the hepatic portal system.

• Advantage: Faster onset of action and higher bioavailability for drugs susceptible to hepatic metabolism.
• Examples: Nitroglycerin, certain benzodiazepines.

4. Key Pharmacokinetic Parameters Affected by Binding

While protein binding influences free drug levels, it also indirectly impacts other parameters:

• Volume of distribution (Vd): Highly bound drugs often have a lower Vd because they remain in the vascular compartment.
• Clearance: Only the free fraction is cleared by the kidneys or metabolized by the liver.
• Half‑life: Reduced free drug can prolong apparent half‑life if clearance is limited.

Integrating Cardiac Physiology and Pharmacology

Understanding how conduction mechanisms intersect with drug actions enables clinicians to predict therapeutic and adverse effects.

1. Drugs Influencing AV‑Node Conduction

Agents such as β‑blockers, calcium‑channel blockers, and digoxin slow AV‑node conduction, prolonging the PR interval. This can be therapeutic in atrial fibrillation but may precipitate AV‑block in susceptible patients.

2. Managing Escape Rhythms

When an escape rhythm emerges, clinicians may use atropine to increase heart rate by enhancing sympathetic tone, or temporary pacing if the rhythm is too slow.

3. Pharmacokinetic Considerations in Cardiac Drugs

Many cardiac medications are highly protein‑bound (e.g., warfarin). Monitoring albumin levels and potential drug‑drug interactions is crucial to maintain therapeutic efficacy.