Dromotropy – Cardiac Conduction Velocity
Dromotropy describes the conduction velocity of electrical impulses through the heart muscle. It is a key parameter in cardiac physiology and clinical cardiology.
Things worth knowing about "Dromotropy"
Dromotropy describes the conduction velocity of electrical impulses through the heart muscle. It is a key parameter in cardiac physiology and clinical cardiology.
What is Dromotropy?
Dromotropy (also referred to as dromotropism) is a medical term describing the ability of cardiac tissue to conduct electrical impulses at a specific speed. The word derives from the Greek dromos (race, course) and tropos (turn, direction). In practical terms, dromotropy is a measure of the conduction velocity of electrical signals through the heart.
Dromotropy is one of four classic physiological properties of the heart, alongside chronotropy (heart rate), inotropy (contractile force), and bathmotropy (excitability). All four properties can be influenced by the autonomic nervous system and various pharmacological agents.
Physiological Background
In a healthy heart, the electrical impulse originates in the sinoatrial (SA) node and is transmitted through the atrioventricular (AV) node, the bundle of His, the bundle branches, and the Purkinje fibers to the ventricular myocardium. Conduction velocity differs depending on the cardiac structure involved:
- AV node: Very slow conduction (approx. 0.05 m/s) – this delay allows coordinated atrial and ventricular contraction
- Bundle of His and bundle branches: Fast conduction (approx. 2–4 m/s)
- Purkinje fibers: Very fast conduction (approx. 4 m/s)
- Working myocardium: Intermediate conduction (approx. 0.3–1 m/s)
This carefully coordinated system ensures that the atria and ventricles contract in the correct sequence, allowing the heart to pump blood efficiently.
Positive and Negative Dromotropic Effects
Substances or conditions that alter the conduction velocity of the heart are described as having a dromotropic effect:
- Positive dromotropic: Increases conduction velocity. Examples include catecholamines such as adrenaline (epinephrine), sympathetic nervous system activation, and atropine.
- Negative dromotropic: Decreases or slows conduction velocity. Examples include parasympathetic (vagal) stimulation, beta-blockers, calcium channel blockers (e.g., verapamil, diltiazem), cardiac glycosides, and adenosine.
Clinical Significance
Disturbances in dromotropy can result in various cardiac arrhythmias, most notably conduction blocks:
- First-degree AV block: Prolonged but intact AV conduction (prolonged PR interval on ECG)
- Second-degree AV block: Intermittent failure of atrial impulses to conduct to the ventricles
- Third-degree AV block (complete heart block): Complete dissociation between atrial and ventricular activity
- Bundle branch block: Delayed or interrupted conduction in one of the bundle branches
These abnormalities are detectable on the electrocardiogram (ECG) and may significantly affect cardiac output and hemodynamic stability.
Diagnosis
Dromotropic function is primarily assessed using the ECG (electrocardiogram). The following parameters reflect conduction velocity:
- PR interval: Reflects conduction time from the SA node through the AV node to the ventricles (normal range: 120–200 ms)
- QRS complex duration: Reflects ventricular conduction time (normal: less than 120 ms)
Prolongation of these intervals indicates reduced (negative dromotropic) conduction velocity.
Therapeutic Relevance
Pharmacologically modifying dromotropy is an important strategy in the management of cardiac arrhythmias. Negative dromotropic drugs such as beta-blockers and calcium channel blockers are used, for example, in atrial fibrillation to slow AV nodal conduction and reduce excessive ventricular rate. Conversely, in cases of high-degree AV block, a cardiac pacemaker may be required to artificially restore conduction and maintain adequate cardiac output.
References
- Silbernagl, S. & Despopoulos, A. (2015). Color Atlas of Physiology. 7th edition. Thieme Publishers.
- Katz, A.M. (2010). Physiology of the Heart. 5th edition. Lippincott Williams & Wilkins.
- Fuster, V. et al. (2022). Hurst's The Heart. 14th edition. McGraw-Hill Education.
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