Baseline: Normal Rhythm
Normally, in an intact heart without ligatures, excitation originates in the primary pacemaker—the venous sinus (analogous to the sinuatrial node, Remak's ganglion). The impulse propagates sequentially to the atria and then to the ventricles.
The kymograph records a normal sinus rhythm with the maximum heart rate (HR) for this myocardium. Two distinct waves are visible:
- Small wave: Atrial systole (AS).
- Large wave: Ventricular systole (VS).
First Stannius Ligature (Isolating)
The first ligature is applied in the sulcus between the venous sinus and the atria. It is tied tightly, making it an isolating ligature.
- Effect: The venous sinus is completely separated from the rest of the heart.
- Result: The sinus continues to contract at its normal rhythm (though this is often unrecorded on the myogram due to isolation). Meanwhile, the atria and ventricles stop in diastole, showing a flat line (asystole) on the tracing.
- Physiological significance: This clearly demonstrates that the venous sinus is the primary pacemaker. The underlying chambers stop because their intrinsic pacemakers have low lability. They require a "warm-up" period (pre-automatic pause) because they were previously suppressed by the high-frequency impulses from the sinus (overdrive suppression).
Second Stannius Ligature (Irritating/Compressing)
The second ligature is applied along the atrioventricular sulcus, leaving the first ligature in place. Crucially, this thread is not tied tightly; it applies only light pressure, acting as an irritating ligature.
- Effect: Mechanical irritation of the secondary pacemaker—the atrioventricular junction (Bidder's ganglion)—while isolating it from inhibitory influences.
- Result: The ventricles resume spontaneous contractions. The atria and ventricles contract simultaneously, merging into a single wave on the cardiogram.
- Rhythm frequency: Becomes slow—approximately twice as slow as the baseline sinus rhythm.
- Physiological significance: The atrioventricular junction possesses intrinsic automatism, but its impulse generation rate is lower. Note: Sometimes this junction may "escape" on its own after some time following the first ligature—a phenomenon known as an escape rhythm.
Third Stannius Ligature (Isolating the Apex)
The third ligature is applied to the lower third of the ventricles, isolating the cardiac apex. Previous ligatures remain intact. This ligature is isolating once again.
- Effect: Separation of the working (contractile) myocardium of the apex from the conduction system.
- Result: The isolated apex does not contract; the myogram records a flat line. Very rare twitches may occasionally occur in other parts of the myocardium due to the automatism of Purkinje fibers.
- Physiological significance: Typical cardiomyocytes of the apex lack pacemaker cells (P-cells). Without impulses from above, the apex lacks automaticity.
Law of the Gradient of Automatism
The overall conclusion of the experiment is that the degree of automatism decreases from the base of the heart to its apex:
- The sinus-venosus region possesses maximal automatism (primary pacemaker).
- The atrioventricular region possesses moderate automatism (secondary pacemaker).
- The cardiac apex lacks automatism.