Pacemaker Potentials, Conduction and Reading an ECG
The sinoatrial node normally sets the heart's rhythm through pacemaker potentials. Excitation crosses the atria, slows at the atrioventricular node, then reaches the ventricles. An ECG records combined electrical activity, not the contractions themselves.
What a learner can do afterwards
- Names P, QRS and T and links them to atrial depolarization, ventricular depolarization and ventricular repolarization.
- Explains why the delay at the atrioventricular node is useful rather than a flaw.
- Calculates ventricular rate from regular R-R spacing at a stated paper speed and identifies unequal R-R intervals.
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The neuronal spike used sodium entry for its rapid rise. Pacemaker cells in the sinoatrial (SA) node have no steady resting voltage between spikes. A changing balance of inward and outward ion currents brings them to threshold. Calcium entry drives the main upstroke, and potassium exit helps repolarize the cell. The SA node normally sets the heart's rhythm; nerves and hormones can change its rate.
Excitation spreads from the SA node across the atria, the upper chambers. Slow conduction through the atrioventricular (AV) node lets atrial contraction add blood to the ventricles before they contract. The signal then passes through the His bundle, bundle branches and Purkinje fibres to ventricular muscle. The ventricles are the lower chambers.
An electrocardiogram (ECG) records voltage differences at the skin from combined cardiac electrical activity. P marks atrial depolarization, QRS ventricular depolarization, and T ventricular repolarization. Depolarization triggers contraction but is not contraction itself. Atrial repolarization is usually hidden by QRS. Lead choice changes wave size and direction, so QRS is not always a tall upward spike.
For a regular ventricular rhythm recorded at 25 mm/s, each large square spans 0.2 s. Rate in beats per minute = 300 / large squares between successive R peaks in QRS complexes. For example, 6 squares span 1.2 s, so rate = 300 / 6 = 50 beats per minute. The dots below mark R-peak times, not wave shapes.
Compare successive R-R intervals to check ventricular rhythm. Equal intervals show regular timing; unequal intervals show irregular timing if the recording is reliable. Do not use the regular-rhythm 300 rule for an irregular strip. These intervals alone do not identify the cause. An ECG also does not directly measure pumping strength.
Stroke volume is blood ejected by one ventricle per beat: volume before contraction minus volume left afterwards. If early contraction reduces filling while the remaining volume stays unchanged, stroke volume falls. Cardiac output is stroke volume times heart rate, so output falls too if heart rate stays unchanged. A change in conduction timing alone does not establish the change in output.
Electrodes are skin contacts; leads are electrical views formed from their signals. A standard 12-lead ECG uses 10 electrodes. Trained staff obtain consent, use standard landmarks and prepare the skin. Movement or poor contact can add noise. Misplacement can change wave shapes and mimic abnormalities. A regular trace is not proof of a healthy heart. This lesson does not teach diagnosis or unsupervised electrode placement.
P, QRS and T mark electrical events. AV delay supports filling. Use 300 / large squares only for regular ventricular rhythms recorded at 25 mm/s.
Lesson, questions, worked solutions and flashcards written for this dataset.
Readings and flashcards written for this dataset.
More to read
Why the next beat starts
A neuron usually needs input to bring its membrane to threshold. SA node pacemaker cells behave differently between action potentials: their voltage moves towards threshold again without a new nerve signal for each beat. This gradual change is the pacemaker potential.
Several changing ion currents contribute to that drift. Once threshold is reached, calcium entry produces the main upstroke. Potassium leaving helps the voltage fall again. The sequence repeats, allowing the SA node to set the normal rhythm. Nerves and hormones can alter the rate.
The impulse spreads across the atria, passes slowly through the AV node, and then travels through the His bundle, bundle branches and Purkinje fibres to ventricular muscle. The neuronal explanation based on a sodium-driven upstroke cannot be applied unchanged to the SA node action potential.
What the AV delay allows
Most ventricular filling occurs while the ventricles are relaxed. Atrial contraction adds blood before ventricular contraction begins. Slow conduction through the AV node leaves time for this contribution. The delay does not mean that all blood flow stops.
A timing change does not by itself tell us the resulting cardiac output. We also need to know the volumes and heart rate. Stroke volume is the amount present before ventricular contraction minus the amount left afterwards. Reducing the starting amount reduces stroke volume if the remaining amount stays the same. If heart rate also stays unchanged, cardiac output falls.
These are conditional relationships. An ECG records electrical activity, so it cannot supply the ventricular volume measurements needed for this calculation.
Electrical events and the time between them
The P wave represents atrial depolarization. QRS represents ventricular depolarization, and T represents ventricular repolarization. Contraction follows electrical activation, but an ECG does not directly measure the contraction. Atrial repolarization is usually hidden during QRS. The appearance of these features also depends on the lead being recorded.
Paper speed tells us how much time a horizontal distance represents. At 25 mm/s, a large square spans 0.2 seconds. On a regular strip, four large squares between successive R peaks mean 0.8 seconds between ventricular activations. The rate is 300 / 4 = 75 beats per minute.
Compare several R-R intervals before choosing that method. Unequal intervals show irregular ventricular timing on a reliable recording. A single interval then does not represent the whole strip, and the regular-rhythm 300 rule is unsuitable. Irregular spacing alone does not identify its cause.
Contacts, views and recording quality
An electrode touches the skin. A lead is an electrical view formed from electrode signals. A standard 12-lead ECG uses ten electrodes, including six chest contacts. The number of views therefore differs from the number of contacts.
Chest contacts have standard landmarks. Moving a contact can change the recorded wave pattern without changing the heart's electrical activity. Poor skin contact or body movement can add unwanted signals. Recording quality must be considered before interpreting an apparent abnormality.
Trained staff obtain consent, protect privacy and follow the recording procedure. This reading does not provide placement instructions for use on another person. Regular spacing on a trace also does not prove that the heart is healthy or pumping effectively.
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