--- title: "Pacemaker Potentials, Conduction and Reading an ECG" description: "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 combine" canonical: https://lightmysky.com/learn/science/pacemaker-potentials-conduction-and-reading-an-ecg-mt_D-WKFc7BZm source: https://lightmysky.com/learn/science/pacemaker-potentials-conduction-and-reading-an-ecg-mt_D-WKFc7BZm.md retrieved: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # 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. Subject: Science · Area: The Human Body · Ages 19 to 20 Page: https://lightmysky.com/learn/science/pacemaker-potentials-conduction-and-reading-an-ecg-mt_D-WKFc7BZm ## Ready when they can - 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. ## Lesson: Reading the heartbeat line 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. *(drawing: Read the conduction route in order. This sequence does not show anatomy or time to scale.)* 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. **Example.** 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. **Recap.** 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.* ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Channel Kinetics and the Ionic Basis of the Spike](https://lightmysky.com/learn/science/channel-kinetics-and-the-ionic-basis-of-the-spike-mt_47-LDzgcKp) ## Optional connections - [The Cardiac Cycle and Pressure in the Heart](https://lightmysky.com/learn/science/the-cardiac-cycle-and-pressure-in-the-heart-mt_iOSVyoV3Pj) ## Opens up - [Preload, Afterload and the Frank-Starling Relationship](https://lightmysky.com/learn/science/preload-afterload-and-the-frank-starling-relationship-mt_WK8o6OPZ-O)