Overview
This lecture explains the cardiac cycle, how electrical activity of the heart is measured with an electrocardiogram (ECG), interpretation of normal and abnormal ECGs, and basics of blood pressure related to heart function.
Cardiac Cycle Basics
- The cardiac cycle includes one complete sequence of heart contraction and relaxation (atria and ventricles).
- Atria contract (atrial systole) while ventricles are relaxed (ventricular diastole), moving blood into the ventricles.
- Ventricles contract (ventricular systole) while atria relax, ejecting blood into arteries.
- The cycle features phases: atrial contraction, isovolumetric contraction (ventricles build pressure, no volume change), ventricular ejection, isovolumetric relaxation, and passive filling.
- All four heart valves open and close during the cycle; heart sounds occur when valves close.
Electrocardiogram (ECG) and Cardiac Cycle Correlation
- ECG measures electrical changes (depolarization/repolarization) in the heart using chest electrodes.
- P-wave: atrial depolarization and contraction begin.
- PQ interval: atria are contracted and ventricles are filling.
- QRS complex: ventricular depolarization, atria repolarize (relax), and ventricles contract.
- ST segment: time when ventricles are contracting (ejection).
- T-wave: ventricular repolarization (relaxation).
- One cardiac cycle on ECG is from one P-wave to the next.
Interpreting Normal and Abnormal ECGs
- Normal ECG: regular P-QRS-T sequence.
- Abnormal ECGs indicate conduction problems:
- Partial block: missing QRS complexes after some P-waves.
- Atrial fibrillation: irregular and missing P-waves.
- Ventricular tachycardia: fast, abnormal QRS complexes, little ventricular filling.
- Ventricular fibrillation: chaotic, no organized contraction.
- Third degree block: disorganized P-waves and QRS complexes.
Blood Pressure and Its Relationship to the Cardiac Cycle
- Systolic blood pressure: pressure during ventricular contraction (higher number, e.g., 120 mmHg).
- Diastolic blood pressure: pressure during ventricular relaxation (lower number, e.g., 80 mmHg).
- Mean arterial pressure (MAP): average pressure, closer to diastolic because the heart spends more time relaxed; calculated as MAP = diastolic + 1/3(systolic - diastolic).
- Arteries have higher pressure and thicker walls compared to veins.
- Pulmonary blood pressure (in the lungs) must be measured invasively.
Heart Sounds & Pressure Changes
- "Lub" (first sound): AV valves close at the start of ventricular contraction.
- "Dub" (second sound): semilunar valves close at the end of ventricular ejection.
- Most of the heartbeat time is spent in relaxation (diastole).
Key Terms & Definitions
- Cardiac Cycle — one complete heartbeat including contraction and relaxation of atria and ventricles.
- ECG (Electrocardiogram) — test measuring electrical activity of the heart.
- P-wave — atrial depolarization/contraction.
- QRS complex — ventricular depolarization/contraction, atrial repolarization.
- T-wave — ventricular repolarization/relaxation.
- Systolic pressure — arterial pressure during heart contraction.
- Diastolic pressure — arterial pressure during heart relaxation.
- Mean arterial pressure (MAP) — average arterial blood pressure during a single cardiac cycle.
- Isovolumetric contraction/relaxation — phases with no change in ventricular volume but pressure changes.
Action Items / Next Steps
- Review ECG wave segments and corresponding heart events.
- Practice identifying normal and abnormal ECG patterns.
- Memorize how to calculate mean arterial pressure (MAP).