Cardiac cycle-1

Cardiac Cycle Overview

• The heart is a pump with two separate systems:

  • Right heart: pumps blood through the lungs.

  • Left heart: pumps blood to peripheral organs.

• Cardiac contraction and relaxation are rhythmic:

  • Systole: the contractile phase.

  • Diastole: the relaxation phase.

  • Atrial events can also be prefixed with 'atrial' to distinguish from ventricular events.

Learning Objectives

• By the end of the lesson, a learner should be able to:

  • Explain events of the cardiac cycle including:

    • Atrial systole and diastole.

    • Ventricular systole and diastole.

  • Relate heart sounds to specific events in the cycle.

  • Define cardiac output and describe influencing factors.

  • Explain adjustments of stroke volume and cardiac output at different activity levels.

Introduction to Cardiodynamics

• Cardiodynamics focuses on mechanical events associated with:

  • Pressure changes in:

    • Ventricles

    • Atria

    • Aorta

  • Volume changes in ventricles.

  • Valvular events contributing to heart sounds.

Cardiac Cycle Defined

• The cardiac cycle involves a series of coordinated events during each heartbeat, including:

  • Systole: heart contracts and pumps blood through arteries.

  • Diastole: heart relaxes and fills with blood.

• Each cycle is 0.8 seconds at a normal heart rate of 75 beats/min.

Phases of Cardiac Cycle

Atrial Cycle

• Atrial systole lasts 0.1 seconds; atrial diastole lasts 0.7 seconds.

  • Atrial systole coincides with rapid filling phase of ventricular diastole.

  • 105 ml (75%) of blood flows into ventricles before contraction.

  • Atrial contraction adds 25 ml (25%), resulting in 130 ml end-diastolic volume (EDV).

Ventricular Cycle

• Ventricular systole lasts 0.3 seconds and consists of:

  • Isovolumic contraction (0.05 seconds).

  • Ventricular ejection (0.25 seconds total):

    • Rapid ejection (0.1 seconds).

    • Slow ejection (0.15 seconds).

• Ventricular diastole lasts 0.5 seconds and includes:

  • Protodiastole (0.04 seconds).

  • Isovolumic relaxation (0.06 seconds).

  • Rapid passive filling (0.11 seconds).

  • Reduced filling (0.19 seconds).

  • Last rapid filling phase coinciding with atrial systole (0.1 seconds).

Heart Sounds

First Heart Sound (S1)

• Caused by closure of AV valves during isovolumic contraction.

• Characteristics: 0.15 seconds duration, heard over mitral and tricuspid areas.

Second Heart Sound (S2)

• Caused by closure of semilunar valves at the start of diastole.

• Characteristics: short, loud, high-pitched sound.

• Coincides with end of T wave in ECG.

Third Heart Sound (S3)

• Caused by blood rush during rapid filling in diastole.

• Normally inaudible in adults, appears between T and P waves in ECG.

Fourth Heart Sound (S4)

• Caused by vibrations during atrial systole.

• Typically inaudible.

Cardiac Output and Venous Return

• Cardiac output (CO) is the amount of blood pumped by each ventricle per minute.

  • Calculated using: CO = Stroke Volume (SV) × Heart Rate (HR).

  • Normal CO: 5-6 L/min.

  • Stroke volume: blood pumped per contraction.

• Cardiac index: CO related to body surface area (normal ~3.2 L/min/m²).

Factors Affecting Cardiac Output

• CO is influenced by:

  • Venous return

  • Myocardial contractility

  • Peripheral resistance

  • Heart rate

Regulation Mechanisms

Intrinsic Regulation (Frank-Starling Mechanism)

• The force of cardiac contractions is proportional to preload (EDV).

Extrinsic Regulation

• Influenced by:

  • Sympathetic nervous system: increases SV and HR.

  • Hormonal effects: T3/T4 and glucagon enhance contractility.

Physiological Variations in Cardiac Output

• Variations due to:

  • Age, Sex, Diurnal changes.

  • Environmental temperature and exercise (can increase CO up to 700%).

  • Anxiety, excitement, and pregnancy can increase CO.

Pathological Variations in Cardiac Output

• Conditions increasing CO:

  • Fever, anemia, hyperthyroidism.

• Conditions decreasing CO:

  • Rapid arrhythmias, congestive heart failure, cardiac shock, hemorrhage, etc.

Conclusion

• The cardiac cycle consists of a series of coordinated events that manage blood flow and pressure changes efficiently. Understanding these elements is vital for grasping cardiovascular physiology and pathophysiology.