Cardiac Output (Preload, Afterload, Pressure-Volume Diagram) | Heart Physiology

Cardiac Output QUIZ:    • Cardiac Output QUIZ  

Content:
0:00 Introduction
0:42 What is Cardiac Output?
02:08 The Equation
02:45 Changes in Heart Rate
03:24 Changes in Stroke Volume
04:42 Normal Values of CO
04:47 Left Ventricular Pressure-Volume Diagram
05:34 Maximum Pressure-Volume Relationship
07:25 Normal Pressure-Volume Relationship
09:46 Why make graphs?
10:35 Preload
10:57 Afterload
11:51 Contractility
12:30 Next video
13:00 QUIZ

Welcome to Taim Talks Cardio! In this segment of our Complete Cheat Code for Heart Physiology series, we dive deep into the topic of cardiac output.

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Complete Cheat Code for Heart Physiology:
1st Video: Types of cardiac muscle, action potentials of pacemaker cells and contractile myocardium, and general properties of cardiomyocytes.
2nd Video: Detailed exploration of the cardiac cycle, including phase-by-phase events, valve operations, and pressure differences.
3rd Video: Cardiac output.
4th Video: Regulation of heartbeat

What is Cardiac Output?
Definition: The amount of blood ejected from each ventricle during one minute.
Calculation: Cardiac output = Heart rate (beats per minute) x Stroke volume (liters per beat).
Units: Measured in liters per minute (or ml/m).

Visualizing Cardiac Output
Heart Anatomy: Right and left atria, right and left ventricles, aorta.
Cardiac Cycle Phases:
Atrial systole
Isovolumetric contraction
Ejection phase (systolic phase)
Isovolumetric relaxation
Passive filling phase (common diastolic phase)
Stroke Volume: Amount of blood ejected from ventricles during ejection phase, approximately 50-100 ml per beat for both ventricles.
Heart Rate: Number of cardiac cycles (beats) per minute.

Factors Determining Cardiac Output
Heart Rate:
Increased heart rate typically increases cardiac output.
Exception: At rates above 150 bpm, cardiac output may decrease due to insufficient ventricular filling.
Stroke Volume:
Increased stroke volume increases cardiac output.
Stroke volume depends on:
Venous Return: More blood into the heart means more can be ejected.
Contractility: Stronger contractions push more blood out.
Aortic Resistance: Higher resistance (e.g., from hypertension or plaque) decreases stroke volume.

Normal Cardiac Output

At Rest: 5-7 liters per minute.
During Physical Activity: 15-30 liters per minute.

Pressure-Volume Relationship

Graph: Pressure-volume curve for the left ventricle.
Diastolic Phase: Ventricle fills with blood, pressure increases.
Systolic Phase: Ventricle contracts, pressure rises then falls.
Maximum Pressure-Volume Relationship:
Diastolic Pressure: Stable until end-diastolic volume (approx. 150 ml), then rises.
Systolic Pressure: Increases with end-diastolic volume until overstretched.
Normal Physiological State: Detailed phases of cardiac cycle shown on the pressure-volume curve.
Isovolumetric Phases: Contraction and relaxation phases where volume remains constant but pressure changes.

Clinical Relevance

Assessment: Pressure-volume curves help evaluate heart function, muscle efficiency, and valve health.
Abnormal PV Loops:
Shift to Right: Indicates increased preload (more blood in, more ejected).
Increased Afterload: Higher pressure in aorta, heart strains to eject less blood.
Increased Contractility: Stronger contractions, more blood ejected.

Factors Affecting Cardiac Output

Preload: Volume of blood in ventricles before contraction.
Afterload: Resistance ventricles must overcome to eject blood.
Contractility: Strength of ventricular contraction.

Regulation of Heartbeat

Mechanisms: Myogenic regulation, neural regulation, and humoral regulation.

Conclusion

Summary: Covered cardiac output, pressure-volume curve, and effects of preload, afterload, and contractility.
Next Video: Regulation of heartbeat.

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#cardiacphysiology #heartfunction #cardiacoutput #ecg #medicaleducation #usmlepreparation
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Sources:
University lectures and notes
Barrett, K. E., Barman, S. M., Boitano, S., & Brooks, H. L. (2016). Ganong's review of medical physiology (25th ed.). McGraw-Hill Education.
Hall, J. E. (2016). Guyton and Hall textbook of medical physiology (13th ed.). Elsevier.
Mohrman, D. E., & Heller, L. J. (2014). Cardiovascular physiology (11th ed.). McGraw-Hill Education.
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