Ninja nerd Cardiac output

Cardiac Output

• Definition: Cardiac output (Co) is defined as the product of heart rate (HR) and stroke volume (SV).
  $Co=HRXSV$

Heart Rate (HR)

• Units: Measured in beats per minute (bpm).

• Normal Range: Typical heart rate ranges between 60 to 100 bpm.

  • Average is approximately 70 bpm.

• Tachycardia: Defined as a heart rate greater than 100 bpm.

• SA Node: The Sinoatrial node (SA node) generates and regulates the sinus rhythm, typically maintaining a heart rate of 60 to 80 bpm.

• Factors Influencing Heart Rate:

  • Sympathetic Nervous System:

  • Increases heart rate through the release of norepinephrine and epinephrine which act on beta-1 adrenergic receptors.

  • This action is termed positive chronotropic effect, enhancing heart rate.

  • Parasympathetic Nervous System:

  • Decreases heart rate via acetylcholine acting on muscarinic receptors, termed negative chronotropic effect.

  • Hormones:

  • Thyroid hormones (T3 and T4) significantly increase heart rate and also metabolic rate.

  • Body Temperature: Higher temperatures typically increase heart rate, while lower temperatures reduce it.

  • Ionic Influence:

  • Calcium: High calcium (hypercalcemia) generally increases heart rate; low calcium (hypocalcemia) decreases it.

  • Potassium: High levels (hyperkalemia) can inhibit heart rate leading to potential cardiac arrest.

• Chemoceptors Influence:

  • Low blood oxygen (partial pressure), high carbon dioxide (partial pressure), or low pH stimulate chemoceptors, which can also increase heart rate.

• Age Factor:

  • Fetal Heart Rate: Very fast, typically 120-140 bpm.

  • Adult Heart Rate: Average adult heart rate ranges from 60-80 bpm.

  • Gender Differences:

  • Males (60-72 bpm) vs. Females (72-80 bpm).

Stroke Volume (SV)

• Definition: The volume of blood ejected by the heart ventricles with each contraction, measured in milliliters (mL).

• Equation: $SV = EDV - ESV$

  • End-Diastolic Volume (EDV): Volume of blood in the ventricles before contraction, normally around 120-140 mL.

  • End-Systolic Volume (ESV): Volume of blood remaining in the ventricles after contraction, typically about 50-70 mL.

• Normal Stroke Volume: Average stroke volume is approximately 70 mL per beat.

• Determinants of Stroke Volume:

  • Preload: The degree of stretch of cardiac muscle fibers just before contraction.

  • Contractility: Strength of contraction influenced by sympathetic stimulation and intracellular calcium levels.

  • Afterload: The resistance the heart must overcome to eject blood; influenced by systemic vascular resistance and aortic pressure.

Preload

• Definition: Reflects the degree of stretch of cardiac muscle; determined by EDV.

• Influencing Factors:

  • Increased venous return improves preload.

  • Methods to increase venous return include:

    • Muscle pump: Skeletal muscle contraction helps push blood in veins.

    • Respiratory pumps: Changes in thoracic and abdominal pressure during breathing facilitate blood flow.

    • Vasomotor tone: Sympathetic influence enhances venous return through constriction.

• Filling Time: Adequate time for heart filling is critical; high heart rates can hinder filling and, subsequently, preload.

Contractility

• Definition: The ability of the heart muscle to contract.

• Regulators:

  • Sympathetic nervous system (increases contractility via norepinephrine).

  • Hormones such as thyroid hormones (T3/T4) enhance contractility by increasing receptor expression.

  • Positive Ionotropic Agents: Calcium, epinephrine, norepinephrine, etc., enhance contractility.

  • Negative Ionotropic Agents: Conditions like hyperkalemia and acidosis impede contractility.

Afterload

• Definition: The resistance the ventricle must overcome to eject blood.

• Clinically Relevant Issues: Conditions like hypertension increase afterload, making it more challenging to eject blood from the heart.

• Influences of Afterload:

  • Valvular stenosis or occlusion due to plaque can increase resistance.

  • Increased systemic vascular resistance affects the pressure gradient, increasing afterload.

• Relationship with Stroke Volume:

  • Increased afterload leads to decreased stroke volume; conversely, decreased afterload improves stroke volume.

• Clinical Notes:

  • Atrial Bainbridge Reflex: Increased venous return/stretch can stimulate increased heart rate via the cardiac accelerator center.

Conclusion

• Overview of cardiac output, heart rate, and stroke volume highlights their interrelations, physiological regulations, and clinical implications.

• Importance of understanding how various factors influence these parameters is crucial for application in clinical settings and understanding cardiovascular physiology.