Cardiac Output and Myocardial Performance Quiz

Cardiac Output (CO) and Myocardial Performance

• Cardiac output (CO) is the blood volume pumped by each ventricle per minute. It's a key indicator of heart function, adjusting to the body's needs.
• CO = Heart Rate (HR) x Stroke Volume (SV) (ml/min = beats/min x ml/beat). Example: 75 beats/min x 70 ml/beat = 5250 ml/min.
• Myocardial performance is the heart's ability to meet bodily demands by providing adequate blood flow to meet metabolic needs. It involves managing CO, SV, peripheral resistance, venous return, HR, and cardiac contractility.

Measuring Cardiac Output

• The Fick principle states that total substance uptake/release by an organ equals blood flow through that organ multiplied by the arteriovenous concentration difference of the substance.
• The oxygen Fick method calculates CO using oxygen consumption rate and arterial/venous oxygen concentrations: CO = (Rate of O2 absorbed by lungs) / ([O2]la - [O2]rv).

Heart Rate (HR)

• HR varies physiologically with age: infants (160+ bpm), young adults (60-85 bpm), and it rises again in the elderly.
• Tachycardia: resting HR > 100 bpm (stress, anxiety, drugs, heart disease, fever).
• Bradycardia: resting HR < 60 bpm (sleep, trained athletes).
• Pulse reflects arterial pressure surges.

Extrinsic Innervation of the Heart

• The medulla's cardiac center contains:
  • Cardioaccelerator center (sympathetic stimulation, increased HR).
  • Cardioinhibitory center (parasympathetic stimulation, decreased HR).
• Receives input from higher centers (hypothalamus) monitoring blood pressure and dissolved gas levels.

Stroke Volume (SV)

• Stroke volume is the difference between end-diastolic volume (EDV) and end-systolic volume (ESV): SV = EDV - ESV.

Frank-Starling Law of the Heart

• The force of contraction is directly proportional to the degree of myocardial fiber stretch (length-tension relationship).
• Increased diastolic stretch (increased EDV) leads to stronger systolic contraction and increased SV. This is an intrinsic property of the myocardium; beyond a certain limit, contractility decreases.

Preload and Afterload

• Preload (EDV): a critical factor controlling SV; increased preload increases myocardial stretch, which increases the force of contraction and therefore SV.
• Afterload (End Systolic Pressure): the pressure against which ventricles contract; determined by peripheral resistance, affected by pleural pressure, vascular compliance, and vascular resistance.

Regulation of Cardiac Output

• Sympathetic stimulation increases contractility, SV (decreasing ESV), and HR (positive chronotropic effect). It's the most significant factor in CO adjustment.
• Parasympathetic stimulation primarily influences the heart at rest, reducing HR and returning it to baseline after stress/exercise.

Contractility

• Increased force of contraction not attributable to the Frank-Starling mechanism; represents the vigor of contraction. It's the change in developed force at a given resting fiber length.

Factors Affecting Stroke Volume, and Cardiac Output (Summary of Mechanical Factors)

• Increased venous return stretches cardiac muscle fibers (increased EDV), leading to stronger contraction and increased SV (Frank-Starling Law).
• The Bainbridge reflex: increased EDV stimulates atrial stretch receptors and directly affects the sinoatrial node, increasing HR.

Factors Affecting Cardiac Output

• Exercise: activates the sympathetic nervous system, increasing HR, contractility, SV, and venous return (via skeletal muscle contraction).
• Cardiac reserve: the difference between resting and maximal CO; allows for dramatic CO increases during activity.
• Sudden blood pressure drop: reduces venous return (decreasing SV), but sympathetic activity increases HR to maintain CO.
• Rising blood pressure: reduces sympathetic activity, decreasing HR and CO (to lower blood pressure).
• Sudden blood volume drop: reduces venous return and SV.

Test your knowledge on cardiac output, myocardial performance, and the factors influencing heart function. This quiz covers key concepts such as the Fick principle and the oxygen Fick method. Enhance your understanding of how the heart meets the body's metabolic needs.