ICU Primary Prep: Cardiovascular Physiology

Overview

• Discussion with Dr. Mike Clifford on cardiovascular physiology focusing on determinants of cardiac output.
• Importance of understanding fundamental physiological concepts for exams.

Cardiac Output Basics

• Definition: Quantity of blood pumped out of the left ventricle each minute.
• Standard Value: Approximately 5 liters for a 70 kg man at sea level.
• Formula: Cardiac Output (CO) = Heart Rate (HR) x Stroke Volume (SV).

Determinants of Cardiac Output

Heart Rate

• Controlled by the autonomic nervous system:
  • Sympathetic Nervous System: Outflow from T1 to T4 via cardiac plexus.
  • Parasympathetic Nervous System: Vagal fibers affecting the atria and nodes.
• Maximal Heart Rate: Age-dependent, limited by diastolic filling time.
• Primarily increases with physical activities before stroke volume becomes a factor.

Stroke Volume Components

Preload

• Degree to which myocardium is stretched before contraction.
• Governed by Frank-Starling Law: Muscle fiber length proportional to end-diastolic volume.
• Increased venous return enhances both contraction strength and heart rate (e.g., Bainbridge reflex).

Afterload

• Resistance to blood ejection into the aorta.
• Influenced by:
  • Ventricular wall tension
  • Left ventricular end-diastolic and end-systolic volumes
  • Aortic valve, blood inertia, systemic vascular resistance
• Generally, increased afterload reduces cardiac output and stroke volume if preload is constant.

Contractility

• Myocardial fibers' intrinsic ability to contract independently of preload and afterload.
• Driven by intracellular calcium influenced by sympathetic stimulation and preload.
• Factors reducing contractility include hypercapnia, hypoxia, and acidosis.

Myocardial Compliance (Lusitropy)

• Enhanced by sympathetic activation, influencing venous return and diastolic stretch.

Clinical Application

• Understanding these factors aids in handling pathologies in clinical practice by adjusting heart rate, preload, afterload, and contractility.
• Framework helps in pharmacological evaluations and understanding impacts of drugs on cardiac output.

Examination Strategy

• Key: Organize response into sections covering heart rate, preload, afterload, and contractility.
• Avoid over-writing; focus on concise, structured responses.
• This "cardiac grid" approach is useful for addressing cardiovascular pharmacology and exam preparation.

Conclusion

• The discussion emphasized the importance of understanding fundamental physiology.
• Encouraged strategic exam preparation by focusing on core concepts.
• Advice: Write concisely, move to other questions after covering fundamental points adequately.