Cardiac physiology

• Cardiac blood pressures (measured via Swan-Ganz catheterization)
  • Right atrium: < 5 mm Hg
  • Right ventricle (pulmonary artery pressure): 25/5 mm Hg
    • During systole, pressures in the right ventricle are much lower than in the left ventricle (only ~25 mm Hg compared to ~120 mm Hg). As a result, coronary perfusion pressure is able to overcome right ventricular wall pressure throughout the cardiac cycle, leading to relatively constant blood flow to the right ventricular myocardium.
  • Left atrium (pulmonary capillary wedge pressure): < 12 mm Hg (higher than left ventricular pressure in mitral stenosis)
  • Left ventricle: 130/10 mm Hg
  • Coronary perfusion pressure
    • The driving pressure that forces blood into the coronary arteries during diastole
    • Calculated as the difference in pressure between the aorta and left ventricle during diastole

Cardiac Conduction System

Component	Location	Intrinsic Rate (bpm)	Conduction Velocity (m/s)
SA Node	Upper wall of the right atrium at the junction with the SVC.	60-100 (Primary Pacemaker)	~0.5
Atrial Myocardium	Spreads from SA node across both atria.	N/A	~1.0
AV Node	Interatrial septum, near the opening of the coronary sinus.	40-60 (Secondary Pacemaker)	~0.05 (Slowest)
Bundle of His	Continuation of the AV node; travels through the interventricular septum.	40-60	~2.0
Bundle Branches	Right and Left branches within the interventricular septum.	20-40	~2.0
Purkinje Fibers	Subendocardial surface of the ventricular walls.	~20-40 (Tertiary Pacemaker)	~4.0 (Fastest)
Ventricular Myocardium	Spreads from Purkinje fibers throughout the ventricles.	N/A	~0.3-0.5

Triangle of Koch: Small anatomical region within the atrioventricular septum superior to the coronary sinus. The AV node is located at its apex.

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Cardiac valve locations on chest x-ray

• Most Anterior/Superior: Pulmonic
• Central/Anterior: Aortic
• Most Posterior: Mitral
• Anterior/Inferior to Mitral: Tricuspid

Cardiac and vascular function curves

• Y-Axis: Cardiac Output (CO) or Venous Return (VR).
• X-Axis: Right Atrial Pressure (RAP) / Preload.
• Intersection Point: The steady-state operating point where CO = VR.

A. Changes in Inotropy (Contractility)

Referencing Graph A

• Affected Curve: Cardiac Function Curve (Red).
• Vascular Curve: Unchanged.

1. Increased Inotropy (Shift Up/Left)
   • Mechanism: $\uparrow$ Stroke volume for a given preload.
   • Causes: Catecholamines (Sympathetic stimulation), Digoxin, Exercise.
   • Result: $\uparrow$ CO, $\downarrow$ RAP.
2. Decreased Inotropy (Shift Down/Right)
   • Mechanism: Heart muscle cannot pump effectively; fluid backs up.
   • Causes: Decompensated Heart Failure (Systolic dysfunction), Narcotic overdose, Myocardial Infarction, Beta-blockers/CCBs.
   • Result: $\downarrow$ CO, $\uparrow$ RAP.

B. Changes in Blood Volume or Venous Tone

Referencing Graph B $\text{Venous Return}=\frac{\text{Mean Systemic Filling Pressure}-\text{Right Atrial Pressure}}{\text{Resistance to Venous Return}}$ Slope is $1/Resistance$

• Affected Curve: Vascular Function Curve (Blue).
• Cardiac Curve: Unchanged.
• Key Concept: Mean Systemic Filling Pressure (MSFP) is the X-intercept (where the blue line touches the X-axis). It represents the pressure in the system if the heart stops.

1. Increased Volume / Venous Tone (Shift Right/Up)
   • Mechanism: Increases MSFP.
   • Causes: IV Fluids, Transfusion, Sympathetic venoconstriction (increases tone).
   • Result: $\uparrow$ CO, $\uparrow$ RAP.
2. Decreased Volume / Venous Tone (Shift Left/Down)
   • Mechanism: Decreases MSFP.
   • Causes: Hemorrhage, Dehydration, Venodilators (e.g., Nitrates).
   • Result: $\downarrow$ CO, $\downarrow$ RAP.

C. Changes in Total Peripheral Resistance (TPR)

Referencing Graph C

• Affected Curve: BOTH curves rotate.
• Key Concept: Changes in TPR alter the slope of the vascular curve but generally do not change the MSFP (X-intercept).

1. Increased TPR (Vasoconstriction)
   • Cardiac Curve: Rotates down ($\uparrow$ Afterload makes it harder to pump).
   • Vascular Curve: Rotates down/left (Arteriolar constriction resists blood flow back to the heart).
   • Causes: Vasopressors (Phenylephrine), Hypertension.
   • Result: $\downarrow$ CO, RAP stays relatively similar (or slight decrease).
2. Decreased TPR (Vasodilation)
   • Cardiac Curve: Rotates up ($\downarrow$ Afterload makes it easier to pump).
   • Vascular Curve: Rotates up/right (Less resistance allows faster return of blood).
   • Causes: Exercise, AV Fistula (classic USMLE example), Sepsis (early).
   • Result: $\uparrow$ CO, $\uparrow$ RAP.