More On Hemodynamic: A Study Notes

1. Preload

Definition:

• The volume of blood in the ventricles at the end of diastole (end-diastolic volume).

• Reflects ventricular filling pressure.

Clinical Examples:

• ↑ Preload: Seen in congestive heart failure (CHF) → fluid volume overload.

• ↓ Preload: Seen in hypovolemia or shock states → inadequate filling.

Interventions:

• To Decrease Preload:

  • Diuretics → reduce intravascular volume.

  • Venous vasodilators (e.g., Nitroglycerin, Morphine sulfate) → dilate veins, reducing venous return to the right atrium.

• To Increase Preload:

  • Volume replacement (IV fluids, blood products) → increases intravascular volume.

Measurement:

• Central Venous Pressure (CVP) = Right Atrial Pressure (RAP)

  • Normal CVP: 2–6 mmHg

2. Afterload

Definition:

• The resistance the ventricles must overcome during systolic ejection to circulate blood.

• Represented as systemic vascular resistance (SVR).

Clinical Examples:

• ↑ Afterload: Hypertension → increased ventricular workload.

• ↓ Afterload: Distributive shock (e.g., septic, anaphylactic, neurogenic shock) → vasodilation and poor perfusion.

Interventions:

• To Decrease Afterload:

  • ACE inhibitors (e.g., “-pril” drugs) → prevent angiotensin II–mediated vasoconstriction.

  • Vasodilators (e.g., Nitroprusside).

• To Increase Afterload:

  • Vasopressors (e.g., Norepinephrine, Vasopressin) → cause vasoconstriction to restore perfusion pressure.

Measurement:

• Systemic Vascular Resistance (SVR) via PA (Swan-Ganz) catheter

  • Calculated using MAP, CVP, and Cardiac Output.

  • Normal SVR: 800–1200 dyn·s/cm⁵

3. Cardiac Output (CO)

Definition:

• Amount of blood pumped by the heart per minute.

  $$CO = Stroke\ Volume\ (SV) \times Heart\ Rate\ (HR)$$

Normal Values:

• CO: 4–8 L/min

• SV: ~60–100 mL/beat

Clinical Context:

• ↑ CO: Sepsis (hyperdynamic phase)

• ↓ CO: Heart failure, hypovolemia, cardiogenic shock

4. Cardiac Index (CI)

Definition:

• Cardiac output adjusted for body surface area (BSA)--gives a more individualized assessment.

  $$CI = \frac{CO}{BSA}$$

Normal Range:

• 2.5–4.0 L/min/m²

Clinical Example:

• The Rock (large BSA): CO of 4 L/min → CI ≈ 2 (inadequate)

• Charlie Sheen (smaller BSA): CO of 4 L/min → CI ≈ 4 (adequate)

5. Mean Arterial Pressure (MAP)

Definition:

• Average arterial pressure during a single cardiac cycle-It is the best indicator of organ perfusion.

  $$MAP = \frac{SBP + (2 \times DBP)}{3}$$

Normal Value:

• ≥ 65 mmHg (minimum needed for organ perfusion)

Example:

• BP = 104/60 → MAP = 74 mmHg (adequate perfusion)

Clinical Use:

• Key in shock management and critical care monitoring.

6. Contractility (Inotropy)

Definition:

• The force of myocardial contraction independent of preload and afterload.

Types of Inotropic Effects:

• Positive Inotropes (↑ contractility):

  • Dobutamine → improves CO in heart failure.

  • Digoxin (Digitalis): ↑ contractility, ↓ HR (positive inotrope, negative chronotropic).

• Negative Inotropes (↓ contractility):

  • Beta-blockers (e.g., Metoprolol): cardioprotective post-MI; reduce myocardial oxygen demand.

7. Pulmonary Capillary Wedge Pressure (PCWP)

Definition:

• Indirect measure of left atrial pressure using a Swan-Ganz (PA) catheter.

• Balloon at catheter tip “wedges” in a pulmonary artery branch, isolating left heart pressures.

Normal Range:

• 6–12 mmHg

Clinical Use:

• ↑ PCWP: Left-sided heart failure, fluid overload.

• ↓ PCWP: Hypovolemia.

Note:

• In pulmonary hypertension, PCWP may not reflect left heart pressures accurately → requires left atrial (LA) line.

8. Swan-Ganz Catheter Waveforms

As the catheter advances:

1. Right Atrium: Low pressure waveform (CVP 2–6 mmHg)

2. Right Ventricle: Higher spikes (RV systolic 15–30 mmHg)

3. Pulmonary Artery: Pulsatile waveform (PASP 15–30 mmHg, PADP 8–15 mmHg)

4. Wedge (PCWP): Flat waveform (6–12 mmHg)

Quick Summary

Parameter	Definition	Normal Range	Measurement Method	Clinical Example
Preload (CVP)	End-diastolic volume	2–6 mmHg	Central line	↑ in CHF
Afterload (SVR)	Resistance to ejection	800–1200 dyn·s/cm⁵	PA catheter	↑ in HTN
CO	Blood pumped/min	4–8 L/min	Thermodilution	↓ in shock
CI	CO ÷ BSA	2.5–4.0 L/min/m²	Calculated	Individualized CO
MAP	Mean perfusion pressure	≥ 65 mmHg	BP cuff/arterial line	Goal in shock
Contractility	Force of contraction	—	Clinical, Echo	Affected by inotropes
PCWP	Left atrial pressure	6–12 mmHg	PA catheter	↑ in LHF