Hemodynamics

 Hemodynamics: Study Notes

Definition

Hemodynamics refers to the movement of blood and the forces involved in circulation.
It’s not just about blood pressure — a normal BP does not necessarily mean good hemodynamic stability.

Core Concept

Two patients can have BP = 120/80, but:

• Patient A: No drips, good volume → stable.

• Patient B: On 3 vasopressors → unstable.

Therefore, hemodynamics ≠ blood pressure.

The Three Pillars of Hemodynamics

1. Preload

Definition:
Pressure created by blood volume in the ventricles at the end of diastole (ventricular filling).

• Right-side measurement:
  CVP (Central Venous Pressure) = 4–10 mmHg

• Left-side measurement:
  PAOP / PCWP (Pulmonary Artery Occlusion/Wedge Pressure) = 8–12 mmHg

Measured via Swan-Ganz catheter, which passes through the right heart into the pulmonary artery.

Interpretation:

Preload Status	Meaning	Common Causes	Treatment
↑ Preload	Volume overload	Heart failure, renal failure	Diuretics, dialysis, CRRT
↓ Preload	Volume deficit	Dehydration, hemorrhage	IV fluids, blood, albumin

Albumin Notes:

• 5% Albumin: Mild volume expander

• 25% Albumin: Strong oncotic pull → draws interstitial fluid into vascular space (useful in third spacing, liver failure)

2. Contractility

Definition:
The force or strength of ventricular contraction (“squeeze”).

Measurement:

• Ejection Fraction (EF) = % of blood ejected from the ventricles

  • Normal: ~65%

  • <50% → indicates systolic dysfunction / heart failure

Medications Affecting Contractility:

Type	Effect	Examples
Positive Inotropes	↑ Contractility	Dopamine, Dobutamine, Milrinone (Primacor), Digoxin, Epinephrine
Negative Inotropes	↓ Contractility	Beta-blockers, Calcium Channel Blockers

3. Afterload

Definition:
The resistance the ventricles must overcome to eject blood.

Think: trying to push open a door with a heavy rock behind it — more resistance = higher afterload.

Measurements:

Chamber	Measurement	Normal Range (dynes·sec·cm⁻⁵)
Right Heart	PVR (Pulmonary Vascular Resistance)	150–250
Left Heart	SVR (Systemic Vascular Resistance)	900–1400

Formulas:

• PVR = (Mean PA - PCWP) / CO × 80

• SVR = (MAP - CVP) / CO × 80

Interpretation & Management:

Afterload Status	Associated With	Treatment
↑ Afterload	Vasoconstriction (e.g., hypertension)	Vasodilators — Nitro, Morphine, Cardene
↓ Afterload	Vasodilation (e.g., septic shock)	Vasopressors — Levophed, Vasopressin, Epinephrine, Dopamine, Neo-synephrine

Relationship Between Hemodynamic Factors

Stroke Volume (SV) = Blood pumped per beat (≈ 60–130 mL)
Cardiac Output (CO) = SV × HR (≈ 4–8 L/min)

Thus:

Preload, Contractility, and Afterload → all determine Stroke Volume → which determines Cardiac Output → which influences Blood Pressure

Heart Rate and Hemodynamics

• Tachycardia: ↓ filling time → ↓ preload → ↓ CO

• Bradycardia: ↓ HR → ↓ CO (even if SV is normal)

Treatments:

• Tachycardia (symptomatic): Adenosine, Amiodarone, synchronized cardioversion

• Bradycardia (symptomatic): Atropine, Dopamine drip, pacing (transcutaneous or transvenous)

Index Values

Index = Value adjusted for Body Surface Area (BSA)
Used to normalize data for patient size.

Parameter	Normal Range	Significance
Cardiac Index (CI)	2.5–4.0 L/min/m²	CO adjusted for BSA
Stroke Volume Index (SVI)	33–47 mL/m²	Measures effectiveness of each beat
SVR Index / PVR Index	Adjusted vascular resistance values	Helps fine-tune management in small vs. large patients

Example: A normal CO of 4 L/min may be fine for an average person but inadequate for someone the size of Shaquille O’Neal — index values correct this discrepancy.

Frank-Starling Law

• Relationship between preload and stroke volume.

• As preload increases, stroke volume increases — up to a point.
  Beyond that, the heart becomes overstretched → no further increase in stroke volume → pulmonary edema risk.

Clinical Application:

• Give small fluid boluses while monitoring response (BP, stroke volume variation).

• If improvement stops → stop fluids and reassess.

Clinical Decision Flow

Scenario	Key Finding	Intervention
↓ Preload	Low volume	Give fluids / blood / albumin
↓ Contractility	Weak pump	Give positive inotrope
↓ Afterload	Vasodilation	Give vasopressor
↑ Afterload	Vasoconstriction	Give vasodilator
Poor response to fluids	Check stroke volume variation	Switch to inotrope or vasopressor

Takeaways

• Hemodynamics is a dynamic interaction of preload, contractility, and afterload.

• Blood pressure alone doesn’t define stability.

• Always assess cardiac output and perfusion indicators (mental status, urine output, skin temp, lactate).

• “Fill your tank before pressing the gas” — optimize preload before starting inotropes or pressors.

• Overfilling leads to wet lungs and pulmonary edema.