PERFUSION & HEMODYNAMICS STUDY NOTES : The Dynamics of Blood Flow and Oxygen Delivery

I. PURPOSE OF PERFUSION

• Goal: Optimize oxygen delivery (DO₂) at the tissue level.

• Shock = Imbalance between oxygen delivery and consumption.

• Types/Causes of shock:

  • Hypovolemic

  • Cardiogenic

  • Obstructive

  • Distributive (e.g., septic, anaphylactic)

  • Dissociative (e.g., carbon monoxide poisoning)

II. OXYGEN DELIVERY (DO₂) COMPONENTS

DO₂ = Cardiac Output (CO) × Hemoglobin (Hb) × Arterial O₂ Saturation (SaO₂)

Component	Description
Cardiac Output	Heart’s pumping ability (HR × Stroke Volume)
Hemoglobin (Hb)	Oxygen carrier in blood
Arterial O₂ Saturation (SaO₂)	% of Hb saturated with O₂

When DO₂ < VO₂ (oxygen consumption) → tissue hypoxia → shock.

III. CARDIAC OUTPUT BASICS

CO = Stroke Volume (SV) × Heart Rate (HR)

Determinants of Stroke Volume:

1. Preload – Volume returning to heart (measured via CVP/wedge).

2. Contractility – Ventricular strength.

3. Afterload – Vascular resistance opposing ejection.

IV. ARTERIAL PRESSURE MONITORING

Continuous Arterial Pressure (A-line) is ideal for evaluating perfusion.

Components:

1. Systolic Pressure (SBP) → reflects ventricular ejection efficiency

   • ↑ after giving volume → ventricle is responsive.

2. Diastolic Pressure (DBP) → reflects vascular tone

   • Low DBP → needs vasopressor, not volume.

3. Pulse Pressure (PP = SBP - DBP) → indirect indicator of stroke volume

   • Narrow PP → decreased SV → possible hypovolemia.

4. Mean Arterial Pressure (MAP)

   • MAP = (SBP + 2×DBP) / 3

   • Represents average perfusion pressure in one cardiac cycle.

   • Goal MAP ≥ 65 mmHg.

V. SYSTEMIC PERFUSION PRESSURE

Perfusion Pressure (SP) = MAP - CVP

• Indicates pressure gradient driving blood across capillaries back to right heart.

• Goal: SP ≥ 65 mmHg.

• High CVP or low MAP → poor tissue perfusion.

VI. CENTRAL VENOUS PRESSURE (CVP)

• Reflects right heart filling pressure (not blood volume).

• CVP < 5 mmHg: Suggests hypovolemia.

• CVP > 12–15 mmHg: Often due to ↓ compliance of RV or fluid in wrong compartments.

Factors Affecting CVP:

• Positive pressure ventilation → ↑ intrathoracic pressure → ↑ CVP.

• RV failure or pulmonary hypertension → ↑ CVP.

• Hypovolemia → ↓ CVP, tachycardia, narrow pulse pressure.

Interpretation Tip:

“Pressure ≠ Volume” — CVP reflects compliance, not fluid status alone.

VII. CLINICAL APPLICATIONS

Volume Challenge Test

• Give 250–500 mL under pressure over ~3 min.

• Evaluate:

  • ↑ SBP or PP → ventricular responsiveness.

  • No change → consider inotrope or vasopressor instead.

Response Patterns:

Parameter	Interpretation
↑ SBP, PP	Volume responsive
↑ DBP	↑ vascular tone (often after vasopressor)
↓ PP, Tachycardia	Hypovolemia / poor stroke volume
↑ CVP, no ↑ SBP	Poor compliance / RV failure

VIII. MANAGING SHOCK STATES

A. Hypovolemic Shock

• ↓ Preload, ↓ Stroke Volume, ↓ CO

• Tx: Volume replacement

B. Cardiogenic Shock

• ↓ Contractility, ↑ CVP

• Tx: Inotrope (dobutamine)

C. Distributive Shock (e.g., Sepsis)

• ↓ Vascular tone, ↓ DBP, ↓ MAP

• Tx: Vasopressor (norepinephrine) + volume

D. Obstructive Shock

• ↓ Filling due to obstruction (PE, tamponade)

• Tx: Remove obstruction

IX. FLUID MANAGEMENT GUIDELINES

• Purpose: To increase stroke volume → improve cardiac output.

• Evaluate systolic response or stroke volume index after bolus.

• If no improvement, stop fluids → consider vasopressor or inotrope.

Quick Bedside Rule:

If CVP < 5 → Give volume.
If CVP ↑ but BP doesn’t → Add inotrope.
If DBP ↓ → Add vasopressor.

X. INTERACTIONS: FLUIDS, INOTROPES, VASOPRESSORS

Drug Type	Example	Primary Effect
Volume	NS, LR	↑ Preload, ↑ SV
Inotrope	Dobutamine	↑ Contractility, ↑ SV
Vasopressor	Norepinephrine	↑ Afterload, ↑ MAP

Caution: Overuse of vasopressors can ↑ afterload → ↓ CO → worsen perfusion.

XI. CASE STUDY SUMMARY

Patient: 50F trauma, hemorrhagic shock.

• Initial: HR 130, MAP ↓, CVP low → volume given → ↑ BP, ↑ SV.

• Added dobutamine → ↑ CO, ↓ lactate → improved perfusion.

• Started norepinephrine → ↑ BP but ↓ SV, ↑ CVP → iatrogenic heart failure.

Key Lesson:
Treat perfusion, not just pressure.
Vasopressors raise BP but can reduce cardiac function if preload is inadequate.

XII. KEY TAKEAWAYS

• Perfusion = DO₂ - VO₂ balance.

• Always interpret MAP + CVP + PP together.

• Low pulse pressure + tachycardia = early hypovolemia.

• MAP - CVP = systemic perfusion pressure (goal > 65).

• Optimize stroke volume first, then address contractility or tone.

• Final goal: Restore tissue oxygenation and correct metabolic acidosis.