Invasive Hemodynamic Monitoring

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Arterial Line Lowdown - Pulse Points Probed

  • Purpose: Continuous real-time BP; frequent ABGs; vasoactive drug guidance.
  • Common Sites:
    • Radial (preferred; Allen's test prior)
    • Femoral (Central; ↑infection risk)
    • Brachial, Axillary, Dorsalis pedis
  • Waveform Insights:
    • Upstroke, peak, dicrotic notch (aortic valve closure), runoff.
    • MAP: $MAP = DBP + 1/3 (SBP - DBP)$. Normal 70-100 mmHg.
    • Pulse Pressure (PP): $PP = SBP - DBP$.
  • Damping (Square Wave Test):
    • Optimal: 1.5-2 oscillations.
    • Overdamped: Blunted, ↓SBP, ↑DBP. Causes: air, clots, kinks.
    • Underdamped: Sharp, ↑SBP, ↓DBP. Causes: stiff tubing, whip. Arterial waveforms: Normal, overdamped, underdamped
  • Complications: Ischemia, thrombosis, hematoma, infection, nerve injury, pseudoaneurysm.

Exam Favourite: Dicrotic notch on arterial waveform: aortic valve closure.

Central Venous Access - Core Pressure Cues

Central Venous Pressure (CVP) estimates right atrial pressure (RAP) & RV preload.

  • Normal: 2-8 mmHg. Zero at phlebostatic axis (mid-axillary, 4th ICS).
  • Read at end-expiration.

CVP Waveform: (Phases of cardiac cycle) 📌 Mnemonic: All Cows Eat Very Young grass (a, c, x, v, y)

  • a wave: Atrial systole. (Absent: AFib; Cannon a: AV dissociation)
  • c wave: RV isovolumetric contraction (tricuspid bulge).
  • x descent: Atrial relaxation.
  • v wave: Atrial filling (tricuspid closed). (Large v: Tricuspid Regurgitation - TR)
  • y descent: Early ventricular filling (tricuspid opens). (Blunted: Tamponade, TS)

RA/CVP waveform interpretation with ECG correlation

Interpreting CVP Changes:

  • ↑ CVP: Fluid overload, RV failure, PE, tamponade, PEEP.
  • ↓ CVP: Hypovolemia, vasodilation.

⭐ Prominent v waves on CVP trace strongly suggest significant tricuspid regurgitation.

Pulmonary Artery Catheter - Swan's In-Depth Scoop

  • A.k.a.: Swan-Ganz catheter.
  • Function: Monitors Right Atrial (RA), Right Ventricular (RV), Pulmonary Artery (PA) pressures, Pulmonary Capillary Wedge Pressure (PCWP ≈ LVEDP), Cardiac Output (CO), Mixed Venous O2 Saturation (SvO2).
  • Key Uses: Complex cardiac surgery, ARDS, cardiogenic/septic shock, guiding fluid/inotrope therapy.
  • Catheter Ports:
    • Distal (PA port - typically yellow): Measures PAP, PCWP (balloon inflated), mixed venous blood sampling (SvO2).
    • Proximal (RA port - typically blue): Measures CVP, injectate for CO, fluid/drug administration.
    • Balloon inflation port (typically red): Inflate with ≤1.5 mL air for PCWP; passive deflation.
    • Thermistor connector: Detects temperature change in PA for thermodilution CO.
  • CO Measurement: Thermodilution principle; cold saline injectate (e.g., 10 mL at <8°C) in RA, temperature change sensed by PA thermistor.
  • Pressure Waveform Progression (During Insertion): RA → RV → PA → PCWP.
  • Normal Values (mmHg):
    • RA: 0-8
    • RV: 15-30 / 0-8
    • PA: 15-30 / 8-15 (Mean 10-20)
    • PCWP: 6-12 (approximates Left Atrial Pressure)
  • SvO2 (Mixed Venous Oxygen Saturation): Normal 60-80%. Reflects global tissue O2 extraction. ↓SvO2 indicates ↑O2 demand or ↓O2 delivery (e.g., ↓CO, anemia, hypoxia, sepsis).
  • Complications: ⚠️ PA rupture (potentially fatal), arrhythmias (VT, RBBB during insertion), pulmonary infarction, catheter knotting, valvular damage, thromboembolism, infection.

Pulmonary Artery Catheter Insertion and Waveforms

⭐ For accurate PCWP measurement, the catheter tip must be in West’s Zone 3 of the lung (where Pa > Pv > PA), ensuring an uninterrupted column of blood between the catheter tip and the left atrium when the balloon is inflated.

High‑Yield Points - ⚡ Biggest Takeaways

  • Arterial lines: Continuous BP, ABG access. Allen’s test vital before radial cannulation.
  • CVP: Measures RA pressure (normal 2-6 mmHg), guides fluid therapy.
  • PAC (Swan-Ganz): Measures PAP, PCWP (estimates LVEDP), CO, SvO2.
  • PCWP (normal 6-12 mmHg): Reflects LV preload; elevated in LV failure.
  • Low CVP/PCWP: Suggests hypovolemia. High CVP/PCWP: Suggests fluid overload/cardiac dysfunction.
  • PAC risks: Arrhythmias, PA rupture, thrombosis, infection.
  • Arterial waveform: Overdamping ↓systolic BP; underdamping ↑systolic BP.

Practice Questions: Invasive Hemodynamic Monitoring

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