Hemodynamic Monitoring: A Crash Course in Swan-Ganz Catheters and Interpreting Waveforms
The pulmonary artery catheter (PAC), commonly called the Swan-Ganz, remains a valuable tool for managing complex shock states—even as non-invasive alternatives have emerged. Understanding hemodynamic monitoring makes you a more effective critical care provider.
When PACs Are Still Useful
Despite controversy about outcomes, PACs provide uniquely valuable data in:
- Cardiogenic shock (differentiating causes)
- Mixed shock states (septic + cardiogenic)
- Severe pulmonary hypertension
- Post-cardiac surgery complications
- Right heart failure assessment
- Guiding complex fluid management
Anatomy of the PAC
The standard 4-lumen catheter includes:
- Distal (PA) port: Measures PA pressure, mixed venous blood
- Proximal (RA) port: Measures RA/CVP, delivers injectate
- Thermistor: Near tip, measures cardiac output
- Balloon port: Inflates for PAWP measurement
Normal Pressures: Know Your Numbers
| Location | Systolic | Diastolic | Mean |
|---|---|---|---|
| Right Atrium (CVP) | - | - | 2-6 mmHg |
| Right Ventricle | 15-30 | 0-8 | - |
| Pulmonary Artery | 15-30 | 8-15 | 10-20 |
| PAWP (Wedge) | - | - | 6-12 mmHg |
Reading the Waveforms
RA (CVP) Waveform
Three peaks: a, c, v waves
- a wave: Atrial contraction (follows P wave)
- c wave: Tricuspid bulging during isovolumetric contraction
- v wave: Atrial filling against closed tricuspid
Clinical pearls:
- Cannon a waves: AV dissociation
- Giant v waves: Tricuspid regurgitation
- Absent a waves: Atrial fibrillation
RV Waveform
Steep upstroke, rapid descent
- Systolic pressure = PA systolic
- Diastolic approaches zero (unlike PA)
PA Waveform
- Dicrotic notch marks pulmonic valve closure
- PA diastolic approximates PAWP in normal conditions
PAWP (Wedge) Waveform
Looks like RA waveform but delayed
- Represents left atrial pressure
- a wave follows ECG P wave by ~200ms
- v wave occurs after T wave
Giant v waves in wedge: Mitral regurgitation, acute MR can have v waves >2x mean PAWP
Obtaining an Accurate Wedge
- Inflate balloon slowly until PA waveform dampens to wedge
- Confirm wedge pattern (should look atrial, not PA)
- Read at end-expiration
- Deflate balloon immediately after reading
- Never leave balloon inflated—risk of PA rupture
Wedge should be lower than PA diastolic. If wedge > PA diastolic, you're overwedged or have damping issues.
Calculated Hemodynamic Parameters
Cardiac Output (CO) and Index (CI)
- CO: Liters/minute (normal 4-8 L/min)
- CI: CO / BSA (normal 2.5-4.0 L/min/m²)
- Measured by thermodilution or calculated methods
Systemic Vascular Resistance (SVR)
SVR = [(MAP - CVP) / CO] × 80
- Normal: 800-1200 dynes·sec/cm⁵
- High SVR: Vasoconstriction (cardiogenic shock, hypovolemia)
- Low SVR: Vasodilation (septic shock, anaphylaxis)
Pulmonary Vascular Resistance (PVR)
PVR = [(mPAP - PAWP) / CO] × 80
- Normal: <250 dynes·sec/cm⁵
- Elevated in pulmonary hypertension, PE, hypoxemia
Stroke Volume (SV) and Index (SVI)
SV = CO / HR
- Normal SV: 60-100 mL/beat
- Normal SVI: 33-47 mL/beat/m²
Shock State Differentiation
Cardiogenic Shock
- CI: ↓ (<2.2)
- PAWP: ↑ (>18)
- SVR: ↑
- Mixed venous O2: ↓
Septic Shock (Warm)
- CI: ↑ or normal
- PAWP: Low to normal
- SVR: ↓↓
- Mixed venous O2: ↑ (poor extraction)
Hypovolemic Shock
- CI: ↓
- PAWP: ↓
- SVR: ↑
- Mixed venous O2: ↓
RV Failure
- CI: ↓
- CVP: ↑ (CVP > PAWP)
- PAWP: Low to normal
- PA pressures may be elevated
Mixed Venous Oxygen Saturation (SvO2)
Obtained from PA port (truly "mixed" venous blood)
| SvO2 | Interpretation |
|---|---|
| >75% | Normal or hyperdynamic state |
| 60-75% | Compensated/early shock |
| 40-60% | Significant oxygen supply-demand mismatch |
| <40% | Severe shock, imminent arrest |
Falling SvO2 = Increased O2 extraction = Inadequate DO2
Troubleshooting Common Problems
Damped Waveform
- Air bubbles in line
- Clot at catheter tip
- Catheter against vessel wall
- Kinked catheter
Fix: Flush, reposition, aspirate if needed
Can't Wedge
- Balloon leak (check for blood return)
- Catheter migration (check position)
- PA too dilated to occlude
Permanent Wedge (Won't Unwedge)
- Catheter has migrated too far distally
- Danger: Risk of PA rupture, infarction
- Action: Deflate balloon, pull back catheter
Arrhythmias During Insertion
- PVCs during RV passage are common
- Usually resolve when catheter enters PA
- Have lidocaine ready, consider prophylaxis in irritable hearts
Clinical Decision-Making Examples
Case: Hypotension Post-MI
Data: CI 1.8, PAWP 24, SVR 1800
Interpretation: Cardiogenic shock—low output, elevated filling pressures, compensatory vasoconstriction
Action: Inotropes (dobutamine), consider IABP, revascularization if ongoing ischemia
Case: Septic Shock, Not Responding to Fluids
Data: CI 4.5, PAWP 14, SVR 550
Interpretation: Hyperdynamic sepsis with profound vasodilation
Action: More vasopressors (not more fluid), source control
Case: Post-Cardiac Surgery, Low BP
Data: CI 2.0, PAWP 8, CVP 18
Interpretation: CVP > PAWP suggests RV failure—could be RV infarct, PE, or air embolism
Action: Avoid excessive volume, consider pulmonary vasodilators, inotropic support for RV
The Bottom Line
Hemodynamic monitoring isn't about the numbers in isolation—it's about integrating multiple data points with clinical context to guide therapy. A PAC gives you a window into cardiovascular physiology that can be invaluable in complex cases.
Master the waveforms, know the normal values, and always correlate with what you see at the bedside.