Pulse Contour Analysis (PCA)

Definition

Pulse contour analysis is a minimally invasive hemodynamic monitoring technique that derives stroke volume (SV)and cardiac output (CO) by analyzing the arterial pressure waveform obtained from an arterial catheter.

It is based on the principle that:

The contour (shape and area) of the arterial pressure waveform during systole reflects the volume of blood ejected by the left ventricle.


Physiological Basis

1. Relationship between Pressure and Flow

  • Blood flow into the arterial system during systole creates a pressure waveform
  • Stroke volume is proportional to the area under the systolic portion of the arterial waveform
  • This relationship is influenced by:
    • Arterial compliance
    • Systemic vascular resistance (SVR)
    • Wave reflection

2. Windkessel Model

Pulse contour analysis is rooted in the Windkessel model of the arterial system.

Classic 2-element Windkessel

  • Compliance (C)
  • Resistance (R)

Modern 3-element Windkessel

  • Compliance
  • Resistance
  • Characteristic impedance

These models explain how:

  • Large arteries act as elastic reservoirs
  • Convert pulsatile cardiac output into continuous peripheral flow


What Pulse Contour Analysis Measures

Primary Parameters

  • Stroke Volume (SV)
  • Cardiac Output (CO)
  • Cardiac Index (CI)

Derived Dynamic Parameters (Key for ICU)

  • Stroke Volume Variation (SVV)
  • Pulse Pressure Variation (PPV)
  • Systemic Vascular Resistance (SVR)
  • Cardiac Power Output (CPO) (some systems)


Arterial Waveform Analysis

Key Components of the Arterial Waveform

  1. Upstroke (Anacrotic limb)
    – LV ejection and contractility
  2. Systolic peak
    – Peak arterial pressure
  3. Dicrotic notch
    – Aortic valve closure
  4. Downstroke (Catacrotic limb)
    – Peripheral runoff and vascular tone

Factors Affecting Waveform Shape

  • Arterial compliance
  • Vascular tone
  • Heart rate
  • Arrhythmias
  • Damping of arterial line


Types of Pulse Contour Analysis Systems

1. Calibrated Pulse Contour Systems

Require external calibration to correct for changes in vascular tone.

Examples

  • PiCCO®
    • Calibrated using transpulmonary thermodilution
  • LiDCOplus®
    • Calibrated using lithium dilution

Advantages

  • More accurate in:
    • Septic shock
    • Vasopressor use
    • Altered vascular tone

Limitations

  • Requires central access
  • Intermittent recalibration needed


2. Uncalibrated Pulse Contour Systems

Use population-based nomograms or proprietary algorithms.

Examples

  • FloTrac® / Vigileo
  • ProAQT®
  • ClearSight® (non-invasive finger cuff)

Advantages

  • Easy setup
  • No indicator injection
  • Faster deployment

Limitations

  • Less reliable in:
    • Septic shock
    • Extreme SVR
    • Liver failure
    • Cirrhosis
    • ECMO


Individual System Overview (Exam-Relevant)

PiCCO System

  • Requires:
    • Central venous line
    • Femoral or axillary arterial line
  • Calibration: Transpulmonary thermodilution

Additional parameters

  • GEDV (Global End-Diastolic Volume)
  • EVLW (Extravascular Lung Water)
  • PVPI (Pulmonary Vascular Permeability Index)

Best use

  • Septic shock
  • ARDS
  • Complex ICU patients


FloTrac / Vigileo

  • Uses standard arterial line (radial/femoral)
  • Algorithm adjusts for:
    • Age
    • Sex
    • Height
    • Waveform characteristics

Known limitations

  • Early generations inaccurate in sepsis
  • Improved with later software versions


Dynamic Indices from PCA

Stroke Volume Variation (SVV)

  • Reflects preload responsiveness
  • Calculated from respiratory variations in SV

Pulse Pressure Variation (PPV)

  • Variation in pulse pressure during mechanical ventilation

Valid only when:

  • Controlled mechanical ventilation
  • Tidal volume ≥ 7–8 mL/kg
  • Sinus rhythm
  • No spontaneous breathing
  • Closed chest


Clinical Applications

1. Fluid Responsiveness Assessment

  • PCA allows functional hemodynamic monitoring
  • Helps avoid blind fluid loading

2. Shock Differentiation

  • Septic shock: High CO, low SVR
  • Cardiogenic shock: Low SV, low CO
  • Hypovolemic shock: Low preload-dependent SV

3. Goal-Directed Therapy (GDT)

  • Perioperative optimization
  • Major surgery
  • ICU resuscitation


Advantages of Pulse Contour Analysis

  • Continuous real-time monitoring
  • Less invasive than PAC
  • Provides dynamic parameters
  • Enables individualized fluid therapy


Limitations and Pitfalls

Physiological Limitations

  • Altered arterial compliance
  • Vasopressor-induced tone changes
  • Severe aortic regurgitation
  • Intra-aortic balloon pump
  • ECMO

Technical Limitations

  • Over-damped or under-damped arterial line
  • Poor waveform quality
  • Arrhythmias (AF)

NOTE:-

  • PCA does NOT directly measure CO – it estimates CO from arterial waveform
  • Accuracy decreases with changing SVR
  • SVV and PPV assess fluid responsiveness, not volume status
  • Calibrated systems outperform uncalibrated systems in septic shock
  • Always assess arterial waveform quality before trusting numbers