Invasive Arterial Pressure Monitoring (IAPM)

Definition

Invasive arterial pressure monitoring involves direct, continuous measurement of arterial blood pressure by cannulating an artery and connecting it to a fluid-filled catheter–transducer–monitor system.
It provides beat-to-beat blood pressure, arterial waveform analysis, and arterial blood sampling.

 

Indications

Absolute / Strong Indications

  • Hemodynamically unstable patients
    • Shock (septic, cardiogenic, hypovolemic, obstructive)
  • Vasopressor or inotrope therapy
  • Major surgery
    • Cardiac, vascular, neurosurgery
    • Major trauma
  • Tight BP control needed
    • TBI, SAH, stroke
    • Aortic dissection
    • Hypertensive emergencies
  • Frequent ABG sampling
  • Mechanical circulatory support
    • IABP, ECMO, LVAD

Relative Indications

  • Difficult or unreliable NIBP readings
  • Morbid obesity
  • Arrhythmias (AF, frequent ectopics)

 

Common Sites of Arterial Cannulation

Site

Advantages

Disadvantages

Radial (most common)

Superficial, collateral flow

Dampened waveform, vasospasm

Femoral

Central pressure, large artery

Infection, thrombosis

Brachial

Accurate waveform

No collateral circulation

Axillary

Lower infection

Technically difficult

Dorsalis pedis / Posterior tibial

Alternative access

Lower systolic pressure

 

Pre-procedure Assessment

Allen’s Test (Radial artery)

  • Assesses ulnar collateral circulation
  • Normal: Hand flushes within ≤5–7 seconds
  • Limitation: Poor predictor of ischemic complications (not mandatory but commonly performed)

 

Components of the Arterial Monitoring System

1. Arterial Cannula

  • Size: 20G (radial), 18G (femoral)
  • Length affects damping and waveform quality

2. Fluid-Filled Tubing

  • Non-compliant, short, stiff tubing preferred
  • Filled with heparinized saline

3. Pressure Transducer

  • Converts mechanical pressure → electrical signal
  • Must be zeroed and leveled

4. Monitor

  • Displays numeric values and waveform

 

Zeroing and Leveling

Zeroing

  • Transducer opened to air
  • Sets atmospheric pressure as zero reference

Leveling

  • Transducer positioned at phlebostatic axis
    • 4th intercostal space, mid-axillary line
    • Corresponds to right atrium
  • Error:
    • Too high → falsely low BP
    • Too low → falsely high BP

 

Arterial Pressure Waveform

Normal Waveform Components

  1. Systolic upstroke (anacrotic limb)
  2. Peak systolic pressure
  3. Dicrotic notch
    • Aortic valve closure
  1. Diastolic runoff

 

Determinants of Mean Arterial Pressure (MAP)

MAP=Diastolic+1/3 (Pulse Pressure)

MAP reflects organ perfusion and is less affected by waveform distortion.

 

Dynamic Response of the System

Key Concepts

  • Natural frequency
  • Damping coefficient

Fast-Flush (Square Wave) Test

Performed by opening flush device briefly.

Interpretation

Response

Finding

1–2 oscillations

Optimal damping

Multiple oscillations

Underdamped system

No oscillation

Overdamped system

 

Underdamping vs Overdamping

Feature

Underdamped

Overdamped

Systolic BP

Falsely high

Falsely low

Diastolic BP

Falsely low

Falsely high

MAP

Usually accurate

Usually accurate

Waveform

Tall, sharp

Blunted, slurred

 

Factors Affecting Accuracy

Causes of Underdamping

  • Long tubing
  • Stiff, non-compliant system
  • Excessive stopcocks

Causes of Overdamping

  • Air bubbles
  • Blood clots
  • Kinked tubing
  • Vasospasm
  • Loose connections

 

Central vs Peripheral Arterial Pressure

  • Peripheral systolic pressure > central systolic pressure
  • Amplification increases with:
    • Young age
    • High SVR
  • MAP remains similar across sites
  • Femoral artery reflects central pressure better in shock

 

Interpretation in Special Conditions

Septic Shock

  • Wide pulse pressure
  • Low diastolic pressure
  • MAP target ≥ 65 mmHg (individualized)

Aortic Regurgitation

  • Bounding waveform
  • Wide pulse pressure

Intra-Aortic Balloon Pump

  • Diastolic augmentation
  • Balloon inflation during diastole

 

Complications

Local

  • Hematoma
  • Thrombosis
  • Ischemia
  • Pseudoaneurysm
  • Infection

Systemic

  • Embolism (air, thrombus)
  • Sepsis (prolonged catheterization)

 

Prevention of Complications

  • Aseptic insertion
  • Smallest effective catheter
  • Regular site inspection
  • Remove when no longer indicated