5.2 Invasive Hemodynamic Monitoring

5.2 Invasive Hemodynamic Monitoring

Invasive hemodynamic monitoring is a core competency in critical care transport, providing continuous, real-time assessment of intravascular pressures, cardiac performance, and systemic vascular tone. Accurate measurement requires strict technical setup, understanding fluid dynamic physics, and mastering waveform interpretation. Errors in transducer leveling, zeroing, or dynamic response testing directly lead to inappropriate vasopressor and fluid administration.

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Arterial Line Setup, Zeroing, & Leveling

Indwelling arterial catheters (most commonly placed in the radial, femoral, or brachial arteries) allow continuous beat-to-beat blood pressure monitoring and arterial blood gas sampling. The monitoring system consists of an intravascular catheter, rigid non-compliant fluid-filled tubing, a continuous flush device (delivering 3 mL/hr of normal saline under 300 mmHg pressure), a pressure transducer, and an electronic monitor.

+--------------------------------------------------------------------------+
|                       HYDROSTATIC TRANSDUCER LEVELING                    |
|                                                                          |
|   Transducer Above Axis  --->  Falsely LOW BP  (-0.74 mmHg per cm high)  |
|   Phlebostatic Axis      --->  ACCURATE BP     (4th ICS, Mid-Axillary)   |
|   Transducer Below Axis  --->  Falsely HIGH BP (+0.74 mmHg per cm low)   |
+--------------------------------------------------------------------------+

Leveling at the Phlebostatic Axis

• Anatomical Definition: The phlebostatic axis is located at the intersection of the fourth intercostal space and the mid-axillary line (representing the anatomical position of the right atrium).
• Hydrostatic Pressure Errors: The fluid column between the patient's arterial bed and the transducer exerts hydrostatic pressure.
  • If the transducer is positioned below the phlebostatic axis, gravity adds fluid column pressure, resulting in falsely elevated blood pressure readings (+0.74 mmHg per cm of height discrepancy below the axis).
  • If the transducer is positioned above the phlebostatic axis, falsely decreased readings occur (-0.74 mmHg per cm of height discrepancy above the axis).
• Transport Re-leveling: Every time the patient's stretcher height or backrest position is adjusted, or during aircraft tilt, the transducer MUST be re-leveled to the phlebostatic axis.

Zeroing the Transducer

Zeroing eliminates atmospheric pressure from the measurement system so that displayed values reflect only intravascular pressures. The stopcock adjacent to the transducer is opened to air (atmosphere), and the electronic monitor is calibrated to read zero. Zeroing must be performed upon setup, during transport transitions, and whenever pressure values appear anomalous.

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Dynamic Response Testing: Overdamped vs. Underdamped Circuits

The accuracy of fluid-filled pressure monitoring systems relies on their natural frequency and damping coefficient. Critical care clinicians evaluate this using the Square Wave Test (Fast Flush Test) by pulling the flush device snap-tab to generate a high-pressure square wave.

NORMAL DYNAMIC RESPONSE:
   ┌──────┐
───┘      └─/
            \/
             └───/
                \─────────── (1 to 2 oscillations, visible dicrotic notch)

OVERDAMPED RESPONSE:
   ┌──────┐
───┘      └───────────────── (Sluggish return, no oscillations, lost dicrotic notch)

UNDERDAMPED RESPONSE:
   ┌──────┐
───┘      └─/
            \/
             \/
              \/
               └───/─────── (> 2 oscillations, false high SBP, false low DBP)

Overdamped Waveforms

• Waveform Characteristics: The square wave returns to baseline sluggishly with no post-flush oscillations. The arterial pulse contour loses its sharp systolic peak and dicrotic notch.
• Numerical Impact: Falsely underestimates Systolic Blood Pressure (SBP) and falsely overestimates Diastolic Blood Pressure (DBP); Mean Arterial Pressure (MAP) usually remains relatively unchanged.
• Common Causes: Air bubbles in the transducer or tubing, blood clots at the catheter tip, soft or compliant tubing, excessive stopcocks, or loose tubing connections.
• Corrective Actions: Purge all air bubbles, flush blood from the line, eliminate soft extension tubing, and tighten connections.

Underdamped Waveforms

• Waveform Characteristics: The square wave produces multiple rapid, ringing oscillations ($> 2\text{ undershoots}$) before returning to baseline.
• Numerical Impact: Falsely overestimates SBP and falsely underestimates DBP.
• Common Causes: Excessively long tubing ($> 3-4\text{ feet}$), small catheter diameter, hyperdynamic circulation, or hypothermia.
• Corrective Actions: Remove unnecessary extension tubing, insert a line damper, or use shorter tubing circuits.

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Central Venous Pressure (CVP) Waveform Analysis

Central Venous Pressure measures the hydrostatic pressure in the thoracic vena cava or right atrium, reflecting right ventricular preload and systemic volume status. Normal CVP ranges from 2 to 6 mmHg (or $3 - 8\text{ cmH}_2\text{O}$). CVP is measured at end-expiration to eliminate breathing-induced intrathoracic pressure fluctuations.

CVP Waveform Components

         c
   a    / \     v
  / \  /   \   / \
 /   \/     \ /   \
/     x      x'    \ y
──────────────────────────────────────

1. a Wave: Represents Right Atrial Contraction during late atrial diastole. Correlates with the end of the P wave on ECG.
   • Abnormalities: Cannon 'a' waves occur when the atrium contracts against a closed tricuspid valve (AV dissociation, 3rd-degree heart block, junctional rhythms, or VT). Absent 'a' waves occur in atrial fibrillation.
2. c Wave: Represents Tricuspid Valve Closure and bulging into the right atrium during early right ventricular isometric contraction. Correlates with the end of the QRS complex.
3. x Descent: Represents Right Atrial Relaxation and downward displacement of the tricuspid valve ring during right ventricular ejection.
4. v Wave: Represents Passive Venous Filling of the right atrium against a closed tricuspid valve during right ventricular systole. Correlates with the end of the T wave.
   • Abnormalities: Giant 'v' waves occur in severe tricuspid regurgitation, where RV systolic blood regurgitates backwards into the right atrium.
5. y Descent: Represents Opening of the Tricuspid Valve and passive emptying of blood from the right atrium into the right ventricle.
   • Abnormalities: A steep, deep 'y' descent occurs in constrictive pericarditis (Friedreich's sign). A blunted or absent 'y' descent is characteristic of cardiac tamponade, because high intrapericardial pressure prevents passive ventricular filling.

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Pulmonary Artery Catheter (PAC / Swan-Ganz) Waveforms & Pressures

The Pulmonary Artery Catheter is floated through the central venous system, right heart chambers, and pulmonary arterial tree to measure central hemodynamics and estimate Left Ventricular End-Diastolic Pressure (LVEDP).

Sequential Waveform Tracing During PAC Insertion:

   Right Atrium         Right Ventricle        Pulmonary Artery         PAWP (Wedge)
  ┌──────────────┐     ┌──────────────┐     ┌──────────────┐     ┌──────────────┐
  │ 2 - 6 mmHg   │ ──► │ 20-30/0-5    │ ──► │ 20-30/8-15   │ ──► │ 4 - 12 mmHg  │
  │ (Mean 4)     │     │ mmHg         │     │ (Mean 10-20) │     │ (Left Atrium)│
  └──────────────┘     └──────────────┘     └──────────────┘     └──────────────┘
                                              Dicrotic Notch!

Chamber Pressures & Waveform Characteristics

1. Right Atrium (RA): Pressure 2 - 6 mmHg. Displays typical a, c, v atrial waveforms.
2. Right Ventricle (RV): Systolic 20 - 30 mmHg, Diastolic 0 - 5 mmHg. Displays a sharp, tall vertical upstroke during systole and a rapid fall to near zero in diastole. Warning: Ventricular ectopy (PVCs, VT) frequently occurs as the catheter tip irritates the RV endocardium.
3. Pulmonary Artery (PA): Systolic 20 - 30 mmHg, Diastolic 8 - 15 mmHg (Mean 10 - 20 mmHg). Characterized by the immediate appearance of a dicrotic notch on the downstroke, representing closure of the pulmonic valve, alongside an elevated diastolic pressure ($8-15\text{ mmHg}$ vs RV diastolic $0-5\text{ mmHg}$).
4. Pulmonary Artery Wedge Pressure (PAWP / PCWP): Pressure 4 - 12 mmHg. Inflating the balloon with $\le 1.5\text{ mL}$ of air allows the catheter to lodge in a small branch of the pulmonary artery (Zone 3), occluding anterograde flow. The distal tip sees through the pulmonary capillary bed to measure Left Atrial Pressure, providing an accurate index of Left Ventricular End-Diastolic Volume (preload).
   • Safety Rule: Never keep the balloon inflated for longer than 15 seconds or 2-3 respiratory cycles. Continuous wedge waveforms indicate spontaneous wedging, carrying high risks of pulmonary artery rupture or pulmonary infarction.

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Core Derived Hemodynamic Formulas

Critical care transport clinicians must master quantitative hemodynamic calculations to guide vasopressor and inotropic titration.

1. Cardiac Output (CO) & Cardiac Index (CI)

$\text{CO} = \text{Heart Rate (HR)} \times \text{Stroke Volume (SV)} \quad (\text{Normal: } 4.0 - 8.0\text{ L/min})$ $\text{CI} = \frac{\text{Cardiac Output (CO)}}{\text{Body Surface Area (BSA)}} \quad (\text{Normal: } 2.5 - 4.0\text{ L/min/m}^2)$

2. Systemic Vascular Resistance (SVR) & Index (SVRI)

$\text{SVR} = \left( \frac{\text{MAP} - \text{CVP}}{\text{CO}} \right) \times 80 \quad (\text{Normal: } 800 - 1200\text{ dynes}\cdot\text{sec/cm}^{-5})$ $\text{SVRI} = \left( \frac{\text{MAP} - \text{CVP}}{\text{CI}} \right) \times 80 \quad (\text{Normal: } 1900 - 2400\text{ dynes}\cdot\text{sec}\cdot\text{m}^2/\text{cm}^{-5})$

3. Pulmonary Vascular Resistance (PVR)

$\text{PVR} = \left( \frac{\text{MPAP} - \text{PAWP}}{\text{CO}} \right) \times 80 \quad (\text{Normal: } 100 - 250\text{ dynes}\cdot\text{sec/cm}^{-5})$

4. Mean Arterial Pressure (MAP)

$\text{MAP} = \text{DBP} + \frac{1}{3}(\text{SBP} - \text{DBP}) = \frac{\text{SBP} + 2(\text{DBP})}{3} \quad (\text{Normal: } 70 - 105\text{ mmHg})$

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Hemodynamic Profiles Across Shock States

Comparing measured filling pressures (CVP, PAWP), pump output (CI), vascular resistance (SVR), and tissue oxygen extraction (SvO2) allows precise shock classification.

Hemodynamic Equalization: In cardiac tamponade, CVP, RV diastolic pressure, PA diastolic pressure, and PAWP all equalize within $1-2\text{ mmHg}$ of each other.