1.2 Invasive Hemodynamic Monitoring & Waveform Interpretation

1.2 Invasive Hemodynamic Monitoring & Waveform Interpretation

Clinical Scenario: A 58-year-old female with severe septic shock and ARDS is admitted to the ICU on mechanical ventilation. She has an indwelling radial arterial line and a pulmonary artery catheter. The bedside monitor displays an arterial waveform with a blunted systolic peak and absent dicrotic notch, recording a blood pressure of 82/52 mmHg (MAP 62 mmHg). However, an automated non-invasive cuff measurement reads 106/66 mmHg (MAP 79 mmHg). Fast-flush testing reveals a sluggish return to baseline without post-flush oscillations.

Arterial Catheter Dynamics & The Fast-Flush Assessment

The arterial pressure waveform consists of a steep systolic upstroke, a systolic peak, a dicrotic notch (marking aortic valve closure and the start of diastole), and diastolic runoff.

Level the transducer at the phlebostatic axis (fourth intercostal space, mid-axillary line, corresponding to the right atrium). Vertical displacement introduces hydrostatic error: for every 2.5 cm (1 inch) displacement above the axis, measured pressure decreases ~1.86 mmHg (falsely low); positioning below causes falsely elevated readings.

The Square Wave (Fast-Flush) Test

Assessing dynamic response via the fast-flush test determines whether the monitoring circuit accurately reproduces pressure dynamics:

Dynamic Fluid Responsiveness

Cyclic cardiopulmonary interactions during positive-pressure ventilation induce fluctuations in arterial pulse pressure: $\text{PPV (\%)} = \frac{\text{PP}_{\text{max}} - \text{PP}_{\text{min}}}{(\text{PP}_{\text{max}} + \text{PP}_{\text{min}}) / 2} \times 100$ A threshold $\ge 12–13\%$ indicates fluid responsiveness ($\ge 15\%$ stroke volume increase with volume challenge). Validity criteria: volume-controlled ventilation, tidal volume $\ge 8$ mL/kg PBW, absence of spontaneous efforts, normal sinus rhythm, closed chest.

Central Venous Pressure (CVP) Waveforms & Pathological Morphologies

Central venous pressure reflects right ventricular end-diastolic filling. The normal CVP waveform exhibits three positive waves and two descents:

• a wave: Right atrial contraction (end-diastole, follows ECG P-wave).
• c wave: Bulging of tricuspid leaflets into right atrium during RV isovolumetric contraction (follows QRS).
• x descent: Atrial relaxation and downward displacement of tricuspid valve during ventricular systole.
• v wave: Passive filling of right atrium against closed tricuspid valve in late systole (coincides with T-wave).
• y descent: Opening of tricuspid valve with rapid passive emptying into RV in early diastole.

Key Pathological Waveforms

• Cannon a Waves: Tall peaked waves from atrial contraction against a closed tricuspid valve in AV dissociation (complete heart block, VT, junctional rhythm, pacing).
• Giant v Waves (Lancisi Sign): Severe tricuspid regurgitation; retrograde systolic ejection obliterates x descent, merging into a massive v wave.
• Blunted y Descent with Elevated CVP: Cardiac tamponade (elevated pericardial pressure impairs diastolic ventricular filling).
• Steep y Descent ("Dip and Plateau"): Constrictive pericarditis (rapid early filling arrested abruptly by rigid pericardium).
• Absent a Wave: Atrial fibrillation (loss of organized atrial contraction).

Pulmonary Artery Catheter (PAC) Profiles & Hemodynamics

During insertion, the PAC encounters distinctive chamber waveforms:

1. Right Atrium (RA): Mean 2–6 mmHg; waveform matches CVP.
2. Right Ventricle (RV): Systolic 15–30 mmHg, diastolic 0–8 mmHg (no dicrotic notch).
3. Pulmonary Artery (PA): Systolic 15–30 mmHg, diastolic 4–12 mmHg (dicrotic notch on downslope; mean 10–20 mmHg).
4. Pulmonary Capillary Wedge Pressure (PCWP): Balloon occlusion creates static column reflecting left atrial pressure (mean 6–12 mmHg; a and v waves). Inflate $\le 1.5$ mL air for $< 15$ sec to prevent PA rupture.

Hemodynamic Reference Ranges

Oxygen Delivery, Consumption & Extraction Balance

Invasive hemodynamic assessment ensures adequate cellular oxygenation:

• Arterial Oxygen Content ($CaO_2$): $CaO_2 = (\text{Hb} \times 1.34 \times SaO_2) + (PaO_2 \times 0.0031) \approx 20\text{ mL } O_2/\text{dL}$
• Oxygen Delivery ($DO_2$): $DO_2 = \text{CO} \times CaO_2 \times 10 \approx 950–1150\text{ mL/min}$
• Oxygen Consumption ($VO_2$): $VO_2 = \text{CO} \times (CaO_2 - CvO_2) \times 10 \approx 200–250\text{ mL/min}$
• Oxygen Extraction Ratio ($O_2ER$): $O_2ER = \frac{VO_2}{DO_2} = \frac{SaO_2 - SvO_2}{SaO_2} \approx 20–30\%$

Clinical Interpretation of SvO2

• Low SvO2 (< 60%): Indicates increased extraction of delivered oxygen because delivery is reduced or demand has increased. Causes include cardiogenic shock, hypovolemia, anemia, hypoxemia, fever, shivering, seizures, or agitation. Assess lactate and organ perfusion to determine whether oxygen delivery is inadequate.
• High SvO2 (> 75–80%): Indicates impaired microvascular oxygen extraction or mitochondrial poisoning (septic shock, cyanide toxicity), hypothermia, paralysis, left-to-right shunt (VSD), or catheter wedge artifact (sampling oxygenated capillary blood).