2.1 Hemodynamic Monitoring and Interpretation

Hemodynamic monitoring is fundamental in the ICU to assess cardiovascular function, fluid status, and tissue perfusion. It provides real-time data to guide fluid resuscitation, vasopressor use, and inotropic support.

Arterial Lines

Arterial lines are the most common invasive hemodynamic monitoring device. Placed typically in the radial, femoral, or brachial artery, they provide continuous, beat-to-beat blood pressure monitoring. This is essential for patients requiring vasoactive infusions or those with severe hypotension or hypertension. Furthermore, arterial lines allow for frequent arterial blood gas (ABG) sampling without repeated needle sticks.

The Arterial Waveform

Understanding the arterial waveform is critical. The normal waveform has several components:

• Systolic Upstroke: Represents left ventricular ejection. A steep upstroke indicates strong contractility.
• Systolic Peak: The maximum pressure during systole (systolic blood pressure).
• Dicrotic Notch: Represents the closure of the aortic valve, marking the end of systole and the beginning of diastole.
• Diastolic Run-off: The continuous decline in pressure as blood flows into the peripheral circulation.
• End-Diastolic Pressure: The lowest point before the next upstroke (diastolic blood pressure).

Overdamping (sluggish waveform, loss of dicrotic notch) can underestimate systolic pressure, while underdamping (artifactual spikes) can overestimate it. The mean arterial pressure (MAP), which is the most accurate reflection of tissue perfusion, remains relatively consistent regardless of damping.

Central Venous Catheters (CVCs)

CVCs are used to measure Central Venous Pressure (CVP), which reflects the pressure in the right atrium or the superior vena cava.

• Normal Range: 2-6 mmHg (or 3-8 cm H2O)
• Interpretation: CVP is traditionally used as an indicator of right ventricular preload and intravascular volume status. However, its utility as a sole predictor of fluid responsiveness is limited. A low CVP (< 2 mmHg) may indicate hypovolemia, while a high CVP (> 8-10 mmHg) can indicate right ventricular failure, volume overload, pulmonary hypertension, or cardiac tamponade.

Pulmonary Artery Catheters (PACs)

Also known as Swan-Ganz catheters, PACs provide comprehensive hemodynamic data. Inserted through a central vein, they pass through the right atrium, right ventricle, and into the pulmonary artery.

Key Measurements and Normal Values

Interpreting Shock States with PAC Data

PACs are incredibly useful for differentiating types of shock:

1. Hypovolemic Shock: Low CVP, Low PAWP, Low CO/CI, High SVR (compensatory vasoconstriction).
2. Cardiogenic Shock: High CVP, High PAWP, Low CO/CI, High SVR.
3. Distributive Shock (e.g., Sepsis): Low to normal CVP, Low to normal PAWP, High CO/CI (initially), Low SVR (massive vasodilation).
4. Obstructive Shock (e.g., Massive PE): High CVP, Low PAWP, Low CO/CI, High SVR.

Mixed and Central Venous Oxygen Saturation (SvO2 and ScvO2)

These values reflect the balance between oxygen delivery (DO2) and oxygen consumption (VO2).

• SvO2 (Mixed Venous O2 Saturation): Measured from the distal port of a PAC (pulmonary artery). It represents true mixed venous blood from the superior vena cava, inferior vena cava, and coronary sinus. Normal is 65-75%.
• ScvO2 (Central Venous O2 Saturation): Measured from a CVC in the superior vena cava. It typically runs about 5% higher than SvO2 (normal > 70%).

A low SvO2/ScvO2 indicates inadequate oxygen delivery (low CO, anemia, hypoxia) or increased oxygen consumption (fever, shivering, pain, seizures). A high SvO2/ScvO2 indicates decreased oxygen extraction (e.g., mitochondrial dysfunction in sepsis, cyanide toxicity, hypothermia).

Dynamic Parameters of Fluid Responsiveness

Given the limitations of static measures like CVP, dynamic parameters are increasingly preferred. These include:

• Stroke Volume Variation (SVV) and Pulse Pressure Variation (PPV): Measured via advanced arterial line analytics. Values > 12-13% in mechanically ventilated patients (without spontaneous breaths and in normal sinus rhythm) suggest fluid responsiveness.
• Passive Leg Raise (PLR): Transfers venous blood from the legs to the central circulation (an auto-fluid bolus). A subsequent increase in cardiac output indicates fluid responsiveness.