5.1 Cardiovascular Monitoring

Cardiovascular Monitoring in the ICU

Hemodynamic monitoring is a cornerstone of critical care, providing vital information about a patient's cardiovascular status, fluid balance, and tissue perfusion. For the RRT-ACCS exam, mastering hemodynamic parameters, interpreting invasive monitoring waveforms, and understanding oxygen delivery and consumption are absolutely critical.

Normal Hemodynamic Parameters

Understanding the normal ranges and physiological meaning of hemodynamic values is essential for diagnosing and managing critically ill patients.

Central Venous Pressure (CVP)

• Normal Range: 2-6 mmHg (or 4-12 cmH2O)
• Significance: CVP measures right atrial pressure and reflects right ventricular end-diastolic pressure (RVEDP). It is an indicator of right heart function and systemic venous return (preload).
• Elevated CVP: Indicates right heart failure, cor pulmonale, tricuspid valve disease, pulmonary hypertension, or hypervolemia (fluid overload).
• Decreased CVP: Indicates hypovolemia, vasodilation, or shock.

Pulmonary Artery Pressure (PAP)

• Normal Range: 25/10 mmHg, with a mean of 15 mmHg.
• Significance: PAP reflects right ventricular afterload and pulmonary vascular resistance.
• Elevated PAP: Often seen in pulmonary hypertension, pulmonary embolism, chronic lung disease (COPD), or left heart failure (causing back-pressure).

Pulmonary Capillary Wedge Pressure (PCWP)

• Normal Range: 4-12 mmHg.
• Significance: Also known as Pulmonary Artery Occlusion Pressure (PAOP). It estimates left atrial pressure and left ventricular end-diastolic pressure (LVEDP), reflecting left heart preload.
• Elevated PCWP: A hallmark of left ventricular failure, mitral valve stenosis, or severe hypervolemia. A PCWP > 18 mmHg often correlates with the onset of cardiogenic pulmonary edema.

Cardiac Output (CO) & Cardiac Index (CI)

• Normal CO Range: 4-8 L/min.
• Normal CI Range: 2.5-4.0 L/min/m².
• Significance: CO is the volume of blood pumped by the heart per minute (Heart Rate × Stroke Volume). CI relates CO to the patient's body surface area, providing a more accurate assessment of perfusion adequacy.

Systemic Vascular Resistance (SVR) & Pulmonary Vascular Resistance (PVR)

• Normal SVR Range: 800-1200 dynes·sec/cm⁵.
• Normal PVR Range: 100-250 dynes·sec/cm⁵.
• Significance: SVR measures left ventricular afterload (resistance the left heart must overcome). PVR measures right ventricular afterload. SVR is profoundly affected by vasoactive drugs and distributive shock states.

Arterial Line Waveform Interpretation

Invasive arterial blood pressure monitoring via an indwelling catheter provides continuous, accurate blood pressure readings and easy access for ABG sampling.

Anatomy of the Waveform

1. Systolic Upstroke: Represents rapid left ventricular ejection. A steep upstroke indicates strong contractility.
2. Systolic Peak: The highest point, representing peak systolic pressure.
3. Dicrotic Notch: A small downward deflection on the descending limb. This is a critical exam point: it represents the closure of the aortic valve and marks the beginning of diastole.
4. Diastolic Run-off: The continuous decline in pressure before the next contraction, representing diastolic pressure.

Waveform Abnormalities

• Dampened Waveform (Overdamped): The waveform appears flattened, with a rounded systolic peak and an absent or poorly defined dicrotic notch. It falsely underestimates systolic pressure and overestimates diastolic pressure. Causes: Air bubbles in the tubing, blood clots at the catheter tip, kinked tubing, or loose connections.
• Resonant Waveform (Underdamped/Whip): The waveform is characterized by sharp, narrow, and exaggerated peaks with multiple oscillations. It falsely overestimates systolic pressure. Causes: Tubing that is too long, excessively stiff tubing, or tachycardia.

Mixed Venous Oxygen Saturation (SvO2)

Mixed venous oxygen saturation is continuously measured via a pulmonary artery catheter. It represents the oxygen saturation of blood returning to the right heart after tissue extraction.

• Normal Range: 60-80%.
• Clinical Meaning: SvO2 is the ultimate indicator of the balance between oxygen delivery (DO2) and oxygen consumption (VO2).
• Decreased SvO2 (< 60%): Indicates that tissues are extracting a higher percentage of oxygen, or delivery has dropped. Causes: Decreased cardiac output (heart failure, shock), decreased arterial oxygenation (hypoxia), decreased hemoglobin (anemia), or increased oxygen demand (fever, shivering, seizures).
• Increased SvO2 (> 80%): Indicates decreased tissue oxygen extraction or an inability of tissues to utilize oxygen. Causes: Sepsis (due to microvascular shunting and cellular dysfunction), hypothermia, or a wedged PA catheter.

Clinical Application and Case Example

Imagine a 65-year-old male admitted with severe dyspnea. Hemodynamic values show:

• BP: 85/50 mmHg
• CVP: 15 mmHg (High)
• PAP: 45/25 mmHg (High)
• PCWP: 22 mmHg (High)
• CO: 3.2 L/min (Low)
• SVR: 1600 dynes·sec/cm⁵ (High)

Analysis: The elevated PCWP indicates left ventricular failure. The left heart failure causes a backup of pressure into the pulmonary circulation (elevated PAP), which subsequently causes the right heart to fail (elevated CVP). The low cardiac output and high SVR confirm cardiogenic shock. The high SVR is a compensatory mechanism to maintain blood pressure. Management would involve inotropes (e.g., dobutamine) to improve contractility and diuretics to reduce volume overload.