3.3 Hemodynamic Monitoring & Interpretation
Key Takeaways
- Cardiac Output is a product of Heart Rate and Stroke Volume; Stroke Volume depends on Preload, Afterload, and Contractility.
- Static CVP measurements are poor predictors of fluid responsiveness; rely on dynamic indices like the Passive Leg Raise or Pulse Pressure Variation.
- Cardiogenic shock is characterized by high filling pressures (CVP/PAOP), low cardiac output, and high systemic vascular resistance.
- An abnormally high SvO2 in the setting of sepsis often indicates severe mitochondrial dysfunction and an inability of tissues to extract oxygen, carrying a poor prognosis.
3.3 Hemodynamic Monitoring & Interpretation
Principles of Hemodynamics
A profound understanding of hemodynamics is paramount for the AGACNP to effectively manage critically ill patients. Hemodynamics is the study of the physical principles of blood flow and the forces involved in circulation. The core parameters dictating cardiac performance are preload, afterload, and contractility, which collectively determine Stroke Volume (SV). Stroke volume, combined with heart rate (HR), dictates Cardiac Output (CO), the total volume of blood ejected by the heart per minute. When indexed to body surface area, this becomes the Cardiac Index (CI), providing a more personalized assessment of cardiovascular adequacy.
- Preload: The volume of blood stretching the ventricular myocardium at end-diastole. It is a reflection of venous return and intravascular volume status. Clinically, right-sided preload is estimated by Central Venous Pressure (CVP), while left-sided preload is estimated by Pulmonary Artery Occlusion Pressure (PAOP), also known as the wedge pressure.
- Afterload: The resistance the ventricles must overcome to eject blood. It is largely determined by vascular tone. The Systemic Vascular Resistance (SVR) represents left ventricular afterload, while Pulmonary Vascular Resistance (PVR) represents right ventricular afterload.
- Contractility: The intrinsic strength of myocardial contraction independent of preload and afterload. It is enhanced by sympathetic stimulation and positive inotropes (e.g., dobutamine) and depressed by ischemia, acidosis, and negative inotropes.
Modalities of Hemodynamic Monitoring
Monitoring ranges from non-invasive methods (blood pressure cuffs, echocardiography) to highly invasive catheters.
Arterial Lines: Indicated for continuous, real-time blood pressure monitoring and frequent arterial blood gas sampling. The normal arterial waveform consists of a steep systolic upstroke, a systolic peak, a dicrotic notch (signifying aortic valve closure), and a diastolic runoff. Troubleshooting the waveform is critical.
- An overdamped waveform appears sluggish with a blunted peak and loss of the dicrotic notch; this falsely underestimates systolic pressure and overestimates diastolic pressure. Common causes include air bubbles in the tubing or blood clots.
- An underdamped waveform shows exaggerated peaks and multiple artificial oscillations, falsely elevating systolic pressure.
Central Venous Catheters (CVCs): Utilized to measure CVP, which approximates right atrial pressure and right ventricular preload. Normal CVP is 2-8 mmHg. While traditionally used to guide fluid resuscitation, static CVP alone is a poor predictor of fluid responsiveness; dynamic parameters are now preferred. CVCs also allow for the administration of potent vasoactive medications and the measurement of Central Venous Oxygen Saturation (ScvO2).
Pulmonary Artery (Swan-Ganz) Catheters: Though their routine use has declined in favor of less invasive methods like point-of-care ultrasound (POCUS), Pulmonary Artery Catheters (PACs) remain invaluable in complex shock states (e.g., mixed cardiogenic and septic shock). The PAC provides direct measurement of right atrial pressure, pulmonary artery pressures, PAOP, and thermodilution cardiac output. It also permits calculation of SVR, PVR, and continuous monitoring of Mixed Venous Oxygen Saturation (SvO2).
Hemodynamic Profiles in Shock States Table
Understanding the hemodynamic derangements in various shock states allows for targeted pharmacologic intervention.
| Type of Shock | CVP (Preload) | CO / CI | SVR (Afterload) | SvO2 | Primary Derangement |
|---|---|---|---|---|---|
| Cardiogenic | High | Low | High | Low | Pump failure; impaired contractility. |
| Hypovolemic | Low | Low | High | Low | Loss of intravascular volume. |
| Distributive (Septic) | Low or Normal | High (early) | Low | High | Profound vasodilation; loss of tone. |
| Obstructive (PE, Tamponade) | High | Low | High | Low | Mechanical obstruction to output. |
Interpretation of Oxygen Demand and Delivery (SvO2 and ScvO2)
Global tissue oxygenation is assessed using Mixed Venous Oxygen Saturation (SvO2), measured from the pulmonary artery, or Central Venous Oxygen Saturation (ScvO2), measured from the superior vena cava. Normal SvO2 is 60-75%, and ScvO2 is typically 70-80%.
- A decreased SvO2 (<60%) indicates increased tissue oxygen extraction, suggesting that oxygen delivery is inadequate to meet cellular demands. This can result from low cardiac output, anemia, hypoxia, or dramatically increased metabolic demand (e.g., fever, shivering).
- An elevated SvO2 (>75%) can be a dire prognostic sign in late distributive shock (sepsis), indicating mitochondrial dysfunction where tissues are unable to extract oxygen from the blood, or it can be seen with excessive inotropic support.
Evaluating Fluid Responsiveness
The fundamental question in shock is whether administering fluids will increase stroke volume—a concept based on the Frank-Starling curve. Because static measures (like CVP) are inadequate, dynamic assessments are the standard of care.
- Passive Leg Raise (PLR): An endogenous fluid bolus created by lifting the patient's legs to 45 degrees. A subsequent increase in stroke volume or cardiac output by >10% indicates fluid responsiveness.
- Pulse Pressure Variation (PPV) & Stroke Volume Variation (SVV): These utilize cardiopulmonary interactions during positive pressure mechanical ventilation. A variation >12-13% over the respiratory cycle suggests the patient is on the steep portion of the Starling curve and will respond to volume expansion. These require strict criteria: the patient must be completely sedated, mechanically ventilated with a sufficient tidal volume (usually >8 mL/kg ideal body weight), and in regular sinus rhythm without spontaneous breathing efforts.
A patient in the intensive care unit has a pulmonary artery catheter in place. Hemodynamic numbers reveal: CVP 18 mmHg, Cardiac Index 1.8 L/min/m2, SVR 1800 dynes-sec/cm-5, and SvO2 55%. Which shock state does this profile best represent?
When assessing fluid responsiveness using Stroke Volume Variation (SVV), which of the following patient conditions must be met for the measurement to be accurate?
An arterial line waveform appears flattened, with a sluggish systolic upstroke and absence of the dicrotic notch. The nurse notes the blood pressure reading has dropped from 130/80 to 90/60 mmHg. What is the most appropriate initial action?