What are the pitfalls of measuring Systemic Vascular Resistance (SVR) in post-cardiopulmonary bypass patients?

From the Guidelines

Measuring Systemic Vascular Resistance (SVR) in post-cardiopulmonary bypass patients is fraught with pitfalls that can lead to inaccurate clinical assessments, primarily due to the instability of hemodynamic parameters and the effects of vasoactive medications and mechanical circulatory support devices. The primary challenge is that SVR calculations rely on assumptions that are often invalid in the post-bypass setting, as seen in studies such as 1. SVR measurements depend on a formula using mean arterial pressure, central venous pressure, and cardiac output, but these parameters are highly unstable following cardiopulmonary bypass due to inflammatory responses, temperature fluctuations, and vasodilation. Some key points to consider include:

• Many post-bypass patients have significant arteriovenous shunting and heterogeneous regional blood flow that isn't captured in standard SVR calculations.
• Vasoactive medications commonly used during this period (such as norepinephrine, vasopressin, or milrinone) further complicate interpretation as they affect different vascular beds non-uniformly, as discussed in 1.
• The presence of mechanical circulatory support devices like intra-aortic balloon pumps can also invalidate standard SVR calculations.
• Clinicians should recognize that normal SVR values (800-1200 dyn·s/cm5) may not represent optimal tissue perfusion in these patients, and treatment decisions should incorporate multiple parameters including lactate trends, urine output, and end-organ function rather than targeting specific SVR values, as suggested by 1.
• Dynamic assessment of fluid responsiveness and tissue perfusion often provides more clinically relevant information than isolated SVR measurements in the post-cardiopulmonary bypass period. It's also important to consider the potential for pulmonary hypertension and right ventricular failure in these patients, as highlighted in 1, which can further complicate the interpretation of SVR measurements. Overall, a comprehensive approach to hemodynamic monitoring and management is necessary in post-cardiopulmonary bypass patients, taking into account the complexities and pitfalls of SVR measurement.

From the Research

Pitfalls of Measuring Systemic Vascular Resistance (SVR) in Post-Cardiopulmonary Bypass Patients

• The measurement of SVR in post-cardiopulmonary bypass patients can be challenging due to various factors, including the use of vasoactive medications and the presence of systemic inflammatory response syndrome 2.
• One of the pitfalls of measuring SVR is the potential for inaccurate readings due to the use of invasive monitoring techniques, such as pulmonary artery catheterization, which can be affected by factors such as catheter position and calibration 3.
• Noninvasive measurement techniques, such as Doppler echocardiography, can provide a reliable assessment of SVR, but may not be suitable for all patients, particularly those with significant mitral regurgitation or left ventricular outflow tract obstruction 4.
• The use of vasoconstrictors, such as norepinephrine, to treat hypotension in post-cardiopulmonary bypass patients can affect SVR measurements, and alternative agents, such as terlipressin, may be more effective in maintaining systemic vascular tone without increasing pulmonary vascular resistance 5, 6.
• The timing of SVR measurements is also important, as values can vary significantly over time, particularly in the early postoperative period 2.

Factors Affecting SVR Measurements

• Systemic inflammatory response syndrome: can cause a low SVR state, making it challenging to interpret measurements 2.
• Vasoactive medications: can affect SVR measurements, particularly if not accounted for in the measurement technique 5, 6.
• Invasive monitoring techniques: can be affected by factors such as catheter position and calibration, leading to inaccurate readings 3.
• Noninvasive measurement techniques: may not be suitable for all patients, particularly those with significant mitral regurgitation or left ventricular outflow tract obstruction 4.
• Timing of measurements: values can vary significantly over time, particularly in the early postoperative period 2.