From the Guidelines
Normal systemic vascular resistance (SVR) is typically considered to be between 800 to 1200 dynes·sec/cm⁵, as it represents the resistance that the left ventricle must overcome to eject blood into the systemic circulation. This measurement is crucial for assessing cardiovascular function and guiding treatment in various clinical scenarios, such as those discussed in the 2020 ACC/AHA guideline for the management of patients with valvular heart disease 1. SVR is calculated using the formula: SVR = [(Mean Arterial Pressure - Central Venous Pressure) ÷ Cardiac Output] × 80.
Importance of SVR in Clinical Scenarios
Understanding a patient's SVR helps determine whether cardiac output issues stem from preload, contractility, or afterload problems, allowing for targeted interventions to optimize hemodynamics and tissue perfusion. Elevated SVR indicates increased afterload on the heart, commonly seen in hypertension or heart failure, while decreased SVR occurs in conditions like sepsis or anaphylaxis. Clinicians use SVR to guide vasopressor or vasodilator therapy in critically ill patients, as seen in patients with severe aortic stenosis (AS) who may require aortic valve replacement (AVR) to improve survival and reduce symptoms 1.
Clinical Application of SVR
In the context of valvular heart disease, such as severe AS, understanding SVR is essential for managing patients and determining the best course of treatment. For example, patients with severe AS and normal LV systolic function have a similar survival rate to age-matched controls during the asymptomatic phase, but those with a low LVEF and severe AS have better survival rates with AVR than with medical therapy 1. By considering SVR in the management of these patients, clinicians can make informed decisions about the timing and type of intervention, such as surgical or transcatheter AVR, to improve patient outcomes.
Key Considerations
Key considerations in the management of patients with valvular heart disease include the mean pressure gradient, which is a strong predictor of outcome after AVR, and the presence of contractile reserve, which can influence the decision to proceed with AVR 1. By taking into account these factors, as well as SVR, clinicians can provide personalized care to patients with valvular heart disease, ultimately improving morbidity, mortality, and quality of life.
From the Research
Normal SVR Classification
- The study 2 classified systemic vascular resistance (SVR) into three categories: below normal (SVR < 900 dyn s cm(-5)), normal (900 ⩽ SVR ⩽ 1200 dyn s cm(-5)), and above normal (SVR > 1200 dyn s cm(-5)).
- The classifier used features derived from the finger photoplethysmogram (PPG) waveform and other routine cardiovascular measurements, such as heart rate and mean arterial pressure.
- The optimum subset of features in the Gaussian model consisted of PPG waveform variability features, including the low-frequency to high-frequency ratio (LF/HF) and the normalized mid-frequency power (MF(NU)), in addition to the PPG pulse wave features.
SVR in Different Conditions
- In pediatric patients with pulmonary hypertension, the ratio of pulmonary-to-systemic vascular resistance decreased in response to arginine vasopressin, but not consistently with phenylephrine or epinephrine 3.
- In a model of systemic inflammation in humans, vasopressin mitigated the decrease in diastolic blood pressure and stabilized SVR and cardiac output, whereas norepinephrine and phenylephrine had limited effects 4.
- In septic shock patients, norepinephrine plus dobutamine was associated with a lower risk of 28-day mortality, while dopamine was associated with a higher risk of 28-day mortality 5.
Vasopressor Therapy
- Vasopressors are administered to critically ill patients with vasodilatory shock not responsive to volume resuscitation, and less often in cardiogenic shock, and hypovolemic shock 6.
- Norepinephrine is the first-choice vasopressor in vasodilatory shock after adequate volume resuscitation, while vasopressin or epinephrine may be added in norepinephrine-refractory patients.
- Angiotensin II and dopamine have limited indications, and novel vasopressors, such as selepressin, are in development.