How is systemic vascular resistance managed?

Management of Systemic Vascular Resistance

In critically ill patients, systemic vascular resistance (SVR) should be maintained higher than pulmonary vascular resistance (PVR) to ensure adequate right ventricular coronary perfusion and prevent right ventricular ischemia. 1

Definition and Calculation

• Systemic vascular resistance represents the resistance to blood flow generated by all systemic vasculature, excluding pulmonary vasculature 1
• SVR is calculated as (systemic mean arterial blood pressure minus right atrial pressure) divided by cardiac output 1
• The major determinant of SVR is arteriolar tone, but blood viscosity and vascular capacitance are also contributing factors 1
• SVR is typically measured in dynes/s/cm² 1

Clinical Significance of SVR

• SVR is a critical hemodynamic parameter used to guide vasopressor and vasodilator therapy in critically ill patients 1, 2
• Maintaining appropriate SVR is essential for:
  • Ensuring adequate coronary perfusion pressure 1
  • Preventing right ventricular ischemia 1
  • Supporting cardiac output and tissue perfusion 3
• SVR abnormalities are central to various cardiovascular pathologies:
  • Increased SVR is seen in hypertensive crisis and pulmonary edema 3
  • Decreased SVR occurs in septic shock and during exercise 4

Management Principles for SVR

Assessment of SVR

• Direct measurement via pulmonary artery catheter provides the most accurate assessment of SVR 1
• Central venous pressure monitoring is helpful in detecting and treating abnormal SVR 1
• Cardiac power index (CPI) and SVR together provide better diagnostic accuracy for different types of acute heart failure than conventional hemodynamic indices alone 3

Pharmacological Management of Low SVR

• Vasopressors are indicated when SVR is inadequate to maintain organ perfusion 1, 5
• Norepinephrine is the preferred agent for increasing SVR:
  • Start with 2-3 mL/min (8-12 mcg/min) of diluted solution (4 mg in 1000 mL of 5% dextrose) 5
  • Titrate to maintain systolic BP 80-100 mmHg or 40 mmHg below baseline in previously hypertensive patients 5
  • Average maintenance dose ranges from 0.5-1 mL/min (2-4 mcg/min) 5
• Vasopressin at replacement doses can be used to offset potential drops in SVR when using inotropes like dobutamine 1

Management of High SVR

• Vasodilators are indicated when SVR is excessively elevated 1, 3
• Short-acting calcium channel blockers are preferred over preload-reducing agents like nitroglycerin in patients with aortic stenosis 1
• In patients with regurgitant valve lesions (AR or MR), decreasing systemic afterload will augment systemic LV output and reduce regurgitant volume 1

Special Considerations

SVR in Congenital Heart Disease

• SVR assessment is crucial in decision-making for congenital heart defect repairs:
  • Surgical repair is recommended when pulmonary vascular resistance is less than one-third of SVR 1
  • Closure of atrial septal defects should not be performed when pulmonary vascular resistance is greater than two-thirds of SVR 1

SVR in Critical Care Settings

• In ICU settings, maintaining SVR greater than PVR is a major guideline for selecting inotropes and vasopressors 1
• Inotropes with neutral or beneficial effects on PVR include dobutamine, milrinone, and epinephrine 1
• Inhaled nitric oxide can be used to decrease PVR without affecting SVR in hypotensive pulmonary hypertension patients 1

SVR During Anesthesia

• Anesthetic management should take into account the effects on SVR, particularly in patients with valvular heart disease 1
• Regional anesthetics may be beneficial in patients with regurgitant valve lesions due to favorable systemic vasodilation 1
• In patients with aortic stenosis, maintaining adequate SVR is critical to ensure coronary perfusion 1

Pitfalls and Caveats

• Exercise-induced sympatholytic vasodilation can significantly reduce SVR, which may complicate resuscitation efforts in patients who were engaged in immediate pre-morbid exercise 4
• Blood viscosity components should be distinguished from vascular components when interpreting SVR values 2
• Conventional SVR measurements may need correction (C-SVR) to provide a better picture of true vascular changes, especially during hypothermia 2
• Isolated systolic hypertension with normal mean arterial pressure suggests normal peripheral resistance despite elevated pulse pressure 6