How to manage abnormal Systemic Vascular Resistance (SVR) measurements on Flowtrac?

Management of Abnormal Systemic Vascular Resistance (SVR) on FloTrac

The management of abnormal SVR on FloTrac should target a normal SVR range of 800-1200 dynes·sec·cm⁻⁵, with specific interventions based on whether SVR is high or low. 1

Understanding SVR Measurements on FloTrac

FloTrac is a minimally invasive cardiac output monitoring system that calculates SVR by measuring arterial pressure waveforms. SVR represents the resistance that must be overcome to push blood through the systemic circulatory system.

Normal Values and Clinical Significance

• Normal SVR range: 800-1200 dynes·sec·cm⁻⁵ 1
• Low SVR: <800 dynes·sec·cm⁻⁵
• High SVR: >1200 dynes·sec·cm⁻⁵

Management of Low SVR

Low SVR is commonly seen in:

• Post-cardiopulmonary bypass (affecting up to 44% of patients) 2
• Sepsis
• Systemic inflammatory response syndrome
• Anaphylaxis
• Mesenteric traction syndrome during abdominal surgery 3

Step 1: Assessment

• Confirm low SVR reading with repeat measurement
• Check for clinical signs of vasodilation (warm extremities, bounding pulses)
• Assess volume status using central venous pressure (CVP) and stroke volume variation (SVV)
  • SVV <9% suggests adequate volume status 3
  • SVV >13% suggests hypovolemia 3

Step 2: Treatment Algorithm for Low SVR

1. If hypovolemic (SVV >13% or low CVP):

   • Administer fluid bolus (crystalloids or colloids)
   • Reassess SVR after fluid administration

2. If euvolemic or hypervolemic (SVV <9% or normal/high CVP):

   • First-line vasopressor: Norepinephrine starting at 0.05 mcg/kg/min, titrating up to 2 mcg/kg/min as needed 1

   • Second-line vasopressor: Vasopressin 0.01-0.04 units/min 4

     • Particularly effective for low SVR states as it causes vasoconstriction by binding to V1 receptors on vascular smooth muscle 4
     • Target SVR 800-1200 dynes·sec·cm⁻⁵ 5

3. For refractory low SVR:

   • Consider epinephrine (0.05-0.5 mcg/kg/min) which increases SVR through α1-mediated peripheral vasoconstriction 6
   • Consider hydrocortisone if adrenal insufficiency is suspected

Management of High SVR

High SVR is commonly seen in:

• Hypertensive crisis
• Cardiogenic shock
• Heart failure
• Aortic stenosis

Step 1: Assessment

• Confirm high SVR reading with repeat measurement
• Check for clinical signs of vasoconstriction (cool extremities, weak pulses)
• Assess cardiac output and blood pressure

Step 2: Treatment Algorithm for High SVR

1. If blood pressure permits reduction:

   • First-line vasodilator: Sodium nitroprusside infusion

     • Starting dose: 0.3-0.5 mcg/kg/min
     • Titrate to effect (maximum: 10 mcg/kg/min)
     • Mechanism: Relaxation of vascular smooth muscle causing dilatation of peripheral arteries and veins 7
     • Monitor for cyanide toxicity with prolonged use

   • Alternative vasodilators:

     • Nitroglycerin (primarily venodilator)
     • Nicardipine (arterial vasodilator)
     • ACE inhibitors (for chronic management)

2. If cardiogenic shock with high SVR:

   • Consider inodilators (e.g., milrinone, dobutamine) that reduce afterload while improving cardiac output 5
   • Target SVR 800-1200 dynes·sec·cm⁻⁵ 5

Special Considerations

Right Ventricular Dysfunction

• Maintain SVR > PVR ratio to prevent right ventricular ischemia 1
• In right ventricular failure, avoid excessive vasodilation that could compromise RV perfusion 5
• The RV prefers euvolemia with CVP of 8-12 mmHg 5

Volume Management

• Avoid excessive fluid administration in patients with elevated CVP as it may worsen right heart function 5
• Consider diuresis if evidence of volume overload is present

Monitoring Response

• Continuously monitor arterial pressure
• Perform serial SVR calculations after interventions
• Consider echocardiographic assessment of ventricular function
• Monitor urine output as an indicator of adequate renal perfusion 1

Common Pitfalls to Avoid

1. Relying solely on SVR without clinical context: SVR should be interpreted alongside other hemodynamic parameters (cardiac output, blood pressure, SVV)

2. Overaggressive fluid administration in low SVR states: FloTrac studies show that many patients with low SVR may not be fluid responsive 3, 8

3. Treating the number instead of the patient: Target SVR values should be adjusted based on the patient's baseline condition and clinical response

4. Neglecting the SVR/PVR ratio: Maintaining appropriate balance between systemic and pulmonary vascular resistance is crucial, especially in right heart dysfunction 1

5. Failing to identify and treat the underlying cause: Always address the primary etiology of abnormal SVR while providing supportive management