## Calculations

**Resistance:** is the amount of work necesary to push blood out into the systemic or pulmonary systems. It is the result of friction between flowing blood and the vascular walls. The diameter of the blood vessels is the principal factor determining resistance.

# Calculations of vascular resistance are applied to both the pulmonary and systemic circulations. The formula to calculate vascular resistance is derived from Poiseuille's Law.

# Use the following to calculate Systemic Vascular Resistance (SVR) and Pulmonary Vascular Resistance (PVR)

# The mean pulmonary capillary wedge pressure (PCWP) can be used as an approximation of left atrial pressure.

# Q can be expressed as cardiac output (L/min).

# These equations yield arbitrary resistance units and are reported in mmHg/(L/min), also known as Hybrid Units. Many may refer to them as __Wood units__ since they were introduced by Dr. Paul Wood.

__Wood units__

# Resistance can be measured as metric resistance units (expressed as Dynes • sec • cm^{-5} by mutiplying the result by 80.

# Hybrid units tend to be used in pediatric cases while Metric units are used in adult cases.

**Normal values for Vascular Resistance**

# Systemic Vascular Resistance = 1170 __+__ 270 Dynes • sec • cm^{-5}.

# Pulmonary Vascular Resistance = 67 __+__ 30 Dynes • sec • cm^{-5}.

### References

- Baim, D.S. (2005)
*Grossman's Cardiac Catheterization, Angiography, and Intervention.*(7th ed.). Philadelphia,Pa: Lippincott, Williams & Wilkins. - Kern, M. et al (2003)
*The Cardiac Catheterization Handbook*(4th ed.). Philadelphia, Pa: Mosby.