Section Four--More about the Swan -Ganz

By this time you have noticed that Swan-Ganz values are sometime inconclusive without clear clinical indicators that point toward a diagnosis. The Swan-Ganz values presented so far may not point toward the correct treatment.

Two additional parameters will now be presented to provide further insight into the pathogenesis of Shock:

1) Cardiac Output

2) Systemic Vascular Resistance

Cardiac Output is usually measured by injecting known volume of Iced cold saline into the proximal port of a properly placed Swan-Ganz Catheter. As stated previously, the thermistor bead, located near the catheter tip, senses the change in temperature of the saline-blood solution. CO can be calculated using the computer on the principle that the rate of warming of the saline bolus is directly proportional to the blood flow. The normal range for Cardiac Output is 4-6 LPM. CO varies with the Body Mass; therefore, CO is standardized (indexed) with respect to Body surface Area.

BSA

An Equation for estimating BSA is:

BSA (M ^2) = Weight (kg) ^ .425 X Height (cm) ^.725 X 0.007184

Nomograms are available which allow estimation of BSA from Height and Weight. The Normal range of Indexed Cardiac Output (Cardiac Index) is 2.8 to 4.2 L/ Min/m ^2

As Cardiac Output = Heart Rate X Stroke Volume, anything that influences either factor influences Left Ventricular function. Some examples of these Influences are:

Of the seven major forces listed above, only the last two can be measured. EKG can measure electrical Activity. SVR reflects what is going on in the vascular bed; if the intravascular spaces are constricted, resistance is high and if they are dilated, SVR is low.

SVR is calculated by dividing Vascular Bed Entrance - Exit pressure by the Cardiac Output. The normal range for SVR is 11-17.5 mm Hg/L/Min.

How did we get these numbers?

SVR = mean aortic pressure - right atrial pressure

Cardiac output

So , for example,

SVR = 90 mm Hg - 4mm Hg

5 L/min

= 17.2 mm Hg/L/min

With the addition of Cardiac Index and SVR to the previously measured hemodynamic parameters, a fairly complete grasp of shock physiology is possible. In order to keep it simple, three parameters, CI, SVR and PCWP will be surveyed.

The Pump--Cardiogenic Shock

PCWP is High --The Left Heart is congested, fluid backs up into the pulmonary vascular bed.

CI is Low-- The Heart muscle is Failing, Heart rate increases which causes further fatigue.

SVR is High--Peripheral Vasculature constricts in a Sympathetic Nervous system Response to low Blood Pressure.

The Volume--Hypovolemic Shock

PCWP is Low--a diminished venous return due to loss of blood and fluids leads to a low left Heart filling pressure.

CI is Low--Low left heart filling Pressure translates into low CI when volume depletion becomes severe.

SVR is High--the Peripheral Vasculature constricts in response to Low Blood Pressure.

The Space--Low Resistance Shock

PCWP is Low--due to leaky capillaries transferring fluid into the interstitial spaces.

CI is High--The Heart is in a hyperdynamic state. The Left Ventricle completely ejects any blood provided by the pulmonary vascular bed.

SVR is Low--The Peripheral Vasculature is dilated, and the sympathetic nervous system has no affect on this Shock.

Review

Normal Values