Afterload Assessment With or Without Central Venous Pressure: A Preliminary Clinical Comparison

Abstract

A clinical comparison, of two methods of afterload assessment, has been made. The first method, systemic vascular resistance index (SVR _i ), is based upon the traditional formula for afterload which utilizes central venous pressure (CVP), as well as cardiac index (C _i ), and mean arterial blood pressure (MAP). The second method, total systemic vascular resistance index (TSVR _i ), also uses MAP and C _i . However, TSVR _i ignores the contribution of CVP. This preliminary examination, of 10 randomly-selected ICU patients, has shown a high degree of correlation (ranging from 90 to 100%) between SVR _i and TSVR _i (P < 0.0001). Furthermore, there was also a high degree of correlation (ranging from 94 to 100%) noted between the hour-to-hour change in SVR _i with the hour-to-hour change in TSVR _i (P < 0.0001). The results, of this pilot study, support the premise that the use of CVP may not always be necessary for afterload evaluation in the clinical setting. Minimally-invasive means of measuring both C _i and MAP, without CVP, may be adequate for use in assessing afterload.

Appendix

The Interrelationship Between ΔSVR _i and ΔTSVR _i

ΔSVR _i is calculated using the definition of SVR _i from Eq. 1:

$$ \Updelta {\text{SVR}}_{i} = \left[ {{\frac{{\left( {{\text{MAP}}_{2} - {\text{CVP}}_{2} } \right)}}{{C_{{i_{2} }} }}}} \right] \cdot 80 - \left[ {{\frac{{\left( {{\text{MAP}}_{1} - {\text{CVP}}_{1} } \right)}}{{C_{{i_{1} }} }}}} \right] \cdot 80 $$

(1A)

Rearrangement yields:

$$ \Updelta {\text{SVR}}_{i} = \left[ {{\frac{{{\text{MAP}}_{2} }}{{C_{{i_{2} }} }}} - {\frac{{{\text{MAP}}_{1} }}{{C_{{i_{1} }} }}}} \right] \cdot 80 + \left[ {{\frac{{{\text{CVP}}_{1} }}{{C_{{i_{1} }} }}} - {\frac{{{\text{CVP}}_{2} }}{{C_{{i_{2} }} }}}} \right] \cdot 80 $$

(2A)

Substituting the definition of TSVR _i from Eq. 2 results in:

$$ \Updelta {\text{SVR}}_{i} = \Updelta {\text{TSVR}}_{i} + \left[ {{\frac{{{\text{CVP}}_{1} }}{{C_{{i_{1} }} }}} - {\frac{{{\text{CVP}}_{2} }}{{C_{{i_{2} }} }}}} \right] \cdot 80 $$

(3A)

Realizing that:

$$ {\frac{{{\text{CVP}}_{1} }}{{C_{{i_{1} }} }}} \approx {\frac{{{\text{CVP}}_{2} }}{{C_{{i_{2} }} }}}\quad {\text{thus:}}\left[ {{\frac{{{\text{CVP}}_{1} }}{{C_{{i_{1} }} }}} - {\frac{{{\text{CVP}}_{2} }}{{C_{{i_{2} }} }}}} \right] \approx 0 $$

(4A)

Finally:

$$ \Updelta {\text{SVR}}_{i} \approx \Updelta {\text{TSVR}}_{i} $$

(5A)

This concept can also be illustrated using the definition of the total differential:

$$ \Updelta {\text{SVR}}_{i} = {\frac{{\partial {\text{SVR}}_{i} }}{{\partial {\text{MAP}}}}}\Updelta {\text{MAP}} + {\frac{{\partial {\text{SVR}}_{i} }}{{\partial C_{i} }}}\Updelta C_{i} + {\frac{{\partial {\text{SVR}}_{i} }}{{\partial {\text{CVP}}}}}\Updelta {\text{CVP}} $$

(6A)

and

$$ \begin{gathered} \Updelta {\text{TSVR}}_{i} = {\frac{{\partial {\text{TSVR}}_{i} }}{{\partial {\text{MAP}}}}}\Updelta {\text{MAP}} + {\frac{{\partial {\text{TSVR}}_{i} }}{{\partial C_{i} }}}\Updelta C_{i} \hfill \\ \hfill \\ \end{gathered} $$

(7A)

Since ${\frac{{\partial {\text{SVR}}_{i} }}{{\partial {\text{CVP}}}}}\Updelta {\text{CVP}} \approx 0$ then $\Updelta {\text{SVR}}_{i} \approx \Updelta {\text{TSVR}}_{i}$. Also: ${\frac{{\partial {\text{SVR}}_{i} }}{{\partial {\text{MAP}}}}} \approx {\frac{{\partial {\text{TSVR}}_{i} }}{{\partial {\text{MAP}}}}}$ and ${\frac{{\partial {\text{SVR}}_{i} }}{{\partial C_{i} }}} \approx {\frac{{\partial {\text{TSVR}}_{i} }}{{\partial C_{i} }}}$.