© 2000 B. Bo Sramek

SYSTEMIC HEMODYNAMICS: FICTIONS AND FACTS

B. Bo Sramek, Ph.D.
International Hemodynamic Society
E-mail: info@hemodynamicsociety.org

Systemic hemodynamics, taught today in medical schools, is based upon the following assumptions, stipulations (and fictions):

  1. In the general patient population, the hemodynamic parameters are the systolic and diastolic blood pressures and the heart rate (HR).
  2. In the high-risk and critically ill patients, cardiac output (CO), central venous pressure (CVP) and pulmonary artery occluded pressure (PAOP) are the supplemental hemodynamic parameters.
  3. Determination of CO is important only in the high-risk and critically ill patients (~2% of the entire patient population).
    Measurement of CVP and PAOP is essential for hemodynamic management decisions in these patients.
  4. A patient with constant oxygen demand (such as a resting, supine, or a hospitalized patient) has a steady-state level of CO, HR and stroke volume (SV). It is, therefore, clinically sufficient to determine CO in these patients only once in a while.
  5. Normal CO (unindexed global blood flow) in resting adults is CO = 5.5 l/min.
    Determination of CO is unimportant and unnecessary in the general population patients.
  6. Systolic and diastolic blood pressures are sufficient clinical determinants in management of hypertension and congestive heart failure.

The facts, however, are:

  1. Adequate oxygen delivery is the primary determinant in survival of high-risk, critically ill patients. An adequate oxygen delivery under all metabolic conditions is a true definition of cardiovascular health. CO is its only dynamic modulator.
  2. Normal CO in all resting mammals is a linear function of their weight at 0.1 l/min/kg.
    Only indexed blood flow parameters exhibit normal ranges and provide information about adequacy of blood flow and oxygen delivery.
  3. Systemic hemodynamics deals with arterial blood pressure and blood flow inter-relationships.
    Both the arterial blood pressure and blood flow have to be measured simultaneously.
  4. Even in a supine, resting patient, the systemic hemodynamics adjusts to a new level for every heartbeat. Hemodynamic state, therefore, is defined by mean values of arterial blood pressure and blood flow over one heartbeat interval, i.e., the mean arterial pressure (MAP) and stroke index (SI). Cardiac Index (CI) is then a perfusion-significant blood flow.
  5. Hemodynamic state (SI @ MAP) is a result of beat-by-beat modulating effect of three hemodynamic modulators (intravascular volume, inotropy and vasoactivity). Only normal levels in these three hemodynamic modulators result in the normohemodynamic state (normotension @ normodynamic flow). The perfusion flow level (CI) is then set by a chronotropic modulation by HR. Normoperfusion state (CI within a normal range) is a result of normochronotropy.
  6. Hemodynamics, perfusion flow dynamics and oxygen delivery dynamics and adequacy have to be determined continuously and in every patient.
  7. Blood pressure in vessels is a result of blood flow through the vessel, vessel resistance and wall compliance.
    Blood pressure in chambers is a result of blood inflow and chamber wall compliance.
    Use of CVP or PAOP for determination of status of intravascular volume, therefore, is flawed.

Reference:

Sramek BB. Systemic Hemodynamics and Hemodynamic Management, 2002, InstantPublisher.com [ISBN 1-59196-046-0]