Cardiovascular Physiology

HUMAN Peripheral Arterial-Venous Fistula Experiment

A peripheral fistula (or 'shunt') is a direct arterial-venous (A-V) connection that bypasses the end purpose of the circulation, which is to supply the tissues with their metabolic (gas and energy) needs. Such a shunt imposes an additional burden on the heart because it forces it to pump extra blood flow (i.e. the shunt flow, AVFLOW) for no useful metabolic use. Such vascular (as opposed to central) shunts can occur in the systemic circulation between an artery and vein or in the lung circulation (e.g. core pulmonale).

A shunt may be created in HUMAN by setting the parameter AVFIST to any value greater then 0.0 (its default). A value of 0.5 will produce a shunt flow size of about half the normal cardiac output and a value equal to the normal cardiac output (quite a serious shunt) can be specified by AVFIST = 1.0. Two values will be employed in this protocol, AVFIST = 0.2 for a long term experiment and AVFIST = 0.5 for a short term experiment. Fistulas are useful tools conceptually because they provide us with an opportunity to study the control of cardiac output in a condition in which it is chronically elevated due to a lowered peripheral resistance (TPR).

When a fistula is introduced, the cardiovascular control systems act to reelevate the useful cardiac output (i.e.. metabolic flow = COL - AVFLOW) back towards normal. Below are instructions for fistula experiments that are follow the consequences for two different periods of time (short and long term). For each condition, set up the experiment, eyeball the data pattern and discuss the pattern of the response by which metabolic flow is maintained at near normal levels

Short Term Response To An A-V Fistula

(note: the instructions below are skeletal and assume prior competence in using web-HUMAN)

Run #1 - Short term response

Begin by running experiment #4 for 0 min. with 0 min. between printouts.

1) Change the value of the parameter AVFIST to 0.5.

2) Change View Output: Tables to read out the following variables

3) Run for 30 min. with 15 min. between printout.

4) Print out and examine your results.

Run #2 - Short term response

1) <Start over> with a freshly initialized model; pick experiment #4.

2) Change the value of the parameter AVFIST to 0.5.

3) Change Tables to read out the following variables

4) Run for 30 min. with 15 min. between printout.

5) Print out and examine your results.

Run #3 - Long Term response To An A-V Fistula

1) <Start over> with a freshly initialized model; pick experiment #1.

2) Change the value of the parameter AVFIST to 0.2.

3) Change Tables to read out the following variables:

AP, COL , TPR , MFLOL , VENT , PRA

Run for 20 min. with 5 min. between printouts.

4) Change Tables so they read as follows:

AP, COL , TPR , ECFV, PO2A , PRA

Run for 7 days (7D) with 12 hours (12H) between printouts.

5) Continue for 8 more days with 1 day between printouts.

 

Analysis (A-V Fistula)

Characterize the responses and underlying mechanisms in the short & long term runs.

Possible Logical Framework For Analysis

Acute response

Overall: demonstrate that the immediate effect of the fistula is to increase cardiac output via a decease in peripheral resistance with an additional effect of dropping arterial pressure (slightly but significantly).

- demonstrate that tissue/metabolic flow increases but is still not quite adequate.

- demonstrate that early compensation involves the effects of the sympathetics on the heart and venoconstriction.

Long Term Response

Overall: demonstrate that blood volume compensation finally fully brings metabolic flow back to normal.

- characterize the role of the renin-angiotensin system in the later upwards adjustment of cardiac output.

- demonstrate & explain how arterial pressure is brought back to normal by the volume regulation system.

* This exercise was adapted from one presented in the 'HUMAN" Users Manual by Dr. Coleman in conjunction with concepts from the Guyton text cited below.

* For those interested, a very informative analysis of the fistula as an example of heart failure and compensation can be found in Guyton and Hall's "Textbook of Medical Physiology", 10th Edtion, W.B. Saunders Company, 2000.