Thursday, August 8, 2013

Arteriovenous fistula - What happens to cardiac output and total peripheral resistance and why?

What is an AV fistula?
An arteriovenous fistula is an abnormal connection or passageway between an artery and a vein.

When an arteriovenous fistula is formed involving a major artery like the abdominal aorta, it can lead to a large decrease in peripheral resistance.

Why?
Making an AV fistula is like connecting a new resistance in parallel to the arterial circuit. A parallel arrangement of vessels greatly reduces resistance to blood flow.

This lowered peripheral resistance causes the heart to increase cardiac output in order to maintain blood flow to all tissues.

What increases the cardiac output?
The heart rate and stroke volume increases to increase cardiac output.
Cardiac output is also increased due to increased venous return to the heart.

Mean blood pressure is constant secondary to increased blood volume caused by renal retention of salt and water.

Physical manifestations:
Relatively normal systolic blood pressure
Decreased diastolic blood pressure (blood rapidly exits the arterial system through the fistula)
Resulting in a wide (large) pulse pressure.

Cool fact:
When you compress the AV fistula, you eliminate the resistance added in parallel.
Total peripheral resistance increases.
We know from physiology that BP = CO x TPR
Therefore, to maintain the blood pressure, heart rate decreases (CO = HR x SV)
This is known as Branham sign - bradycardia following compression or excision of an arteriovenous fistula.

That's all!

Eid Mubarak to everyone =)
Have a fun weekend <3


-IkaN


6 comments:

  1. And this is why AV fistulas can end up as a cause of high output cardiac failure.

    ReplyDelete
    Replies
    1. Exactly!! ^__^ Large arteriovenous fistula or multiple small arteriovenous shunts as in Paget's disease of bone can cause high output cardiac failure.

      High-output HF can occur in any patient with reduced systemic vascular resistance.
      Other causes are chronic and severe anemia, hyperthyroidism, pregnancy, beriberi and acutely in septic shock :)

      Delete
  2. Very nice ..can u please put some discussion on TURBULENCE .thank u

    ReplyDelete
    Replies
    1. Thank you.

      No, I'm afraid I don't have the time, but I'll try :)

      Delete

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