ARTERIAL CANNULA

 

·        French designation: measure of the outside diameter at the tip of the cannula

·        Performance index = pressure gradient versus external diameter for any given flow

·        The narrowest portion of the catheter that enters the aorta should be as short as possible, thereafter the cannulae size should enlarge to reduce the gradient

·        Long catheters with a uniform narrow diameter are associated with increased resistance to flow, but may have reduced forward peak velocity of flow and subsequent turbulence in aorta

·        Arterial cannula is usually the narrowest part of the circuit

·        Limit to the size of the arterial cannula is the size of the vessel intubated

·        The use of metal or hard plastic provides the best ID to OD ratio

·        High flows through narrow cannula may lead to high pressure gradients, high velocity of flow (jets), turbulence and cavitation

·        Pressure gradients > 100 mmHg are associated with excessive hemolysis and protein denaturation

·        Jets produced may damage aortic wall, dislodge atheroemboli, cause dissections, and disturb flow into neighbouring arteries [Coanda effect]

·        Right angle configurations may be associated with more haemolysis than straight cannula

·      Excessive pressures increase tendency for tubing to rupture or separate

·      Use of cannula with side holes may reduced pressure gradients, reduce jetting and turbulence

·      Flanges aid in fixation and prevent too deep insertion into aorta; use of a short aortic cannula with a flange may prevent insertion into brachiocephalic arteries

·      Line on cannula indicates direction of beak

·      Beak is directed towards middle of transverse arch to avoid entering arch vessels

·      Straight cannula: designed to sit in the middle of the lumen of the aorta

·      Right angle near tip: designed to point towards aortic arch

·      Long cannula: designed to be fed some distance around the aortic arch

·        Excessive pressures increase tendency for tubing to rupture or separate

·        The aortic pressure upon insertion of cannula is important:

1) Too High - chance of dissection, bleeding, trauma, tears

2) Too low - chance of damaging back wall of aorta, harder to insert cannula

·      Vitally important that patient is heparinised prior to arterial or venous cannulation

·      A pulsatile pressure in the aortic line pressure monitor of at least radial pressure may indicate correct placement of cannula

            Aorta

·      Preferred site for blood return; access is easy, cannulation safe and the incidence of complications low

·      Manipulation of aorta during insertion may dislodge embolic material

·      Beak of cannula inserted pointing towards middle of transverse arch to avoid entering arch vessels

·      On Redo CABG due to location of earlier proximal grafts, have limited space to insert a ascending aortic cannula; therefore may use a descending aortic cannula in the distal ascending aorta but do not insert into descending aorta [is easier to manipulate distal end for surgeon]

 

            Subclavian, Axillary, Descending aorta

·      May provide alternates when unable to cannulate ascending aorta or femoral artery

·      Subclavian artery has potential for cerebral or upper extremity ischaemia

·      Subclavian used during aortic arch surgery to selectively perfuse top of body

·      Descending aorta cannula often is very positional; used when unable to clamp ascending aorta; an alternate to femoral yet still provides antegrade flow

 

 

Ascending Aorta/arch

Femoral/iliac

Accessibility

Easy

Hard

Additional incision

No

Yes

Cannula size

usually unlimited

Limited; therefor increased complications associated with jets and high press

Obstruct ascending aorta

Possible

No

Potential of malposition of arch vessels

Yes

No

Perfusion direction

Antegrade

Retrograde; danger of: excessive afterload on a maginally performing heart; beating heart perfusing brachiocephalic arteries with hypoxic blood [watershed effect]

Leg ischaemia

No

Retrograde perfusion cannula totally occludes artery;

complication associated if > 6 hours CPB

Aortic dissection incidence

< 0.2%

[ascending aorta]

< 3%; caused by direct (cannula insertion into sub intima) or indirect (jet) trauma; esp with atherosclerosis or mediodegeneration

Leg wound & artery complications

0

4%

Indications

Most cases

When aortic cannulation not feasible or desirable, bleeding complications under reentry

Contraindications

Ascending aorta aneurysms, dissections, severe atheroma, diseased aorta

When aortic cannulation feasible; occlusive dx of vessels

Other complications

Inability to insert (plaques, fibrosis, too low a pressure), intramural placement, dislodgment of atheroma, malposition of tip (eg into arch vessel), tip against vessel wall (high line press)

 

 

Sizing:

1.                  Size is determined by choosing the smallest cannula that can provide a calculated flow of less than 100 mm Hg — pressure gradients are provided by the manufacturer [press-drop charts come in packaging]

2.                  Normal adult size

a)                  ascending aorta 22-24 g

b)                 femoral artery 18 -22 g

 

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