VENNOUS CANNULATION

· 90° bent cannula (single or 2-stage): out of operative site, less affected by heart rotation on venous return

· Wire reinforcement: maintain structural integrity & prevent kinking (large diameter - low wall tension) especially due to low pressures and when inadvertently bent

· Closed ended or caged tip cannula are more difficult to occlude than open ended but have increased resistance to blood flow

· Spiral tip cannulae: anti cavitation device

· ‘Light house’ cavoatrial right atrial baskets and IVC tips have less resistance to siphon venous return compared to ‘bullet’ designs

· Cannula designs with smaller drainage area openings may not collapse until the CVP is lower and that ‘light house’ designs promote lateral vessel collapse compared to ‘bullet’ designs at the same CVP & siphon gradient

· Cavoatrial; one third flow via SVC; two thirds via IVC

· Venous drainage is accomplished by siphonage

1) venous reservoir must be below level of patient

2) lines must be full of fluid

3) If siphonage is too great, negative pressure may be created in collapsed cavae or atrium leading to intermittent collapse of vessel wall around end of cannula, causing ‘chattering’ which can actually decrease venous drainage

i) Various types of baskets & fluted tips at the end of venous cannulas to prevent this occlusion by the vascular walls

· Amount of venous drainage is dependent on:

1) Pressure in central veins

[influenced by intravascular volume and venous compliance]

2) Height between patient and top of blood level

[gravity negative pressure = height differential in cm water]

3) Resistance in venous return tubing (cannula, clamps, connectors)

i) Resistance to flow is directly related to length of tubing

ii) Resistance to flow is inversely related to the fourth power of the radius of the tubing

iii) As 2/3 of the venous return comes from the IVC, the cannula in the IVC should be larger than in the SVC

Peripheral venous drainage

· Via femoral or iliac veins

· To maximise drainage:

1) Use as large cannula as possible

2) Advance cannula tip into atrium

· Indications: reoperations to manage potential bleeding during sternotomy, certain thoracic surgery, emergency closed cardioplumonary assist, tamponade

Alternatives

· Pulmonary artery : to augment inadequate femoral drainage

· Internal jugular : eg ECMO

Complications associated with insertion

· Lacerations & bleeding

· Arrhythmia

· Air embolisation [especially with low atrial press & L-R shunt]

· Malposition of tips [insertion into azygous, innominate, hepatic veins, across ASD]

· Once inserted, venous cannula interfere with venous return [esp with cannula tourniquet]

	Bicaval		Single
	Tourniquet	No Tourniquet	Atrial	Cavoatrial
Atrial incisions	2	2	1	1
Cannulation speed	Slowest	Slow	Fast	Fast
Technical difficulty	Very	Moderate	Easiest	Easy
Right heart exclusion	Complete	Incomplete	No	No
Coronary sinus return	Excluded; a real concern during direct coronary plegia as right heart will fill	Partial	Included	Included
Right heart decompression	None	Fair	Good	Best
Right heart decompression with heart lifted up	Bad	Bad	Bad; very sensitive to position	Good
Caval drainage	Best	Good	Moderate; less good for IVC	Good
Caval drainage with heart lifted up	Good	Good	Bad	IVC OK; SVC bad
Potential rewarming of heart by venous return	No	Yes	Yes	Yes
Myocardial preservation	Best; due to reduced rewarming	Good	Suboptimal	Excellent; due to superior heart decompression
Indication	Entry into right heart	Mitral valve surgery; due to retraction distorts cavoatrial junctions

Sizing

1. To determine appropriate size:

a) Required flow

b) ¹/₃ flow from SVC; ²/₃ flow IVC

c) Siphon gradient 40 cm

d) for: 1.8 m² patient: at least

i) SVC 34 F

ii) IVC 38 F

e) Normal adult minimum 30-40 two-stage

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