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Continuous Chronic CO SP #80 |
Chronic CO in the Pulmonary Artery in Sheep (6/00) |
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Courtesy of J.M. Power BVSc, PhD,
Baker Medical Research Institute, Australia
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TS420 Perivascular Module
PAX-Series Probes
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Introduction
We have used Transonic 20A CO
flowprobes for four years and have had outstanding success with them
for implant durations
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up to 15 months. The probes have
coped well with multiple reuse and the only failures that have occurred
have been associated with mechanical damage, either during removal at
PM or animal inflicted. Because the insertion of the aorta is quite
deep in sheep and the available length before division short, we have
always utilized the pulmonary artery (PA) to implant a Transonic CO
flowprobe.
Methods
In our studies we use adult
Merino sheep, 40 to 50 kg BW. The animals are seated vertically on the
floor resting back against the holders knees and a 18g temporary
indwelling catheter is placed in a foreleg bracheal vein. Propofol
(administered as rapidly as possible via the catheter, 20 ml, or 200mg
is usually sufficient for an adult animal) is used for induction and
intubation. Anaesthesia is maintained and the animals positively
ventilated with halothane and oxygen. A large area of the left thorax
is clipped and prepared as a sterile field, extending from the middle
of the back line to the sternum and from the point of the elbow
anteriorly caudal almost to the rib line.
A skin incision is made above
the third left intercostal space and lies approximately in the middle
of the flat area of the thoracic wall immediately behind the point of
the elbow. The length of the incision depends on the skill of the
operator but it should be remembered that the PA lies in the dorsal
third of the thorax. The muscles of the chest wall are dissected (blunt
and with electrosurgical gear) to expose the relevant intercostal
space. The third intercostal space is the best choice in Merino sheep
but in more stocky chested animals the fourth often gives better
access. Check the spaces by located the first rib with the flat of the
hand.
A 5mm midline incision is made
through the dorsal third of the intercostal muscle using a combination
of scissors and blunt dissection. A longer incision will probably be
needed initially until the operator is familiar with the anatomy and
the position of the PA. Some sort of pediatric rib spreader will be
needed to give good exposure. A 4 cm incision is made in the
pericardium above the PA. Dissecting behind the PA to allow access for
the flow probe shell is probably the most difficult part of the
implantation and potentially the most dangerous. Locating the plane of
dissection is best done with a gloved finger. We sometimes use right
angle forceps to extend the dissection once we have opened a track
behind the vessel but mostly we utilize a "finger dissection."
The next task is to place the
flowprobe around the vessel. The probe usually has to be grasped gently
in a pair of straight or right angle forceps to maneuver it around the
vessel. It is important not to damage the probe shell during this
procedure or to attempt to force it through an inadequate track through
the tissue as this may lead to rupture of the vessel and uncontrollable
haemorrhage or crimping of the PA and obstruction of blood flow. Once
the probe is around the vessel the "gate" has to be inserted across the
jaws of the probe. This is most easily done with the gate grasped in a
large pair of straight forceps. Sterile bone wax is rubbed on the
mating surfaces of both the gate and the jaws to lock it in position.
It does not seem to matter which
way the probe lies as long as it does not pinch the PA. For the same
reason we don't close the incision in the pericardium. With very long
term implants the probe body may adhere to the pleura but that does not
seem to cause any problems. The wound is then closed layer to layer
with 0 Dexon or equivalent until the subcutaneous fat/fascia layer is
left. A coarse purse string suture is placed around the exit of the
flowprobe lead so that, when closed, the suture does not make contact
wit the surface of the probe lead. We do not use a chest drain but
merely slightly over inflate the lungs several times and hold at peak
inflation when the purse string is pulled tight.
It is important that the plug
and lead are not tunnelled immediately below the skin but slightly
deeper beneath the fat/fascia layer. Otherwise the lead can erode
through the skin over time. We usually tunnel in two stages almost to
the dorsal midline. A 20 cm loop of plastic coated bell wire is
inserted using a large needle through a fold of skin immediately above
the final lead exit so that approximately 8 cm of the wire is beneath
the skin in an anterior/posterior direction. A small leather coin purse
with a belt loop is held in position by the looped and knotted wire.
The probe lead plug is inserted into the purse through a hole in the
purse back and the hole is closed with a suture. A coarse purse string
(0 silk) is placed around the probe lead at the exit and sutured to the
skin purse string. It is important that the lead exit point is behind
the back of the purse to minimize the exposed length and the
opportunity for the animal to catch the lead with its toe when
scratching. Sheep have surprisingly mobile hind feet! The remainder of
the fascia and the skin is closed over the wound in the thorax.
Antibiotic (1gm ampicillin and
80mg gentamicin sulphate, both i/v) are given prophylactically
immediately after surgery and for three days following. Flunixin
meglumine, 50mg i/m, is given prior to surgery and post-operatively
once daily for two days as an analgesic and anti-inflammatory agent.
Reference
Power, J.M., Raman, J., Dornom,
A., Farish, S.J., Burrell, L.M., Tonkin, A.M., Buxton, B., Alferness,
C.A., "Passive Ventricular Constraint Amends the Course of Heart
Failure: A Study in an Ovine Model of Dilated Cardiomyopathy,"
Cardiovascular Research, Vol. 44, No. 3, p. 549-55, 1999.
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