This model can help answer key questions in the field of neonatal resuscitation and birth asphyxia. The main advantage of this model is that we can accurately study the physiological changes during fetal to neonatal transition with our known instrumentation. The implications of this technique extend towards the therapy of birth asphyxia as well as hypoxic ischemic encephalopathy.
Researchers new to this method may struggle because there are many steps designed specifically for this species and our intended instrumentational goal. Visual demonstration of this method is critical as particular care is needed for the placement of the pulmonary and ductal flow probes during the open-chest procedure. To perform the Caesarian section, use cautery to make a 15 to 18-centimeter abdominal skin incision over the linea alba of an anesthetized pregnant ewe to expose the fascia and use blunt-tipped scissors to create and extend the incision.
After locating and exteriorizing the fetal head, hold the head with one hand and use the cautery to make a 10-centimeter incision through the uterine wall over the animal's forehead. Use a 3 1/2 to 4 1/2-millimeter cuffed endotracheal tube to intubate the partially exposed animal and inflate the cuff, then tilt the head to the side to allow the excess fetal lung fluid to drain passively and occlude the endotracheal tube to limit spontaneous breathing. For neck vessel instrumentation, tent the skin and administer subcutaneous analgesia on each side of the neck of the fetal animal approximately six centimeters distal to the head adjacent to the trachea and make a single, three-centimeter skin incision on each side of the neck by cautery.
Isolate the right external jugular vein and right common carotid artery by blunt dissection of the fascia and place two 20-centimeter zero silk sutures one centimeter apart under both vessels. Gently lift each suture to restrict the blood flow and use Iris surgical scissors to make a partial one to two-millimeter transverse cut into the vessel between the sutures. Insert one 15 or 17 gauge catheter into the right carotid artery caudally toward the aortic arch for blood pressure monitoring and blood sampling.
Insert a 14 or 16 gauge pre-flushed catheter seven to 10 centimeters caudally within the jugular vein towards the right atrium for fluid and medication administration. Then, insert a second catheter approximately five centimeters rostrally to collect blood from the cerebral circulation. Next, extend the left incision one to two centimeters into a T shape.
Isolate the carotid artery and place a two-millimeter perivascular flow probe around the vessel to measure the blood flow. Stabilize the probe with a flexible, one-centimeter polymer sleeve and place the flow probe cable line into the T-shaped incision so that the cable runs parallel with the vessel. Then close the skin incision, tying the cable with a loop to ensure appropriate position and minimal risk of displacement.
For great vessel instrumentation, extend the left forelimb behind the neck to extract the fetal lamb, fully exposing the left side of the chest and cover the animal with polyethylene wrap to prevent heat loss. Subcutaneously infiltrate three milliliters of analgesia along the fourth intercostal space and use cautery to make a six-centimeter skin incision into the same region. Use blunt, curved forceps to carefully pierce the intercostal muscle, circling the tips under the third rib and coming out between the next intercostal space.
Open the forceps to grasp a piece of zero silk and pull the suture back through the incision to encircle the rib, then clamp the ties around the rib. Next, gently pull up both silk ties and insert a cotton-tipped applicator into the chest under the fourth intercostal space to protect the chest contents while using cautery and blunt dissection to extend the intercostal space to six to eight centimeters. Place a rib spreader into the chest opening to reveal the left upper lobes of the lung and of the heart covered by the pericardium.
Envelop the lung with a two-by-two inch gauze and gently push any exposed tissue away from the surgical area. Using forceps, lift the pericardium and cut along the main pulmonary artery, taking care not to cut the vagal nerve. Use an applicator stick to keep the right atrium from interfering with the lateral cut of the pericardium along the vagal nerve.
Now that the pericardium is open, it reveals the main pulmonary artery, which splits into the ductus arteriosus and the left and right branches of the pulmonary artery. Effective transition at birth is evaluated by study of the change in flow as blood passes into the lung rather than continuing fetal circulation through the ductus. Locate the ductal pulmonary artery notch and move Gemini forceps carefully around and behind the left pulmonary artery to encircle the artery carefully, avoiding the right pulmonary artery notch and advancing the forceps only when there is no resistance.
When the tip of the forceps is observed on the other side of the vessel, open the tips just far enough to grab one end of a pre-moistened piece of umbilical tape and gently retract the forceps to pull the tape into position. Ties are placed around vessels in preparation for a flow probe insertion. Gently lift the umbilical tape to allow the L-bracket of a four to eight-millimeter perivascular flow probe to be guided around each vessel along the opening established by the tape and redirect the tape toward the back of the probe to help move the flow probe into place.
When the end of the L-bracket is visible, secure the sliding closure and carefully cut one end of the umbilical tape close to the probe, gently pulling the other end of the tape to remove it from the vessel. Instrumentation complete, measurement of these flows following birth gives insight to lung function and effective transition to air breathing. Use a tapered needle and a 2-0 synthetic monofilament suture to close the chest in layers and a cutting needle and 2-0 silk suture to secure the cables of the flow probes to the outer skin with a loop.
To initiate the experimental protocol, check that the endotracheal tube is occluded and tie and cut the umbilical cord. Move the lamb to a radiant warmer and insert umbilical venous and arterial catheters. After asphyxial arrest, initiate the neonatal resuscitation program guidelines with 30 seconds of ventilation followed by three-to-one chest compressions and epinephrine administration every three minutes.
Following instrumentation, hemodynamic variables can be recorded, analyzed, and interpreted. Frequent blood samples can be collected to allow, for example, the assessment of the pH and the partial pressure of oxygen of the samples at different time points throughout the resuscitation procedure. Occasionally the catheters or flow probes may malfunction or get dislodged, preventing data collection and analysis.
Once mastered, instrumentation can be completed in two hours. It's also important to work with a trained team with assigned roles to ensure that all tasks are completed consistently. The animal in this model has a transitioning circulation and fluid-filled lungs simulating human newborns following delivery and is therefore an excellent model for studying newborn physiology.
This perinatal lamb model resembles a depressed newborn baby in the delivery room. Studies on this model will optimize resuscitation of asphyxiated neonates worldwide.
