Medicine
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Lavage-induced Surfactant Depletion in Pigs As a Model of the Acute Respiratory Distress Syndrome (ARDS)
Chapters
Summary September 7th, 2016
Repeated pulmonary lavages in anesthetized pigs induce lung injury resembling major aspects of human acute respiratory distress syndrome (ARDS). For this purpose the lungs are repeatedly lavaged with 0.9% saline at 37 °C. The goal of the protocol is a reproducible mitigation of gas exchange and hemodynamics for research in ARDS.
Transcript
The goal of this method is to create a model of acute respiratory failure using a pulmonary surfactant washout to induce lung atelectasis, hypoxemia, and pulmonary hypertension, important pathophysiological aspects of human acute respiratory distress syndrome, or ARDS. Hi, I'm Doctor Pickerodt from the Charite in Berlin, and this is my colleague Doctor Russ, and today we're going to show you how we've been working in the ARDS model introduced into science by professor Lachmann back in the 70s to induce severe lung injury by surfactant washout with warm saline and pigs. This model is great because it reassembles much of the respiratory mechanic disturbances and thermodynamic changes that occur in human ARDS as well.
In addition, the main focus for us is to reduce the number of animals needed, implement this model in your lab. After confirming the correct placement of the endotracheal tube into a fully anesthetized adult male pig, start the mechanical ventilation. Adjust the settings to target an end expiratory partial pressure of carbon dioxide of 35 to 40 millimeters of mercury, and a peripheral oxygen saturation above 95%Observe the brachiocephalic and the sternocephalic muscles.
After separating the fascia between the muscles by blunt dissection to expose the external jugular vein, puncture the vein with a needle and advance the needle until venous blood can be aspirated. Advance the guide wire through the needle about 15 centimeters into the vein. And remove the needle.
Then completely advance the introducer sheath for the pulmonary artery catheter. Now remove the guide wire and the dilator from the sheath and aspirate the venous blood to confirm the correct placement of the catheter. Repeat the same procedure to insert a central venous line if needed, being sure to control catheter placement by blood aspiration.
Enclose the wound with sutures. Then place an arterial catheter into the femoral artery using the same technique. To start the monitoring, connect the arterial catheter and central venous line to the transducer system and confirm that the transducer systems are calibrated against the atmosphere, and at 50 or 200 millimeters of mercury for the central venous or arterial lines respectively.
Then place the pressure transducers level with the right atrium. Inflate the balloon, check for possible damage, and deflate before inserting the catheter. Then proceed to connect the catheter to the pressure transducer system after calibrating the transducer system.
And insert the catheter 10 to 15 centimeters into the introducer sheath depending on the length of the sheath. When the balloon has been pushed beyond the sheath, inflate the balloon and advance the catheter further, observing the pressure and typical wave forms. Continue to advance the PAC and identify the typical pressure wave forms as the catheter reaches the right atrium, the right ventricle, and the pulmonary artery.
Stop when the pulmonary capillary wedge pressure curve appears. Then record the pulmonary capillary wedge pressure at the end expiration and deflate the balloon. To measure the cardiac output using the thermodilution technique, first connect the thermistor and the appropriate flow-through housing to the central venous lumen of the pulmonary artery catheter, and connect the distal temperature port of the catheter to the monitor.
Start the measurement and inject five milliliters of four degree celsius normal saline as quickly as possible through the flow-through housing. When the measurement is complete, the corresponding temperature curve will appear on the monitor. After recording the cardiac output value from the monitor, measure the cardiac output five times in a randomized order over the respiratory cycle.
To induce lung injury by lung lavage, first adjust the ventilator settings, then prepare a funnel and disconnect the animal from the ventilator. Afterwards, connect the funnel containing warm saline to the endotracheal tube. Raise the funnel about one meter above the animal and let about 50 milliliters per kilogram of the saline enter the lungs.
When the mean arterial pressure falls below 50 millimeters of mercury or all of the fluid has entered the lungs, lower the funnel to ground level to drain the lavage fluid, and reconnect the animal to the ventilator. Allow no more than five minutes of hemodynamic recovery before repeating the lavage. Repeat the lavages until the partial pressure arterial oxygen to fraction of inspired oxygen ratio is consistently below 100 millimeters of mercury for at least 60 minutes, with the fraction of inspired oxygen at 1.0, and a positive end expiratory pressure of greater than 5 centimeters of water.
Adjust the respiratory rate to keep the arterial pH above 7.25 as necessary. After the induction of the lung injury as demonstrated, the changes in blood gases and hemodynamics may remain stable for hours, deteriorate, or even improve depending on the ventilator settings. The pulmonary artery pressure increases two to threefold after lung lavages due to atelectasis and hypoxic pulmonary vasoconstriction.
Of course, this model doesn't mimic all the features of human ARDS, yet if you standardize your lavages, you will get reproducible results that will allow you to compare your results with the results from other labs. Again, this is a surfactant washout model, so you will induce atelectasis. Therefore, this model is mainly usable to test ventilatory strategies in severely lung-injured animals.
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