The overall goal of this procedure is to assess the pulmonary vascular response to exercise using multiple fraction of inspired oxygen, or FiO2, diffusion capacities and contrast-enhanced echocardiography. This method helps us better understand how the pulmonary circulation responds to the stress of exercise. The main advantage of this technique is that it can be applied to a variety of experimental, clinical, and pharmacological conditions, including during exercise in participants with heart or lung disease.
Demonstrating the breath hold technique will be Wade Michaelchuk and Sophie Collins, graduate students in my lab. Performing the contrast echocardiography technique will be Doctor Sean van Diepen, cardiologist. Before beginning the procedure, use an air oxygen blender system to fill one 60-liter non-diffusing Douglas bag with 40%oxygen, and one with 60%oxygen.
Attach two large bore three-way stopcock valves for modulation of the inhaled gases, and use flexible non-compressing tubing to connect the Douglas bags to the valve system. Then, connect the valve system to a two-way T-shaped non-rebreathing valve connected to the test gas intake assembly of the mass flow sensor. Switch the pre-breathe valve and the carbon monoxide gas selector to the appropriate orientation for the selected FiO2, and instruct the subject to begin cycling at the intensity of interest while maintaining a heart rate of between plus or minus three beats per minute.
Once the steady state has been achieved, affix the nose clips and mouthpiece onto the subject, and have the subject breathe normally for five breaths from the 40%oxygen Douglas bag. After the fifth breath, instruct the subject to inhale to the total lung capacity, and then exhale to the residual volume. When the lung volume begins to plateau, press F1 on the valve system keyboard to switch to the carbon monoxide gas, and instruct the subject to continue exhaling for three seconds, followed by inhalation of the carbon monoxide gas to the total lung capacity.
Take a big breath in, blow it all the way out, keep pushing, three, two, one, and big breath in, up, up, up, up, up, up, up, six, five, four-Count down from six seconds. Three, two, one, blow it out-And instruct the subject to exhale again to the residual volume. Good, breathe normally.
Monitoring the methane tracing during the exhalation to ensure that the slope is horizontal. The success of this step relies on the skill of the tester and the ability of the subject to follow commands and remain calm during the breath hold. We suggest that you rehearse the timing of the breath holds with the subject before attempting this procedure.
Now, remove the mouthpiece, and have the subject cycle for at least two more minutes to allow the residual carbon monoxide to be washed out. Then, repeat the breath hold maneuvers for the remaining FiO2s. To ensure a high-quality maneuver, confirm that the alveolar volume is within 5%of the previous trials, and that the breath hold time is approximately six seconds.
For intrapulmonary arteriovenous anastomosis, or IPAVA, recruitment, first insert a 20-gage intravenous catheter into one of the subject's antecubital veins. Then, use a six-inch piece of extension tubing to attach the catheter to a three-way stopcock, and connect two 10-milliliter syringes to the stopcock. To create the intravenous contrast, combine 10 milliliters of sterile saline with zero point five milliliters of air, and rapidly agitate the two syringes back and forth through the stopcock until the bubbles are finally suspended.
Instruct the subject to begin the cycling exercise at 100 watts of intensity. Once the steady state has been reached, have an experienced cardiologist obtain a standard apical four chamber view of the heart. Continue agitating the saline until the cardiologist is ready to record.
Then, inject the contrast while the cardiologist maintains the four chamber view, and record 15 cardiac cycles following the detection of contrast in the right ventricle. The quality of the ultrasound image depends on the technical ability of the sonographer to maintain an acceptable image of all four chambers of the heart during exercise. Then, have an echocardiographer who is blinded to experimental conditions interpret the echocardiograms for the presence of IPAVAs.
The capillary blood volume and diffusing capacity increase as the exercise intensity increases, consistent with the need for an expanded diffusion capacity for a greater oxygen consumption. Indeed, in this representative experiment, the echocardiographic data demonstrate no IPAVA recruitment at rest, but with an IPAVA score that increases as the exercise intensity increases. In this recording of a subject's heart at rest, the contrast agent can clearly be visualized entering the right ventricle.
Even after five cardiac cycles, no contrast can be observed in the left ventricle, giving this participant a resting IPAVA score of zero. Here, representative tracings of four chamber contrast echocardiographs are shown. Note that as the exercise intensity increases, the intrapulmonary arteriovenous anastomosis score increases from zero at rest to three at the highest exercise intensity.
Once mastered, this technique can be completed within two hours. While attempting this procedure, it's important to remember to coach the volunteer in the proper breathing technique and timing. Following this procedure, other interventions, such as a drug infusion, can be used to answer questions about the pulmonary circulation.
After its development, this technique paved the way for researchers in the field of pulmonary physiology, to explore the determinants of pulmonary diffusing capacity in humans. After watching this video, you should have a good understanding of how to assess the pulmonary vasculature at rest and during exercise. Don't forget that working with carbon monoxide can be extremely hazardous, and that precautions, such as proper room ventilation, should always be taken when performing this procedure.
