Yes, it is important to understand how breathing affects pressure waveforms to obtain the most accurate readings. To do this, we have to revisit the basic mechanics of normal respiration. Let’s bring out an experiment that you should remember from junior high or high school. Hopefully, you recall the bell jar (Figure 1) experiment, used to demonstrate how our lungs work. Simply, this is a large glass dome with some small balloons inside and a large piece of rubber attached to the bottom. The large piece of rubber also has a knob on it that you can hold. In this experiment, we think of the dome as the chest, the small balloons as the lungs and the large piece of rubber as the diaphragm. Most people will think that they suck air in for respirations. While it might seem this way, it is not what happens. Respirations are based upon pressures inside and outside of the chest. When pressures on the inside of the chest are less than the pressures outside of the chest, air moves into the chest. The reverse is also true. When pressures on the inside of the chest are higher than the pressures on the outside of the chest, air is pushed out of the chest. We can simulate the mechanics in the chest with the bell jar. Basically, if we push up on the rubber piece in Figure 2 (diaphragm) we create a positive pressure within the dome (chest) because we have reduced the space that the air in the dome (chest) is occupying, and essentially it is compressed. If there is an opening to the outside environment (trachea and mouth), then the pressures must equalize and air moves out of the balloons (lungs) to the outside environment until the pressures are equalized. An analogy would be a can of soda that has been agitated a little. Pressure builds up in the can, and when you pop open the top, the pressure rushes out of the can until the pressure inside and outside the can are equalized. As we noted, the opposite is true as well. If you pull down on the rubber piece in Figure 2 (diaphragm) you create a negative pressure within the dome (chest) because we have increased the space that the air in the dome (chest) is occupying. If there is the opening to the outside environment (trachea and mouth), then the pressures must equalize and air is pulled into the balloons (lungs) from the outside environment until the pressures are equalized. An analogy would be the jar of pickles sealed with a vacuum. When you turn the lid, you hear a ‘pop’ and air rushes into the jar until the pressures inside and outside are equal. Let's apply this to the chest instead of to a bell jar. If you place your hands on your abdomen during inspiration, you should be able to feel that your diaphragm moves caudal and your chest expands (Figure 4). This creates the negative pressure in the chest that is transmitted to the lungs and allows air to rush in to equalize with the outside. Contracting the diaphragm applies negative intrathoracic pressure to the lungs, causing inspiration. It also applies this negative pressure to the right atrium, causing increased venous return from the abdomen (Figure 6). During expiration (Figure 5), you should be able to feel your diaphragm move cranial, which in turn creates the positive pressure in the lungs allowing air to be expelled. Expiration is passive, because the relaxed diaphragm allows the expanded lungs to empty, just as when you release an expanded balloon, the air is expelled until the pressures both in and out equalize. Like this empty balloon, the lungs are also most relaxed at the end of expiration. Now that we understand how the mechanisms work, how do we know when to record pulmonary artery catheter pressures and why? When we measure pressures, we want the truest and most accurate readings possible. Assuming that equipment and technique are perfect, we want the most reproducible and relaxed pressure possible. Inspiration is always active because the diaphragm contracts, causing negative pressure within the chest and pulling in air. The point between the end of exhalation and the start of inspiration is the most neutral and relaxed time (Figure 6). This is where the outside pressure and the inside pressures are most equal, and this is where we want to obtain our readings. Figure 7 shows this area on an actual recording. We see the slow cycling of pressure levels during expiration as well as inspiration. Remember, during inspiration, there is more negative pressure in the chest, which correlates to the lower pressure readings. Another way to remember this is that when the diaphragm moves DOWN on inspiration, so do the intracardiac pressures. Actually, all the intracardiac pressures cycle with this 5-10 mmHg respiratory variation, because the heart lies within the thorax, just as the lungs do. Ideally, we want to read the pressures at end-expiration, which is the point during exhalation before the pressures dip, because as soon as the patient begins to inspire, the pressures begin to fall. The next logical question is How in the heck do we do this? It’s simple, but it requires a little education of the patient before the procedure. They need to be told how to best relax their breathing when you record pressures. First, try this on yourself.
QUICK HOLD YOUR BREATH!(wait) (wait) Did you take a breath in and hold it? Most patients will do the same as well. Doing so creates artificially high pressures because of the air pressure against a closed epiglottis (we closed the open-to-air passage). The patient then performs a valsalva-type maneuver bad for pressures and cardiac output. The patient needs to be educated to, upon command, let the air out of their lungs passively and then just stop breathing with their glottis still open. This assures that the patient is at the end-expiratory phase as long as possible. Patients can be told to breathe by letting their air out naturally and pausing while you record the pressures, as you will be able to see when they are breathing and when they are at the end-expiratory phase. If you let the patient breathe for 30 seconds between these commands, you will be able to obtain all the pressures of all the chambers within just a few minutes. It must be said that in ventilator-assisted patients, everything we discussed is actually the opposite. That is because air is being forced into the lungs mechanically, instead of through pressure changes in the chest. THAT is a subject for another time. Recording accurate hemodynamics requires you to carefully record and measure cardiac pressures at the right time in the respiratory cycle: end expiration. Acknowledgments: I would like to thank Wes Todd for his contribution to this month’s article.
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