Calling for a Specific Placement Site for Thoracic Effort Belt

Respiratory Inductance Plethysmography (RIP) effort belts are similar to piezo belts in that they help to monitor breathing patterns in patients with respiratory disorders. However, unlike piezo belts that can be placed anywhere on the thorax, RIP belts—which offer greater accuracy and show true movements of breathing—must be specifically placed. The key to successful use of RIP belts and retrieving the accurate data is, therefore, dependent on correct positioning and a basic understanding of anatomy.

The chest cavity is essentially a sealed container with a small constant negative pressure that holds the lungs expanded against the chest wall, gliding on the parenchyma as they are inflated and deflated by normal ventilation. At the bottom, the chest cavity is sealed off from the abdominal cavity by the diaphragm. The diaphragm, which moves down, during inspiration pushes the abdomen out to create a negative pressure in the chest causing air to flow into the lungs.

"The direct rib cage contribution to lung volume change is much less, and that the diaphragmatic contribution is much more than was previously thought."¹

"The first to the sixth ribs are connected with one another by the intercostal muscles, whose fibers run downward and forward. Since the first rib is fixed by the scalene muscles, contraction of the intercostal muscles results in an upward and forward movement of the remaining five ribs. There is very little lateral movement of the first four ribs, which overlie the upper lobes of the lungs, and this portion of the chest cage increases in size primarily in an anteroposterior direction.

"The fifth and sixth ribs, which are situated approximately over the middle lobe of the right lung and the lingular segment of the left lung, differ from the upper four ribs in having a greater radius of curvature. Because of this, inspiratory elevation of these two ribs increases both the anteroposterior and the transverse diameter of that portion of the thoracic cage."²

Finally, excerpts from Clinical Application of Respiratory Care, Barry A. Shapiro, Ronald A. Harrison, Carole A. Trout offer:

"Contraction of the muscle fibers causes the domes of the diaphragm to be pulled down, thereby increasing the volume of the thoracic cavity.
"The diaphragm is the major muscle of ventilation."³

These sources confirm that the chest cage does not press outward as much as it is pressed down by the movement of the diaphragm. This increases the negative pressure in the chest causing an inflow of air. Furthermore, the degree to which the abdomen is displaced by this downward movement is greater than the increase in the circumference of the chest. The signals from circumferential change will always be greater in the abdomen than the chest. Based on this movement, the proper place to position thoracic measuring belts would be at, or below, the fourth intercostal space because the upper chest is restrained anatomically from a great degree of excursion.

Piezo belts can be placed anywhere on the thorax and by being pulled tightly can produce a large signal. Piezo belts produce signals of respiratory effort but they do not reflect an accurate signal of the depth of ventilatory movement. The tension placed on the belt when applied affects the output of the crystal element. Therefore, the tighter the piezo element is, at a preset sensitivity, the larger the signal it produces. Whether it is moved, or merely tightened in place, the output can be adjusted by changing the tension of the belt.

RIP effort belts employ the laws of electronics to provide breathing-pattern information on a patient. A frequency generated and put into the wire of these belts is changed by changing inductance of the movement of breathing underneath the belt. The results are not impacted by the belt's tension, but rather its placement. In fact, tightening the RIP belt will actually degrade the signal because the movement in the restricted area beneath the belt can not be measured. The output of the RIP belt is linear, showing a true relationship breath-to-breath, as there are increases or decreases in the efforts of breathing. Because the RIP belt is accurate and shows true movements of ventilation, it is important that these belts be placed where there is maximal movement of thoracic effort.

As previously noted, the upper half of the rib cage has small movements during ventilation with the maximal excursion of the rib cage being below the fourth or fifth intercostal space. Placing the RIP effort belt above this point will produce small breathing signals because there is very little movement in the chest in this area. In the past, it was common practice to place thoracic effort belts along the nipple line; however, this point varies on each individual thereby limiting consistency in data collection. For this reason, it is now suggested to measure and place belts using intercostal spaces. By using these anatomical reference points, it becomes easier for technologists to place belts accurately to achieve maximum signal capture.

Positioning the thoracic effort belt just below the pectoral muscle of the chest places it at the lower margin of the rib cage where there is maximal movement. This position also helps to minimize effort belt movement on the subject. The pectoral muscles will help keep the belt in place—preventing its movement from the chest to the armpit. Similarly, the abdomen should keep the belt from sliding down over the subject's stomach. This position will keep the belt above or on the xyphoid process, and should be a sufficient distance from the abdominal belt which is placed near the umbilicus. (Note: sensitivity will be required in working with women as the belt will be placed underneath the breasts.)

Figure 1 illustrates how the belt should be placed below the pectoral muscle and either above or on the xyphoid process. The belt is over the part of the chest that has more movement which results in a larger signal than if the belt were placed as in Figure 2. Placement in Figure 2, completely above the fourth intercostal space, will result in minimal chest movement involved with breathing. Using the RIP effort belt in this area will produce smaller effort signals than previously elicited from a piezo belt. Again, this is because RIP reflects the actual movement variances of the area under the belt—the cross section of the body at that point.

It is important to recognize that each person's anatomy is different. Some people have well developed musculature in certain places; others do not. All of the figures presented here are meant to be generalizations and should be used as guidelines designed to illustrate the reasoning for this belt-placement preference. In order to produce a large signal that is more representative of actual respiratory effort, the chest effort belt must be placed where the largest change in circumference occurs.

Understanding how the chest moves during breathing, and RIP technology, should help to explain the importance of RIP belt positioning and the value of this technology. RIP is different from piezo technology but yields a greater degree of accuracy. It is intended to help technologists record a study with appropriate signals and support treatment plans for subjects.

Rick Swanson, RPSGT, CRTT
Pro-Tech, a Philips Respironics Company
Mukilteo, WA

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