Pulmonary Mechanics

Fritz J. Baumgartner

The elastic properties of the lung and chest wall and air flow resistance is related to breathing mechanics. Figure 23.1 shows the lung volumes used in respiratory physiology. Vital capacity is the maximum volume that can be expired after a maximal inspiration. Total lung capacity is the volume of the lungs after a maximal inspiration. Residual volume is that volume remaining in the lungs after a maximal expiration. This is about 25% of the total lung capacity. Functional residual volume is the volume in the lungs at the end of a normal expiration. Tidal volume is the volume of a spontaneous breath. These are the major volumes; the inspiratory capacity, expiratory reserve volume, and inspiratory reserve volume are less commonly used terms.

The pressure volume curve defines the elastic properties of the lung. Figure 23.2 shows such a curve. It shows that with fibrosis, more intrapleural pressure is required to establish lung volume whereas with emphysema much less intrapleural pressure is required to inflate the lung. The elastic properties of the lung reside in the alveolar walls, which contain a thin film of surfactant, resulting in surface tension accounting for lung elasticity. The surface tension increases as the size of the alveolus decreases, and alveolar collapse would result if it were not for this surface tension. Thus, surfactant is extremely important in preventing atelectasis.

Certain factors restrict movement of the chest wall and reduce compliance. This includes diaphragmatic compression caused by abdominal distention such as with obesity, ascites, pregnancy or intra-abdominal tumors. These conditions reduce vital capacity, total lung capacity and functional residual capacity. The loss of elastic properties of the lung itself, such as with emphysema or fibrosis, also can effect the elastic properties of the lung as shown in the pressure volume curve.

The FEV-1 is the forced expiratory volume in one second. This is the commonest flow measurement. The FEV-1 is often reported as a percentage of the vital capacity (FEV-1/VC) as well as the natural volume. It is important to note the value both ways. If the vital capacity is significantly reduced, the ratio of FEV-1/VC may be satisfactory even though the actual volume exhaled is abnormal. The generally accepted minimal postoperative FEV-1 value is 0.8 liters. It can be predicted that postoperative FEV-1 of less than this will result in a pulmonary cripple, although this statement needs to be modified under different circumstances depending on the individual. For example, an elderly woman may require less of an FEV-1 than a young male athlete.

Another commonly used test is the maximal voluntary ventilation (MVV) which is effort dependent and performed by having the patient inhale as deeply and rapidly as possible for about 10 seconds. The MVV usually correlates well

with FEV-1 (FEV-1 x 34 will generally give the MVV). If the actual MVV does not correlate with the calculated value, it suggests poor effort or a tired patient.

Figure 23.3 shows another important test of pulmonary function. This is the flow volume loop. The pressure volume and flow relationship of lung airway obstruction can be determined by measuring maximal expiratory flow since maximal inspiratory flow is relatively independent of effort and primarily dependent on recoil pressure in the lung, which in turn is dependent on lung volume. The measured flow can be related to the lung volume at which it is measured. The classic example is the flow volume loop that results from an obstruction, for example a tracheal tumor or other intraluminal airway obstruction.

Fig 23.1. Lung volumes.

Fig 23.2. Lung pressure-volume curve.

Fig 23.1. Lung volumes.

Fig 23.2. Lung pressure-volume curve.

Fig 23.3. Lung flow volume loop.

Why is pulmonary function worse postoperatively resulting in atelectasis? There are several reasons. 1) Pain causing hypoventilation can result in a poor respiratory effort and can lead to hypoventilation. 2) Decreased lung volumes including decreased TLC, decreased FRC and decreased end respiratory volume. 3) Retained secretions. 4) Increased closing volumes.

A closing volume is that volume required to keep the alveoli open and nonatelectatic. Postoperatively, the closing volume may exceed the expiratory reserve volume leading to atelectasis.

Suggested Reading

1. Cugell DW, Fish JE, Glossroth J. Mechanics of Breathing. In: Shields TW ed. General Thoracic Surgery 3rd ed. Philadelphia: Lea and Febinger, 1989:110-118.

2. West JB. Respiratory Physiology: The Essentials, 2nd ed, Baltimore: Williams and Wilkins, 1979.

3. Rahn H et al. The pressure-volume diagram of the thorax and lung. Am J Physiol 1946, 146:161.

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