Respiratory Output

The mechanical output of the respiratory system may be assessed by measuring respiratory muscle activity and resulting gas flow. The electrical activity of the diaphragm, the most important inspiratory muscle, has been measured in adults with needle electrodes inserted into the eighth or ninth intercostal spaces in the midaxillary line^[205] and in neonates with surface electrodes applied subcostally at the nipple line.^[206] Diaphragmatic electromyograms can be obtained in this manner in neonates, but in adults, they are frequently contaminated by the simultaneous electrical activity of the overlying intercostals.

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Esophageal and gastric pressure simultaneously measured may be used to calculate transdiaphragmatic pressure. A nasogastric tube with esophageal and gastric balloons is commercially available (Viasys Healthcare, Palm Springs, CA). Transdiaphragmatic pressure measurement has been used to assess diaphragmatic function^[209] and loss of diaphragmatic contraction demonstrated in the early postoperative period after upper abdominal surgery.^[210] Loss of transdiaphragmatic pressure generation implies lack of diaphragmatic contraction ^[211] ( Fig. 36-21 ).

Diaphragm function can also be assessed by measuring either the compound action potential or transdiaphragmatic twich pressure after phrenic nerve stimulation.^{[212] [213] [214]} Diaphragm motion can be assessed somewhat more indirectly by observing or monitoring thoracic and abdominal movements during inspiration. The normal outward movement of the chest and abdomen during inspiration is replaced by paradoxical inward movement of the abdomen in the presence of diaphragmatic inaction,^[215] which may be caused by phrenic nerve paresis or diaphragmatic fatigue.^{[216] [217]} External movement of the thorax and abdomen can be monitored in this way by using magnetometers, which can monitor anteroposterior and lateral thoracic and abdominal dimensions,^{[215] [218]} or by strain gauge displacement transducers.^[219] Another method uses two coils of wire that encircle the torso at the thorax and abdomen. The self-inductances of the

Figure 36-21 Examples of transdiaphragmatic pressure monitoring. A, Simultaneous esophageal and gastric pressure waveforms during tidal breathing in a normal individual. Negative esophageal (and therefore pleural) pressure swings are accompanied by positive gastric pressure waves, indicating the development of transdiaphragmatic pressure during inspiration. B, The same waveforms in a patient with phrenic nerve palsy (and therefore absent diaphragmatic contraction). Negative intrathoracic pressure swings (arrowheads) are accompanied by gastric pressure swings in the same direction. Intrathoracic pressure changes are directly transmitted through a passive diaphragm. These changes can also be observed in the early postoperative period in patients who have had upper abdominal surgery. (From Brown KA, Hoffstein V, Byrick RJ: Bedside diagnosis of bilateral diaphragmatic paralysis in a ventilator-dependent patient after open-heart surgery. Anesth Analg 64:1208, 1985.)

Monitoring of breath sounds provides a low-cost, highly reliable, semi-quantitative measure of gas flow into and out of the lung. Continuous monitoring of breath sounds using an earpiece connected to a precordial or esophageal stethoscope allows immediate detection of breathing circuit disconnection in mechanically ventilated patients. It facilitates early detection of decreased tidal volume, changes in respiration rate, and endotracheal tube cuff leaks. It also provides qualitative data that may indicate changes in pulmonary mechanics. Wheezes (i.e., rhonchi) are produced when gas flow and airways interact such that airway walls become apposed. Oscillation of the airways between open and nearly closed causes airway vibration, which results in sound production detected as wheezing. Wheezes may occur when airway diameter is narrowed as a result of smooth muscle contraction (i.e., bronchospasm), mucosal thickening or edema, buildup of secretions, extrinsic compression, or an endobronchial mass such as a tumor or foreign body. Although severe generalized obstruction of airways may occur without wheezing, such as in emphysema, the presence of wheezes correlates with reversibility of airways obstruction.^[222] Crackles (i.e., rales) are discontinuous sounds produced by the sudden opening of small airways during inspiration. They indicate airway closure and subsequent reopening, which may occur in interstitial fibrosis or in any situation resulting in premature closure of small airways, such as pneumonia, pulmonary edema, or low lung volumes. In patients with large amounts of airway secretions, particularly in the large airways, coarse crackles may also

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Minute ventilation and tidal volume are easy to measure directly in patients whose tracheas are intubated, and these parameters are routinely measured by modern mechanical ventilators. However, in the patient whose trachea is not intubated, these parameters can be obtained only somewhat inaccurately by monitoring external dimensions of the thorax and abdomen by direct observation or by using one of the techniques described. Subjective assessment is extremely inaccurate,^[226] commonly done with overestimation of tidal volume by up to 60%. Alternatively, V̇E can be monitored directly by using a head tent.^[227] These methods are difficult to apply for routine clinical use. Measurement of the ventilation rate remains the mainstay of respiratory output monitoring. Mechanical measures of gas flow such as these provide combined assessments of respiratory drive and mechanical function of the respiratory system. However, in the presence of relatively normal thoracic mechanics, respiratory gas movement is considered to be a reasonable monitor of respiratory output.