PHYSICAL ASSESSMENT OF BREATHING ABILITY IN THE ACUTELY III PATIENT
Tests of the patient’s breathing ability are easily performed at the bedside by measuring the respiratory rate (see the previous section “Breathing Patterns and Respiratory Rates”), tidal volume, minute ventilation, vital capacity, inspiratory force, and compliance. These tests are particularly important for patients at risk for developing pulmonary complications, including those who have undergone chest or abdominal surgery, have had prolonged anesthesia, have preexisting pulmonary disease, or are elderly. These tests are also used routinely for mechanically ventilated patients.
Patients whose chest expansion is limited by external restric-tions such as obesity or abdominal distention and who cannot breathe deeply because of postoperative pain or sedation will inhale and exhale a low volume of air (referred to as low tidal volumes). Prolonged hypoventilation at low tidal volumes can produce alve-olar collapse or atelectasis. The amount of air remaining in the lungs after a normal expiration (functional residual capacity) falls, the ability of the lungs to expand (compliance) is reduced, and the patient must breathe faster to maintain the same degree of tis-sue oxygenation. These events can be exaggerated in patients who have preexisting pulmonary diseases and in elderly patients whose airways are less compliant, because the small airways may collapse during expiration.
The volume of each breath is referred to as the tidal volume (see Table 21-1 to review lung capacities and volumes). A spirometer is an instrument that can be used at the bedside to measure volumes. If the patient is breathing through an endotracheal tube or tra-cheostomy, the spirometer is directly attached to it and the exhaled volume is obtained from the reading on the gauge. In other pa-tients, the spirometer is attached to a facemask or a mouthpiece po-sitioned so that it is airtight, and the exhaled volume is measured.
The tidal volume may vary from breath to breath. To make the measurement reliable, it is important to measure the volumes of several breaths and to note the range of tidal volumes, together with the average tidal volume.
Respiratory rates and tidal volume alone are unreliable indicators of adequate ventilation because both can vary widely from breath to breath. Together, however, the tidal volume and respiratory rate are important because the minute ventilation, which is use-ful in detecting respiratory failure, can be determined from them. Minute ventilation is the volume of air expired per minute. It is equal to the product of the tidal volume and the respiratory rate or frequency. In practice, the minute ventilation is not calculated but is measured directly using a spirometer.
Minute ventilation may be decreased by a variety of condi-tions that result in hypoventilation. When the minute ventilation falls, alveolar ventilation in the lungs also decreases, and the PaCO2 increases. Risk factors for hypoventilation are listed in Chart 21-8.
Vital capacity is measured by having the patient take in a maximal breath and exhale fully through a spirometer. The normal value depends on the patient’s age, gender, body build, and weight.
When the vital capacity is exhaled at a maximal flow rate, the forced vital capacity is measured. Most patients can exhale at least 80% of their vital capacity in 1 second (forced expiratory volume in 1 second, or FEV1) and almost all of it in 3 seconds (FEV3 ). A reduction in FEV1 suggests abnormal pulmonary air flow. If the patient’s FEV1 and forced vital capacity are proportionately reduced, maximal lung expansion is restricted in some way. If the reduction in FEV1 greatly exceeds the reduction in forced vital capacity, the patient may have some degree of airway obstruction.
Inspiratory force evaluates the effort the patient is making during inspiration. It does not require patient cooperation and thus is useful in the unconscious patient. The equipment needed for this measurement includes a manometer that measures negative pres-sure and adapters that are connected to an anesthesia mask or a cuffed endotracheal tube. The manometer is attached and the air-way is completely occluded for 10 to 20 seconds while the inspi-ratory efforts of the patient are registered on the manometer. The normal inspiratory pressure is about 100 cm H2O. If the negative pressure registered after 15 seconds of occluding the airway is less than about 25 cm H2O, mechanical ventilation is usually re-quired because the patient lacks sufficient muscle strength for deep breathing or effective coughing.