Chapter 8:	Pulmonary Function Testing
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A patient’s pulmonary function status can deteriorate without exhibiting any changes in symptoms. For example, asthmatic patients recovering from an acute attack can be asymptomatic for some time prior to flow rates returning to normal. The spirometry of PFT can determine the patient’s vital capacity, with expiratory flow rates being useful in identifying airway obstructions.

Ventilatory Status and Parameters

It is important that the attending RCP be aware of their patient’s ability to breathe spontaneously and without any ventilatory assistance. Patients need to be able to increase the depth and rate of their breathing without becoming fatigued. Patients without an adequate ventilatory reserve to deal with normal daily stressors are at risk for developing life-threatening respiratory distress.

The RCP can evaluate this reserve by evaluating the data obtained from performing various bedside assessment procedures, including:

• Tidal Volume (V_T): By having a patient breath normally through a respirometer, the RCP can obtain the patient’s tidal volume. The V_T is obtained by averaging the expired volume over the period of a minute. It can be determined as a calculation of minute ventilation, and normally should be at least 5-7 ml/kg of the patient’s body weight to be sufficient to maintain adequate ventilation without assistance.

• Minute Ventilation (V_E): This can be measured by counting the patient’s respiratory while they exhale when breathing normally through a spirometer for one minute. The total amount of exhaled gas in a minute equals the patient’s minute ventilation. Total average tidal volume is then calculated by dividing V_E by the respiratory rate. 5-10 l/min is considered a normal.

• Forced Vital Capacity (FVC): To ascertain ventilatory reserve under stress (forced vital capacity) have the patient inspire maximally and then exhale as fast and completely as possible through the spirometer. If the FCV is less than 10-15 ml/kg body weight, the patient may need ventilatory assistance.

• Maximal Inspiratory Force (MIF): Patients need to have an acceptable level of respiratory muscle strength in order to maintain an adequate vital capacity. MIF can be measured by using a pressure manometer. Patients need to be strong enough to exert a MIF of at least -20 cm H₂O to be considered normal and adequate.

• Peak Expiratory Flow Rate (PEFR): The existence and degree of airway obstruction and the patient’s response to aerosolized bronchodilators can be measured at bedside by evaluating the patient’s maximum rate of air flow expelled during a forced expiration.