D11 Respirator function

Topic

Spirometry is the fundamental pulmonary function test; it measures airflow and volumes moved during a standardized forced breathing maneuver. The ratio of the volume expelled during the first second to the total exhaled volume allows for the detection of airway obstruction. According to the text, if this ratio remains below 70% after the administration of a bronchodilator, obstruction is present—as seen in asthma or chronic obstructive pulmonary disease—and the lower the value, the greater the severity. If the total exhaled volume is reduced but this ratio remains normal, restriction is suspected; this can occur in conditions such as pulmonary fibrosis, respiratory muscle weakness, or marked obesity. Restriction must be confirmed via plethysmography, which measures total lung volume.

Another test assesses the lung's capacity to transfer gas from the alveoli to capillary blood, typically using carbon monoxide due to its high affinity for hemoglobin. This capacity indirectly reflects pulmonary oxygen transfer and depends on the available surface area of ​​the alveolocapillary membrane, the volume of blood in the pulmonary capillaries, and the amount of available hemoglobin. It may decrease due to loss or alteration of surface area (as in emphysema and fibrosis) or a reduction in the vascular compartment (as in pulmonary hypertension and vasculitis). The Roughton-Forster equation allows for the separation of the contributions made by the membrane and the capillary blood. A reduced transfer capacity accompanied by normal spirometry results may point toward interstitial or pulmonary vascular disorders.

The six-minute walk test measures the distance a patient can cover within that timeframe at the maximum sustainable pace. Unlike tests that focus on a specific pulmonary component, it integrates the functional capacity of both the respiratory and cardiovascular systems during exercise. It is reproducible and provides prognostic information in chronic obstructive pulmonary disease, heart failure, pulmonary hypertension, and pulmonary fibrosis. According to the text, walking less than 300 meters or experiencing a drop in oxygen saturation of more than four percentage points indicates severe functional impairment.

Polysomnography examines breathing during sleep by simultaneously recording brain activity, eye movements, muscle activity, respiratory airflow, oxygen saturation, body position, and snoring. The number of complete apneas and episodes of insufficient breathing per hour allows for the calculation of an index that quantifies the severity of obstructive sleep apnea. According to the text, reaching at least 15 episodes per hour, combined with compatible symptoms such as daytime sleepiness, hypertension, or snoring, indicates a need for continuous positive airway pressure (CPAP) treatment. The cumulative time spent with oxygen saturation below 90% also quantifies nocturnal hypoxemia and is associated with cardiovascular risk and pulmonary hypertension.

In patients undergoing mechanical ventilation, the relationship between pressure and lung volume can also be studied through inflation and deflation maneuvers performed slowly enough to minimize the effect of airway resistance. The resulting curve allows for the identification of a lower limit—below which previously open alveoli begin to collapse—and an upper limit—beyond which overdistension occurs. These points help determine a positive end-expiratory pressure (PEEP) sufficient to keep alveoli open without causing excessive distension. The respiratory system stiffness derived from this relationship also determines the driving pressure, which is used to assess the mechanical stress placed on the lung during ventilation.

Taken together, these tests examine different dimensions of respiratory function. Spirometry distinguishes obstructive patterns and suggests restrictive ones; gas transfer assessment evaluates the alveolocapillary membrane and its vascular component; the six-minute walk test integrates the cardiorespiratory response to exercise; Polysomnography analyzes breathing during sleep, while the pressure-volume curve allows mechanical ventilation to be adapted to the lung's properties.

Together, they enable the study of how the patient moves air, exchanges gases, responds to exercise, and breathes during sleep or while receiving artificial support. Thus, respiratory assessment begins with a relatively simple spirometric maneuver and can be progressively expanded to characterize the lung's functional structure and tailor ventilatory support to each patient's needs.

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