FIGURE 1.

Representative examples of positive hyperventilation provocation test (HVPT) from four patients with normal pulmonary function tests and lung computed tomography scans at evaluation in the outpatient clinic. The hyperventilation manoeuvre (grey area) began at the 3rd min and was interrupted at the 6th min or when clinical intolerance was reached. The first 3 min and the last 6 min characterise the breathing pattern at rest. a) Premature interruption of the hyperventilation manoeuvre. The HVPT provoked a rapid reproduction of daily symptoms with major discomfort that led to premature interruption of the hyperventilation (HV) manoeuvre. The patient's breathing pattern was considered normal, as the mean respiratory rate at rest was <20 breaths·min⁻¹ (upper panel) and the tidal volume remained stable without hyperpnoea or deep sighing (middle panel), allowing quick recovery of the baseline end-tidal carbon dioxide tension (P_ETCO2) after HV (lower panel). b) Hyperventilation. After completion of the HV manoeuvre, an abnormal breathing pattern appeared with persistent tachypnoea that reached 30 breaths·min⁻¹ even after 6 min of resting breathing (upper panel). Tidal volumes were normal (middle panel). The recovery of P_ETCO2 was delayed and it remained below its baseline value at the end of the test (lower panel). c) and d) Deep sighing. The HVPT provoked a rapid reproduction of daily symptoms with major discomfort that led to premature interruption of the HV manoeuvre. The patient's breathing pattern consisted of either normal (c) or increased (d) respiratory rate at rest (upper panel) with frequent deep sighs that resulted in several spikes on the volume–time curve (middle panel) which are mirrored by transient drops in the P_ETCO2 (lower panel).