Which of the following is a sign that ventilatory support might be required?

The option indicating an inability to breathe lying flat, also known as orthopnea, is a significant sign that ventilatory support might be required. This symptom often suggests that a patient is experiencing respiratory distress which may be exacerbated in a supine position. Inability to maintain adequate ventilation and gas exchange when lying flat can be indicative of underlying pulmonary or cardiac conditions that might necessitate more aggressive respiratory assistance.

High forced vital capacity (FVC) levels do not typically indicate ventilatory failure; rather, elevated FVC may suggest good lung function. Elevated negative inspiratory pressure is generally a sign of good respiratory muscle strength and does not imply a need for ventilatory support. Low oxygen saturation is indeed a concern but, in itself, might not directly indicate that ventilatory support is needed unless accompanied by other symptoms or clinical findings suggesting respiratory failure. Thus, orthopnea stands out as a direct indicator of the potential need for assistance in ventilation.