Final answer:
The spirogram value that does not change from a normal patient is the residual volume (D), which prevents alveoli collapse and remains constant as it can't be easily measured.
Step-by-step explanation:
The value in the spirogram that never changed from that of a normal patient is the residual volume (D). The residual volume is the air left in the lungs after the expiratory reserve volume is exhaled. It is critical as it prevents the alveoli from collapsing by maintaining a constant volume within the lungs.
The residual volume cannot be measured directly because it is impossible to empty the lungs of air, therefore, it is calculated rather than measured directly.