Final answer:
The PaCO2 would probably increase if a patient on SIMV with partial ventilatory support stops breathing spontaneously because only the mandatory breaths from the machine would remain, potentially resulting in insufficient alveolar ventilation and carbon dioxide buildup.
Step-by-step explanation:
If the patient is being ventilated via mechanical ventilator using synchronized intermittent mandatory ventilation (SIMV) with partial ventilatory support and suddenly had no spontaneous breathing, PaCO2 would probably increase. This is because in SIMV mode, the ventilator provides a set number of breaths while allowing the patient to breathe spontaneously in between. If the patient ceases spontaneous breathing, only the mandatory breaths provided by the machine remain, which may not be sufficient to maintain the previous level of alveolar ventilation. Therefore, carbon dioxide may accumulate in the bloodstream as a result of decreased ventilation.
When ventilation is low, typically, the respiratory rate will decrease and the tidal volume will either decrease, remain the same, or increase depending on the type of adjustment made by the clinical team.