The sudden onset of tachycardia and hypotension indicates an acute process. Since the patient is being mechanically ventilated with positive pressure, he is at increased risk of a bullous rupture from barotrauma, leading to a pneumothorax. Cardiac arrhythmia could lead to tachycardia and hypotension. Ventricular tachycardia and atrial fibrillation with a rapid ventricular response may cause this from decreased ventricular filling. An ECG would aid in this diagnosis. In the setting of mechanical ventilation, however, a pneumothorax must be excluded first. Bronchial secretions usually have a progressively worsening presentation. Furthermore, the patient would exhibit desaturation, but not necessarily hypotension. Myocardial infarction may lead to cardiogenic shock from failure. However, this would most likely be a bit more progressive and less acute. Infarction must remain high on the differential diagnosis, and the patient may require vasopressors because of the shock. In the immediate setting, the pneumothorax is more likely, given the acuity of onset. Pulmonary embolus is on the differential diagnosis of electromechanical dissociation. This patient's risk of an embolus is increased because of prolonged immobilization. However, the subcutaneous heparin should be adequate prophylaxis against an embolism.