External ventricular drains (EVDs) are pressure-based drainage systems designed to remove cerebrospinal fluid (CSF) from the ventricular system. Much like a chest tube or a lumbar drain, the rate of fluid removed from the ventricular system is determined by setting a pressure scale based on the patient's intracranial pressure (ICP). There are several indications for EVD placement, including acute symptomatic hydrocephalus, subarachnoid hemorrhage, meningitis or ventriculitis, monitoring of increased ICP due to a space occupying lesion (ie stroke, tumor, etc), management of intraoperative brain edema, and as an adjunct for a malfunctioning ventriculoperitoneal shunt (VPS). The main contraindications for EVD placement include bleeding disorders, use of anticoagulant medications, and an active scalp infection or brain abscess.

The most common indication for EVD placement in the United States is post-aneurysmal rupture subarachnoid hemorrhage (aSAH). There is currently no agreed upon protocol for EVD usage in this context, and EVD management is largely an experience-driven artform. Researchers in the Department of Neurosurgery at Massachusetts General Hospital conducted a single institution randomized controlled trial to determine whether continuous or intermittent EVD drainage was more effective at reducing length of EVD placement, ICU admission length, the need for delayed VPS placement, and the frequency of symptomatic vasospasm.

Using the following table, calculate the appropriate test statistic to determine whether the use of an intermittent drainage protocol was associated with reduced EVD duration.

	Continuous Drainage	Intermittent Drainage
N	100	100
Mean EVD Duration	15.2 days	10.6 days
Standard deviation	2 days	3 days

1. t = -12.76
2. t = -1.31
3. z = -12.76
4. z = -1.31

Cannot be determined with the available information.