Vol. 16 •Issue 19 • Page 20
Assessing Cognitive Function in Patients on Mechanical Ventilation
LM is a 72-year-old male who was an unrestrained passenger involved in a motor vehicle accident. He presented to the ED with significant head trauma, chest contusion and a ruptured spleen.
Surgical repair of all abdominal injuries was uneventful; and six days post-injury, the patient was on rather significant ventilatory support, although his oxygen requirements were satisfied with 30 percent O2. The attending therapist had to stir him to open his eyes. When LM was apparently awake, he could not follow simple commands to accomplish bedside tests of weaning readiness. He was receiving minimal sedation and pain control, but his mental status was a significant barrier to weaning from mechanical ventilation.
Sound familiar? Overcoming impaired mentation is a challenge for any health care provider. Combine that problem with mechanical ventilation, and the end result is generally a long stay in the ICU. The AARC’s “Evidenced Based Guidelines for Weaning and Discontinuing Ventilatory Support” addresses the notion that caregivers must first perform a multi-system patient assessment that includes: objective measures of pulmonary, cardiovascular, neurological and psychological variables prior to any attempt to wean patients from mechanical ventilation.
Among examples mentioned in these guidelines are adequate oxygenation (PO2/FIO2 ratio, PEEP level, etc), a stable cardiovascular system (HR, BP, presence of vasopressors) and adequate mentation. There’s that word again. Mentation specifically addresses issues such as whether the patient is arousable, a Glasgow Coma Score greater than 13 and no continuous sedative infusions are present.
Perhaps the most basic and widely used method of assessing the mental status of a patient is the Glasgow Coma Score (GCS). This system is used to assess the level of consciousness following traumatic brain injury and measures three dimensions of alertness:
• Best Eye Response,
• Best Verbal Response, and
• Best Motor Response. (See table 1 for the specific GCS criteria.)
The GCS is scored between 3 and 15 with a score of 13 or higher associated with a mild brain injury; 9 to 12 as a moderate injury; and 8 or less as a severe brain injury. The GCS is a very simple scoring system, which, in part, accounts for its widespread use.
Two other aspects of this and any test are important though. Does the test measure what it is purported to measure? This question addresses the validity of a test. The GCS seems to correlate well with outcome following severe brain injury, so its validity is thought to be acceptable.
A second question also must be asked of any test: Will the test results be the same on subsequent trials and related to this is will the results of the test be the same in the hands of different practitioners? These questions address the issues of reliability and inter-rater reliability respectively. Again, the GCS performs well in these two areas and, importantly, any bedside test must meet these two performance requirements.
GCS results often are reported in the form of a single number, but the most meaningful information is provided when the score for each subcategory is combined with the total score. A patient with a Glasgow Coma Score of 15 would be reported as E4 V5 M6, for example.
Since the patient, in this instance LM, is intubated, his score would be: E4 Vintubated M6. Some caveats of the GCS. Certain types of trauma may render some elements of the GCS useless. For example, spinal cord injury will make the motor scale invalid; and severe eye trauma may make it difficult for the evaluator to assess eye opening.
Some conditions like shock, hypoxemia, drug use and alcohol intoxication also will affect the patient’s level of consciousness and the GCS will cease to reflect the severity of brain injury when those conditions are present.
Another common method of assessing cognitive impairment is the Ramsay Scale. This scale rates the patient’s level of sedation with a scoring system of 1 (anxious) to 6 (unarousable). Scores of 4-6 require therapists to be vigilant in their assessments of the patient’s readiness to wean. (See Table 2 for the complete Ramsay Scale).
Many other instruments exist to assess cognitive function at the bedside. The Subjective Assessment of Sedation and Agitation (SAS), the Motor Activity Assessment Scale (MAAS) and the Vancouver Interaction and Calmness Scale (VICS) are good examples of tests currently used. They score well in measures of validity and reliability, but their use exceeds the scope of this article.
One newly reported test, the Confusion Assessment Method for the Intensive Care unit (CAM-ICU), is enjoying widespread use in the assessment of delirium in the ICU. This test is quick, and it accurately predicts delirium in ICU patients, which may account for fully 80 percent of the ICU patients studied. Further reading for this and other tests is provided in the Resources at the end of the article.
For LM, the weaning process was a bumpy road. With an initial GCS of 9, E3 V3 M3, caregivers assessed him to have significant brain trauma, and this sequela played out in his ventilator course. His follow up GCS score was 15, E4 V5 M6, and his Ramsay score was 3 when he was finally weaned, using very carefully monitored trials of reduced pressure support levels.
Neurologic improvement was better from there, and he left the hospital awake and alert and “feeling fine”.
1.Glasgow Coma Scale: http://www.cdc.gov/masstrauma/resources/gcscale.htm.
2.Other tests to assess cognitive function: http://www.ashp.org; see Clinical Practice Guidelines for the sustained Use of Sedatives and Analgesics for the Critically Ill Adult.
3.Hurford WE. Sedation and Paralysis During Mechanical Ventilation Ramsey Scale, Respir Care (2003; 47, 3: 332-346).
4.Ely EW, et al. Delirium in mechanically ventilated patients: validity and reliability of the Confusion Assessment Method for the Intensive Care unit (CAM-ICU). JAMA (2001; 286, 21: 2703-10).
Eric Bakow is a Pennsylvania practitioner.