BACKGROUND: Pain assessment is a significant challenge in critically ill adults, especially those unable to communicate their pain level. At present there is no universally accepted pain scale for use in the non-communicative (cognitively impaired, sedated, paralyzed or mechanically ventilated) patient. Facial expressions are considered among the most reflexive and automatic nonverbal indices of pain. The facial expression component of pain assessment tools include a variety of facial descriptors (wincing, frowning, grimacing, smile/relaxed) with inconsistent pain intensity ratings or checklists of behaviors. The lack of consistent facial expression description and quantification of pain intensity makes standardization of pain evaluation difficult. Although use of facial expression is an important behavioral measure of pain intensity, precise and accurate methods for interpreting the specific facial actions of pain in critically ill adults has not been identified. OBJECTIVE: The three specific aims of this prospective study were: 1) to describe facial actions during pain in non-communicative critically ill patients; 2) to determine facial actions that characterize the pain response; 3) to describe the effect of patient factors on facial actions during the pain response. DESIGN: Descriptive, correlational, comparative. SETTING: Two adult critical care units (Surgical Trauma ICU-STICU and Medical Respiratory ICU-MRICU) at an urban university medical center. SUBJECTS: A convenience sample of 50 non-communicative critically ill intubated, mechanically ventilated adult patients. Fifty-two percent were male, 48% Euro-American, with mean age 52.5 years (±17. 2). METHODS: Subjects were video-recorded while in an intensive care unit at rest (baseline phase) and during endotracheal suctioning (procedure phase). Observer-based pain ratings were gathered using the Behavioral Pain Scale. Facial actions were coded from video using the Facial Action Coding System (FACS) over a 30 second time period for each phase. Pain scores were calculated from FACS action units (AU) following Prkachin and Solomon metric. RESULTS: Fourteen facial action units were associated with pain response and found to occur more frequently during the noxious procedure than during baseline. These included areas of brow raiser, brow lower, orbit tightening, eye closure, head movements, mouth opening, nose wrinkling, and nasal dilatation, and chin raise. The sum of intensity of the 14 AUs was correlated with BPS (r=0.70, P<0.0001) and with the facial expression component of BPS (r=0.58, P<0.0001) during procedure. A stepwise multivariate analysis predicted 5 pain-relevant facial AUs [brow raiser (AU 1), brow lower (AU 4), nose wrinkling (AU 9), head turned right (AU 52), and head turned up (AU53)] that accounted for 71% of the variance (Adjusted R2=0.682) in pain response (F= 21.99, df=49, P<0.0001). The FACS pain intensity score based on 5 pain-relevant facial AUs was associated with BPS (r=0.77, P<0.0001) and with the facial expression component of BPS (r=0.63, P<0.0001) during procedure. Patient factors (e. g., age, gender, race, and diagnosis, duration of endotracheal intubation, ICU length of stay, and analgesic and sedative drug usages, and severity of illness) were not associated with the FACS pain intensity score. CONCLUSIONS: Overall, the FACS pain intensity score composed of inner brow raiser, brow lower, nose wrinkle, and head movements reflected a general pain action in our study. Upper facial expression provides an important behavioral measure of pain which may be used in the clinical evaluation of pain in the non-communicative critically ill patients. These results provide preliminary results that the Facial Action Coding System can discriminate a patient’s acute pain experience.