In this study, we found that the overall incidence of postoperative delirium to be 48.1%, which was consistent with that found in previous studies [9
]. This relatively high incidence of delirium may be secondary to the older patient population being studied: the mean age of our cohort was 73.9 ± 6.3 years. In addition, the same trained research assistant evaluated patients' cognitive status daily, which may have increased the sensitivity of delirium.
Although postoperative delirium is prevalent and prognostically important, evidence-based preventive therapy is limited. Previous studies have investigated other prophylactic measures in preventing postoperative delirium using pharmacological (e.g., antipsychotics [19
], anesthetic agents [20
], and anticholinergics [21
]) or non-pharmacological intervention (e.g., proactive geriatric consultation program [22
]). Marcantonio et al. [22
] studied patients with hip fracture in acute hospital settings. Patients were randomized in either proactive geriatric consultation group or standard care group. They found that a decrease in the rate of delirium in the geriatric consultation group, however the length of hospital stay was not significantly different in 2 groups. In contrast, results from pharmacological prevention of delirium are not conclusive. According to Larsen et al. [19
], perioperative administration of antipsychotic agent, olanzapine decreased an incidence of postoperative delirium in patients underwent elective orthopedic surgery. Sieber et al. [20
] reported that the use of light propofol sedation during spinal anesthesia decreased incidence of postoperative delirium by 50% compared with deep sedation. In contrast, a study by Gamberini and colleagues [21
] found that administration of anti-cholinesterase inhibitor, rivastigmine did not decrease the rate of postoperative delirium in patients underwent elective cardiac surgery.
Our study is one of the first to investigate a relationship between the use of a femoral nerve block and postoperative delirium. In recent years, anesthesia and postoperative pain management techniques for TKR have changed substantially. Since anticoagulant drugs such as low molecular weight heparin are now commonly used to lower the risk of perioperative venous thromboembolism, femoral nerve block has become a more popular choice over epidural analgesia, as femoral nerve block does not carry the risk of epidural hematoma. Femoral nerve block has additional advantages including: 1) no opioid related side effects (e.g., nausea/vomiting, urinary retention, and pruritus); 2) the contralateral limb will not be affected; and 3) not associated with hypotension as may occur with an epidural.
Our study results showed that pain management was a predictor of postoperative delirium and also showed femoral nerve block reduced the rate of delirium. The current findings did show that the use of femoral nerve block reduced the amount of intraoperative opioid dose, but the opioid sparing effect did not appear to extend to the postoperative opioid. The reduced intraoperative opioid use is likely related to the bolus of local anesthetic (e.g., 30 ml of 0.5% ropivacaine) administered for femoral nerve block during the catheter placement. A single shot femoral nerve block with local anesthetic often lasts up to 12-24 hours. The lack of difference in postoperative opioid usage and pain level may be secondary to the location of the femoral nerve catheter. Capdevila et al. [23
] reported that the course and location of continuous three-in-one block was totally unpredictable. The tip of the nerve catheter reached the lumber plexus in 23% of the patients, lay deep to the medial or lateral part of fascia iliaca in 33%, 37% respectively. They concluded that the quality of sensory and motor blockade depend on the location of the catheter tip. Therefore, femoral nerve catheter may not consistently produce sensory blockade of primary branches of lumber plexus (femoral, obturator and lateral femoral cutaneous). In addition, it spares the sciatic nerve.
We assessed patients' cognitive status and other clinical outcomes for 2 days postoperatively. If a patient had experienced cognitive changes later, the incident may have been missed. However, because the incidence of postoperative delirium is higher and surgical pain is intense the first few days after surgery, we believe that we have captured the most important time period in this study.
Pain assessment was conducted once daily during the patient interview. Because acute postoperative pain is dynamic and may fluctuate, we may not have evaluated the complex relationship between postoperative pain and delirium completely. However, from our experience, pain assessment once in the morning vs. twice a day assessments showed no significant difference in pain scores. In a separate pilot study of 20 patients evaluating the potential differences in pain scores between once a day vs. twice a day assessments, we found that only 2% of pain scores were significantly different between the two methods.
The initial analysis on patients between 2001-2006 showed a trend of lower incidence of postoperative delirium in the femoral nerve block group. This result led to the further analysis including more patients, and it became statistically significant for lower incidence of postoperative delirium and reduced intraoperative opioid consumption, but not statistically significant for the postoperative pain levels and opioid use. Our results should still be considered as pilot since sample size calculation shows that 61 subjects per group (total of 122) will be needed for a future trial, given a power of 80% and level of significance at 0.05. A prospective study that is adequately powered is indicated to confirm this preliminary finding.
Lastly, our study design was a nested cohort, therefore there could be a potential bias with this type of study. It was not randomized, although the two groups of patients were matched with respect to preoperative demographics except for ASA physical status, additional covariates that may have been important may have been missed and need to be considered in future prospective randomized trial.