“Applying robust [natural language processing] methods to the perioperative setting will improve our understanding of how patterns of substance use impact the anesthetic management and perioperative outcomes for millions of patients […].”
Patients commonly use substances that pose risks to their health such as tobacco, alcohol, and other drugs in the perioperative period, with important implications for their anesthetic care. In recent surveys of patients presenting for surgery, more than two in five patients evaluated in the preoperative clinical setting endorsed some level of substance use that poses surgical risk, including use of cannabis products. Clinically, it can be challenging to consistently assess use of different types of substances in an efficient yet reliable manner. Cannabis use presents a complex yet increasingly prevalent practice reported by patients. Most Americans now live in states that allow medical use of cannabis as a treatment for conditions including chronic pain, and many states have legalized recreational cannabis use. Perhaps unsurprisingly, more Americans report daily use of cannabis than daily alcohol consumption. This change in population-level behavior is likely to translate to individual patients who present for surgery. The recent surge in cannabis use has exposed significant gaps in our understanding of how presurgical cannabis consumption affects anesthesia management and patient care.
In this issue of Anesthesiology, Sajdeya et al. examine the association between cannabis use in the 60 days before surgery and the dose of inhalational anesthesia administered in the operating room. Cannabis use was identified using classifications derived from natural language processing methods along with other structured data points such as diagnostic codes. They hypothesized that anesthesia doses would be higher among persons with current cannabis use compared to those with no use. The study focused on persons aged 65 yr and older who received care at one health system during a 3-yr period. After using propensity score matching methods to balance confounding variables, they examined the minimum alveolar concentration (MAC) values for isoflurane and sevoflurane for 1,340 patients, converting MAC values into an equivalent unit that was then averaged over every minute of the case. They found that the difference in the dose of anesthesia was higher for patients with cannabis use (mean MAC, 0.58) when compared to those without (mean MAC, 0.54), a result that was statistically significant although likely not clinically meaningful (mean difference, 0.04; 95% CI, 0.01 to 0.06).
Among strategies to classify cannabis exposure for included participants, natural language processing methods offer one way to glean insights from data within electronic health records. Unstructured free-form text in electronic health record notes often contains crucial clinical information. Analyzing unstructured text from these notes may identify a particular individual as someone who possesses a relevant characteristic or not (e.g., whether they use specific substances). Manual review of unstructured text in a limited number of notes is feasible for researchers. However, analyzing thousands or millions of notes requires automated processing to efficiently scale this size of project. In this study, the approach to determine cannabis use incorporated data from unstructured clinical notes from a patient’s chart documented in the preoperative period up to 60 days before surgery.
Natural language processing builds on a linguistics approach using rule-based models of human language along with statistical, machine learning, and other analytical approaches. Before the analytical approaches may be applied, consensus must exist about the target concept. Defining a comprehensive and exhaustive list of terms and their meaning in a lexicon is a fundamental task. To generate a list of key words, phrases, and concepts that contribute to a relevant lexicon, researchers may review previous literature, query the opinion of experts, and draw from databases built for this purpose. For example, terms for cannabis include not only words like tetrahydrocannabinol, marijuana, and cannabidiol, but also their abbreviations, brand names, and potential misspellings. One concern for accurately identifying use of cannabis, as well as substances of other types, is the common use of different names, nicknames, or slang words that refer to specific drugs, which may evolve over time and introduce challenges in misclassification. Whether pot indicates cannabis or a container for cooking clearly depends on context.
Extracting phrases from charts and rating their status manually allows for researchers to refine their approach to classification, such as whether active cannabis use is present or not. An existing validated assessment or consensus among two or more raters with clinical domain expertise is often used as a benchmark and thus serves as the basis for data on which to train machine learning and deep learning models. In testing these models, researchers often evaluate the precision, sensitivity, specificity, and other scores that indicate performance. Similar to other validation studies, replicating the analysis using a separate database highlights the degree to which findings from classification algorithms may apply more broadly. Externally validating findings from natural language processing algorithms takes time and resources, although without this step, model results may be inaccurate, biased, and not generalizable.
Classifying a patient as having active substance use or not using natural language processing presents a set of challenges compared to doing so in routine clinical settings. While clinicians may tailor questions and follow-up to discern whether a patient is currently using substances, the same is not true when querying data within unstructured notes in the electronic health record. Certainly, some patients will clearly have documentation of active use of cannabis in their chart, given 16% of adults endorse using cannabis in the past month. On the other hand, many patients will also have documentation that confirms that they are not currently using cannabis, an important concept that may also be referred to as negation of cannabis use. Another group of persons will fall into a third category: review of their notes leads to an inability to either confirm or deny current cannabis use. Because of the uncertainty regarding their use, including persons with unknown cannabis use would lead to biased estimates in an analysis, and this group was appropriately excluded by the authors. Similar approaches may be used to classify patients and identify those with no use of any substances, which the authors also performed.
The need for natural language processing methods to identify persons with active cannabis use would be less of a priority if our health systems widely implemented the use of validated screening tools to ascertain whether and to what degree a patient uses risky substances. Alternatively, natural language processing could supplement validated screening tools. Tools such as the Brief Screener for Alcohol, Tobacco, and other Drugs (BSTAD) for adolescents and the Tobacco, Alcohol, Prescription medication, and other Substance use (TAPS) for adults provide efficient means to screen patients in both patient- and clinician-administered formats. Answers to three or four screening questions unveil insights about the use of cannabis, tobacco, and alcohol that are typically more informative than content on substance use found in many “social history” sections. Both patients and surgeons appear receptive to the use of screening tools in the preoperative period. Given gaps in the use of screening tools for surgical patients, natural language processing methods have the potential to unlock meaningful information within unstructured clinical notes in the perioperative setting, permitting a deeper phenotyping of patients beyond the more commonly used approaches based on structured data.
What are some considerations for next steps in natural language processing for substances in the operating room?
- Identifying persons with active, recent, or past use of risky substances. Broad lines of inquiry that examine the associations and implications of active and past substance use on patient-centered and health-related outcomes represent one of the clearest possible benefits of deploying natural language processing methods. A more complete understanding of substance use includes the degree, timing, and frequency with which such exposures may create risk. This has the potential to drive additional investigations, increase awareness among patient and professional communities about how substance use may impact surgical outcomes, and foster proactive change in habits among patients, all of which may advance care for patients in the operating room. This can be particularly challenging with cannabis giving the lack of standardization across different products and formulations. For cannabis, complexity and variability arises from diverse products and individual use patterns, including varying cannabinoid composition. Products contain unique concentrations and ratios of delta-9-tetrahydrocannabinol and cannabidiol, among other active compounds, each of which is expected to have different mechanisms of action. Even among the same compounds, different routes of administration lead to important changes in the bioavailability and resulting side-effect profiles. Altogether, the varying physiologic effects are of high interest within the perioperative period, based on dose, frequency, and timing of exposure relative to surgery.
- Mitigating biased and stigmatizing language in clinical documentation. Narratives and unstructured notes reflect the perspectives of clinicians who author them, including potentially biased or stigmatizing language. Among hypothetical cases, one could imagine interventions that recognize biased or stigmatizing language, bring this to the attention of a note writer, and suggest alternative language. Nudges to change the language that we use may advance efforts to reduce stigma encountered by persons who use risky substances, as well as other groups, and lead to improvements in their health along the way. In addition to stigma, confidentiality and patient privacy considerations arise given patients may not disclose their use status due to fear of legal repercussions, especially when substances may be illegal in their state.
- Identifying persons in need of connection to care for risky substance use. Looking back to identify persons with high risk or disordered use of cannabis, opioids, alcohol, and tobacco products using reliable and validated natural language processing methods tailored to the perioperative setting is an appropriate starting place. Classifying patients for a retrospective study presents one set of challenges, including misclassification and bias. The consequences of misclassification increase significantly when developing risk prediction, where similar methods are applied to classify risky substance use for purposes not of research but of clinical care and real-time clinical decision-making. Using natural language processing tools to assist the screening process will require clinicians to verify and comprehensively assess the output of tools before making decisions and recommendations that directly impact the patient.
- Addressing a broader range of risky substance use before surgery. The development and validation of natural language processing methods enable the examination of a greater breadth and depth of substances. For example, these methods boosted rates for correctly identifying risky alcohol use before surgery from 29% when using diagnosis codes alone to 87% when using natural language processing.8 Analyzing certain types of exposures within a substance use category may be possible to a degree that was previously not feasible. For example, distinguishing among the various routes of cannabis use may reveal differences in edible versus topical forms. Studying substances less commonly used than cannabis, alcohol, and tobacco products is also of interest, such as emerging drugs including xylazine or synthetic cathinones.
Natural language processing methods offer a timely and important tool to identify and quantify aspects of risky substance use. Applying robust methods to the perioperative setting will improve our understanding of how patterns of substance use impact the anesthetic management and perioperative outcomes for millions of patients who undergo surgery every year.