An adjunctive measure of patient risk for ambulatory anesthesia includes specific evaluation and classification of the airway. Mallampati's classification is a simple visual classification system, divided into four categories, which attempts to assess the posterior oropharyngeal airway patency based on the visibility of structures of the posterior oropharynx (uvula, fauces, soft and hard palates). The distance between the hyoid bone and the chin can be estimated as an additional, albeit crude, indicator of airway patency and ease of intubation with shorter mental–hyoid distances indicating greater airway risk. The thyromental distance is used to predict the difficulty of intubation and is measured from the chin to the thyroid cartilage with the head extended (<7.0 cm indicates the possibility of a difficult intubation). In addition, specific characteristics of patient body habitus such as obesity or the presence of a short, thick neck can be general predictors of risk of airway collapse during anesthesia. Head and neck movement can also be assessed and >90° is considered within normal limits. Of course, limited interincisal mouth opening, maxillary incisor proclination, a limited upper lip bite test, and a retrusive mandible may present risks for maintaining an airway and performing endotracheal intubation. Each of these anatomic and functional factors may be used to calculate an “airway score.”
Table 1.1. ASA physical status classification
CVA: cerebrovascular accident; MI: myocardial infarction; CHF: congestive heart failure; COPD: chronic obstructive pulmonary disease; DM: diabetes mellitus; ESRD: end‐stage renal disease.
| ASA I | No systemic disease, no smoking, no or minimal alcohol use |
| ASA II | Mild to moderate systemic disease, well‐controlled disease states; e.g., well‐controlled DM, mild asthma or epilepsy, pregnancy, current smoker, social alcohol use, BMI 30–40 |
| ASA III | Severe systemic disease that limits activity but is not incapacitating; e.g., uncontrolled DM, history of CVA, MI, or CAD with stents >3 mo ago, mild COPD, BMI > 40, alcohol abuse or dependence, cardiac pacemaker, ESRD on dialysis |
| ASA IV | Severe systemic disease that limits activity and is a constant threat to life; e.g., CVA or MI within the past 3 mo, severe CHF, severe COPD, ongoing cardiac ischemia or valve dysfunction, sepsis |
| ASA V | Patients not expected to survive 24 h |
| ASA VI | Organ donors |
Table 1.2. Duke Activity Scale Index
| Functional class | Metabolic equivalents | Specific activity scale |
|---|---|---|
| I | >7 | Patients can perform heavy housework such as moving furniture or scrubbing floors, and can participate in moderate recreational activities such as bowling, dancing, skiing, or doubles tennis. |
| II | >5 | Patients can do light housework such as dusting or washing dishes, can climb one flight of stairs, and can walk on level ground at 4 mph. |
| III | >2 | Patients can dress themselves, shower, make the bed, and walk indoors. |
| IV | <2 | Patients cannot perform activities of daily living without assistance and may be bedbound. |
An increased BMI (>25 kg/m2) and obstructive sleep apnea (OSA) are risk factors for airway obstruction during anesthesia and have had an increased prevalence in the United States. In patients without a known history of OSA, the STOP‐BANG questionnaire can be a useful screening tool to identify at‐risk patients. The questionnaire scores an individual from 0 to 8 based on risk factors such as snoring, daytime sleepiness/tiredness, observed gasping/choking/cessation of breathing during sleep, hypertension, BMI > 35 kg/m2, age over 50 years, neck circumference >16 in. or 40 cm, and male gender. Scores of 0–2 are considered low risk, while scores of 5–8 are moderate to high risk. Intermediate scores of 3–4 can be the most difficult to characterize, but risk increases with male gender and increasing BMI. The STOP‐BANG questionnaire has a moderate risk of false positives; however, it can be useful to the OMS to have an increased suspicion of potentially difficult airways [3].
Obesity, defined as a BMI > 30 kg/m2, is a recognized risk factor for anesthetic‐related complications. Note the BMI classification: <18.5 = underweight; 18.5–24.9 = normal; 25–29.9 = overweight; 30–39.9 = obese; and >40 = severe obesity. Obesity is associated with a decreased respiratory functional residual capacity (FRC) and can lead to an increased incidence of respiratory complications, particularly airway collapse and oxygen desaturation. Obese patients have a fourfold increased risk of respiratory complications during ambulatory anesthesia procedures [4]. In the pediatric population as well, obesity has been recognized as a growing problem. A study by Setzer and Saade found an increased incidence of respiratory complications and unexpected overnight hospital admission in a group of obese pediatric patients undergoing ambulatory anesthesia for dental surgery procedures (compared to a nonobese cohort) [5]. Patient positioning during surgery may play a role in preventing adverse respiratory complications in obese patients, as a study demonstrated an increase in time to desaturation in obese patients who were preoxygenated in an upright (90° sitting) versus supine position prior to induction of general anesthesia [6]. Maintaining obese OMS patients in an upright or semi‐recumbent position is done routinely in outpatient anesthesia and may help to prevent respiratory complications by maximizing FRC and minimizing the effects of gravity on collapse of the posterior oropharyngeal airway.
Age is also an important determinant of anesthetic and surgical risk. Age is easily quantified and there is evidence that increased risk of complications occurs at the extremes of very young and very old age. There is greatly increased risk associated with anesthesia and surgery in the first one month and one year of life [7]. In terms of increasing age and risk of complications, there remains a strong positive correlation though the association is more gradual and progressive. In the very young, much of the increased anesthetic risk can be attributed to the relative anatomical and physiological immaturity of infants and very young children. This makes the mechanics of anesthesia more difficult (e.g., airway management, fluid replacement, patient monitoring), while the decreased therapeutic index of anesthetic drugs in small children greatly increases their toxicity potential. At the other end of the spectrum, advanced age leads to an increase in medical comorbidities and decreased physiological reserve from the normal aging process. This also decreases tolerance for physiological insults and lowers the therapeutic index of many drugs and interventions.
Other than patient characteristics,