It’s no secret the general population is getting bigger. This increase in patient size can make it harder to treat patients and creates some unique medical considerations—most poignantly seen in the EMS and ED environments. The necessity to adapt to this special population has spurred everything from specially built EMS rigs and gurneys to fortifying our CT scanner gantries to tolerate greater weights.

Emergency airway management in the morbidly obese patient can be challenging for a variety of reasons, including more rapid desaturation, difficulty with bag-valve mask (BVM) ventilation and a potential for altered medication kinetics.1 Between 13–35% of intubations in the bariatric surgery setting have been difficult or required the use of difficult airway adjuncts, and is probably higher in the prehospital setting.2

EPIDEMIOLOGY

Morbid obesity (body mass index > 30 kg/m2) has risen to nearly 30% in conservative estimates.2 Obesity affects all body systems and contributes to coronary artery disease, hypertension, diabetes, restrictive lung disease, obstructive sleep apnea with pulmonary hypertension, and arthritis, among other conditions.

While obese patients don’t visit the ED more than non-obese, they tend to be sicker when they do, which means they’re probably sicker when they call 9-1-1.2

RESPIRATORY PHYSIOLOGY

Obesity affects the respiratory physiology in many ways that become exaggerated in the acutely ill patient. When a patient grows more obese, they have an increased metabolic demand because there’s more tissue needing to be oxygenated. However, their lungs don’t grow proportionally; in fact, obese patients have a diminished total lung capacity and vital capacity due to decreased chest wall compliance as well as the weight of the abdominal contents and the chest restricting diaphragmatic movement.3 This mechanical constriction is worsened when the patient is supine and/or sedated, which in turn causes a reduction in the functional residual capacity (FRC).

FRC is the part of the lung that can usually be filled with extra oxygen to create a reserve for the patient to draw upon when they hypoventilate or when they become apneic, such as when we give paralytics for rapid sequence intubation. On a practical level this means these patients will desaturate much more quickly than ideal-bodyweight patients. Even healthy obese patients may desaturate to critical levels in half the time of normal-weight patients—three minutes compared to six minutes—and it stands to reason that critically ill obese patients will desaturate even quicker.4

The patient is placed in the ramped position by using pillows and elevating the head of the stretcher. Ear to sternal notch is achieved.

PHARMACOLOGY

Increased metabolism leads to increased cardiac output and more blood flow across the kidneys. This in turn will decrease the level of any medication that’s cleared by the kidneys. The patient’s larger mass also means they have an inherently larger volume in which the medications are distributed so that achieving therapeutic levels can be challenging. If a medication is soluble in fat then a larger dose will be required to achieve the desired effect as more of the medicine will be hiding in the fat rather than in the bloodstream. On the other hand, the duration of effect may be longer as the drug is slowly released from temporary storage in the fat.

Many drugs used in medication-assisted airway procedures may need to have their dosages adjusted for actual total body weight (ATBW), ideal body weight (IBW) or lean body mass (LBM). The IBW for males is 50 kg 2.3 kg for each inch over 5 feet, and for females is 45.5 kg 2.3 kg for each inch over 5 feet. Succinylcholine, a neuromuscular blocker, is based on ATBW whereas rocuronium, another neuromuscular blocker, is based on IBW. Of the common induction agents, etomidate is based on ATBW whereas ketamine is based on LBM. Of the agents commonly used to maintain analgesia and sedation of the intubated patient, fentanyl and ketamine dosing is based on LBM whereas midazolam and propofol are based on IBW.

This is quite confusing and it’s difficult to remember all of these variations and make accurate calculations in stressful situations. Therefore, it’s useful to utilize an application that can calculate these dosages for you.

AIRWAY ASSESSMENT, PREDICTION & MANAGEMENT

Challenges in managing the airway in the obese patient are multifactorial and weight itself may be a poor predictor of an anatomically difficult laryngoscopy and intubation. In fact, many anesthesia studies would suggest that intubation is not anatomically more difficult in the obese population, unless they have a very increased neck circumference.3,5 Obesity is, however, a predictor of difficult preoxygenation and early desaturation, which will limit the time the provider has to manage the airway before having to abort to bag, insert an extraglottic device or even place a surgical airway. This time factor is often exaggerated in the critically ill so that these cases are frequently functionally difficult even though the anatomy itself would be manageable if time permitted.

Positioning: One of the most important aspects to airway management in the obese patient is positioning. Regardless of level of training and scope of practice, positioning should be carefully considered for any obese patient with respiratory distress or altered mental status. Having patients sit up in a position of comfort if possible is tremendously helpful to respiratory physiology. If the patient’s mental status doesn’t permit a seated position, place them in a ramped position. If the patient is in cervical spine precautions, consider the reverse Trendelenburg position.

The patient’s blood pressure should be considered before raising their head, but the hemodynamic benefits of placing the hypotensive patient flat may be offset by the negative respiratory effects. The seated, ramped and reverse Trendelenburg position all decrease the abdominal interference with diaphragmatic excursion, which opens up functional residual capacity as well as lessens he amount of chest wall weight restricting chest wall compliance; this makes ventilation easier. The seated and ramped positions have the additional benefit of maximally opening the airway and placing the larynx and vocal chords in the ideal position for viewing.5

BVM ventilation: Since obese patients often have prominent cheeks and redundant chin and neck tissue, mask seal may be difficult to obtain. The weight of the chest wall, diaphragmatic interference, redundant tissues and poor lung compliance all make forcing air in to the lungs more difficult. The increased force necessary to get chest rise may also make gastric insufflation more likely. The use of oral and/or nasal airway adjuncts is recommended in patients who’ll tolerate them and for whom they’re not contraindicated. These simple devices will allow for easier passage of air past the redundant tissues and decrease the pressure needed for bagging. It’s recommended that three providers be utilized when possible for BVM ventilation: one to maintain the mask seal, one to squeeze the bag and one to maintain cricoid pressure.

Non-invasive positive pressure ventilation: This technology is now widespread in EMS and is particularly helpful in obese patients. Non-invasive positive pressure ventilation (NIPPV) has a role in both averting the need for further advanced airway management and for pre-oxygenation of patients for whom invasive airway management is unavoidable. EMS crews must be careful as high airway resistance may necessitate the use of higher pressures, which increases the risk of gastric insufflation and potential aspiration.

Intubation: As discussed earlier, one of the major barriers to intubation in the morbidly obese patient is time, so oxygenation strategies become particularly important. The patient should be optimally positioned and a nasal cannula applied under a tight-fitting non-rebreather mask used to deliver highflow oxygen. The nasal cannula—set at 5–15 Lpm—increases oxygen flow and may have an effect on stenting small airways open. The cannula becomes even more important once the mask is removed for intubation as oxygen will continue to diffuse down the airways even if the patient is completely apneic.

This ramped positioning shows that the patient’s head may be elevated beyond the typical captain’s seat configuration. A staged volunteer shows proper BVM application in a stationary ambulance.

If the patient has a low saturation despite high-flow oxygen via a partial non-rebreather mask, they should be placed on NIPPV if available and tolerated. If the patient isn’t a candidate for NIPPV, consider assisted respirations with a self-inflating bag, ideally with an integral or attached positive end-expiratory pressure valve.

Due to body habitus in the obese patient, the traditional “sniffing” position is generally insufficient for viewing the vocal cords; the ramped position is considered superior, but the exact method of ramping isn’t as important as the position.5 Ramping can be achieved with commercially available inflatable wedges, stacking linens behind the patient, placing the gurney in reverse Trendelenburg, or elevating the head of the bed. Whatever technique is utilized, the goal is to get the ear canal and sternal notch at the same horizontal level. One thing to consider is the post-intubation logistics of the ramping. Transferring, repositioning or adjusting a stack of linens from behind the head and shoulders of a now anesthetized obese patient can create its own challenges.3

The intubation itself is similar in obese and non-obese patients, though it may be necessary to insert the handle and blade at 90 degrees to the midline to prevent the handle from impacting redundant chest tissue. Endotracheal intubation confirmation can be difficult in the obese patient as physical exam and visual cues may be unreliable. It’s been reported that esophageal detector devices are less accurate in the obese as well, but qualitative and quantitative end-tidal carbon dioxide monitoring remains reliable and should be used in all cases.6

Some experts recommend preferentially using video laryngoscopy when available for morbidly obese patients and intubating stylets/bougies should be readily available as these are often useful when negotiating redundant tissues.

Rescue: In the case of a second look or a failed airway there are some considerations for the rescue of the obese patient. As discussed above, the three-handed BVM technique may be necessary to achieve adequate ventilation.

Placement of an extraglottic airway is a reasonable next step after BVM ventilation, however, in some cases, the pressure necessary to lift the patient’s chest wall and diaphragm may exceed the pressure at which leakage around the device occurs.

Needle, Seldinger and open surgical cricothyrotomies are all challenging in the obese patient. Extensive redundant tissues and deep airway structures may necessitate a longer incision and extensive dissection when performing surgical airways. Difficulties staying on the midline and dissecting to the trachea can occur. Using ultrasound to identify deep airway structures has been proposed, but it’s not likely to be available in the prehospital environment and there may not be enough time.

SUMMARY

In airway procedures, preparation is 90% of success. For obese patients, positioning is 90% of this preparation. Other considerations include utilizing NIPPV and nasal cannula apneic oxygenation, utilizing multiple providers and airway adjuncts for BVM ventilation, correctly dosing medications based upon ideal, lean or total body weight, and having backup airway devices ready.

RESOURCES

· Members of the International Society for the Perioperative Care of the Obese Patient. Airway management in bariatric surgery: A challenge for anesthesiologists. Bariatric Times. 2012;9(2):28–29.

· O’Neill T, Allam J. Anaesthetic considerations and management of the obese patient presenting for bariatric surgery. Curr Anaesth Crit Care. 2010;21(1):16–23.

REFERENCES

1. Dargin J, Medzon R. Emergency department management of the airway in obese adults. Ann Emerg Med. 2010;56(2):95–104.

2. El-Solh A, Sikka P, Bozkanat E, et al. Morbid obesity in the medical ICU.Chest. 2001;120(6):1989–1997.

3. Zerah F, Harf A, Perlemuter L, et al. Effects of obesity on respiratory resistance. Chest. 1993;103(5):1470–1476.

4. Jense HG, Dubin SA, Silverstein PI, et al. Effect of obesity on safe duration of apnea in anesthetized humans. Anesth Analg. 1991;72(1):89–93.

5. Kristensen MS. Airway management and morbid obesity. Eur J Anaesthesiol. 2010;27(11):923–927.

6. Baraka A, Choueiry P, Salem R. The esophageal detector device in the morbidly obese. Anesth Analg. 1993;77(2):400.