Dr Niraj Mistry

Aug 17, 20227 min read

Ventilation of the Obese Patient

Updated: Mar 20

Dr Niraj Mistry

Emergency Registrar

Peer review: Dr Nick Erskine

Editor: Dr Dave McCreary

THE CASE

You’re on for resus in the middle of a night shift and you receive an alert for a shocked diabetic male with respiratory sepsis and DKA who has been intubated and will be arriving soon. Your initial reaction to this news probably wouldn’t worry you too much (aside from how severe the DKA must be to lead to a tube) knowing that the hard work has been done pre-hospital until the paramedic ends the call with a very casual, “Oh by the way the patient is 280kg.”

Suddenly, managing the patient has become that much harder and you’re in for a night of complicated respiratory physiology and unexpected challenges. I recently encountered just such a situation and the myriad of problems I encountered during the case left a lasting impression that motivated me to share my experiences in this blog post.

FIRST, WHAT IF THE PARAMEDICS HADN’T BEEN SO KIND AS TO TUBE THIS PATIENT FOR ME?

Intubation of the morbidly obese patient is a high-risk situation. You need an experienced airway operator and there are some considerations to optimise pre-oxygenation. Schetz et al.(1) recommend:

At least 5 minutes of non-invasive ventilation (NIV) in an upright seated position with positive end-expiratory pressure (PEEP) support of 5-10cmH20 (3).
Apnoeic oxygenation via the addition of high-flow nasal cannula (NC) to pre-oxygenation with NIV provided more effective pre-oxygenation and subsequently less oxygen desaturation then NIV alone (4).

Furthermore, simple manoeuvres to improve the positioning of the patient such as ramping will optimise your first-pass success when it comes to intubating these tricky airways.

OBESITY, THE RESPIRATORY SYSTEM & MECHANICAL VENTILATION

The main issue with this patient was optimising ventilation. After trialling various modes of ventilation and PEEP settings to little avail I can honestly say that at the end of the night the Hamilton ventilator and I were not the best of friends. The physiological changes to the respiratory system in obesity explain why ventilating can be so difficult in these situations. Including(2):

Increased oxygen consumption and carbon dioxide production
Increased work of breathing
Increased intraabdominal and intrathoracic pressures
Decreased compliance
Decreased functional residual capacity

Schetz et al. eloquently summarise the most effective intubation/pre-oxygenation and ventilation techniques in obese individuals as such:

PROTECTIVE VENTILATION

The key to mechanical ventilation is a protective ventilation strategy using low tidal volumes of 6ml/kg of ideal body weight (as our image above illustrates, inside every obese patient is the non-obese patient we're trying to ventilate) with moderate-to-high PEEP support of >10cmH2O. This approach improves respiratory mechanics, alveolar recruitment, and gas exchange (6). Whilst Schetz et al proposed using PEEP flows of 10 or more it should be noted one study found that PEEPs around this level were inadequate in minimising atelectasis and optimising ventilation in obese ICU patients, and PEEPs as high as 20 cmH2O significantly improved lung volumes and oxygenation (7).

BEWARE OF HIGH PEEP FLOWS

Much like the respiratory system, the cardiovascular system undergoes changes in obesity which are detrimental to the patient, including:

Haemodynamic changes – increase blood volume and cardiac output leading to hypertension and left ventricular hypertrophy
Myocardial fibrosis secondary to myocardial fat accumulation
Coronary atherosclerosis
Pulmonary hypertension and right ventricular failure

These changes ultimately manifest in ventricular dilatation and heart failure (obesity cardiomyopathy) (8).

Whilst using higher PEEPs sounds great on paper, because of these changes to the cardiovascular system it would be careless not to acknowledge the effects high PEEP flows will have on haemodynamics. At higher PEEPs there is an increase in intrathoracic pressure. This compresses the inferior vena cava resulting in decreased venous return to the heart and reduced ventricular filling, a lower end-diastolic volume and less myocardial stretch before systole. As described by (primary exam trigger warning) Starling’s Law, this will result in a weaker strength of myocardial contraction and therefore reduced stroke volume and cardiac output. Ultimately this manifests as lower blood pressure – a situation I encountered first-hand in this case; every time I increased the PEEP the mean arterial pressure would drop so much so that I had to immediately dial up the Noradrenaline! Whilst on the topic of haemodynamics in obese patients it is important to mention that non-invasive blood pressures are not reliable. Blood pressure cuffs of appropriate size may not be readily available, and even when used the blood pressure cuffs are inaccurate in obesity(9). As such invasive blood pressure monitoring with an arterial line is the standard for haemodynamically unstable obese patients and this task should be prioritised.

PATIENT POSITIONING

Simple changes here can work wonders towards combating some of the above problems and improve the effectiveness of your ventilation. The increased body habitus in these patients adds to the intrathoracic pressures so any technique to offload some of that mass off of the thorax should used. In ED the simplest manoeuvre is the reverse Trendelenburg position (Figure 1). In this position, gravity will redistribute the apron of the patient away from the thorax and reduce the intrathoracic pressure exerted by the body habitus and thus reduce the work of breathing.

Figure 1: https://www.hospitalbedscn.com/news-posts/put-a-patient-in-reverse-trendelenburg-position/

THE ULTIMATE REPOSITION - PRONING

A trickier, but more effective approach, would be prone positioning. This is advocated as the positioning of choice when mechanically ventilating obese patients with ARDS. Prone positioning of obese patients has been shown to improve the ratio of arterial oxygen pressure to the fraction of inspired oxygen (PaO2/FiO2) (10). In others words, less oxygen was required to achieve satisfactory arterial oxygenation. Prone positioning provides this benefit because of the dynamic advantages it provides over supine positioning (Figure 2) (11):

Decrease gravitational pressure of the heart and mediastinum on the lungs
Decrease compressive effects of abdominal organs on the lungs
More evenly spread chest wall compliance due to the restriction of the anterior chest wall movements

Figure 2: Venus et al. CMAJ 2020.

However the logistics of proning an unwell, intubated, morbidly obese patient in a busy Emergency Department need to be considered. Pushing the “tilt bed” button on the bed control is significantly easier and safer than proning, and should be the first port of call. [Ed. – proning even an average-build intubated patient is complicated and requires training and experience that generally isn’t available in the ED. Proning is great in theory, but let’s leave this one to the ICU team.]

SUMMARY

Morbidly obese patients present many challenges. High-risk procedures such as intubation and ventilation become even trickier in these patients and failure to acknowledge the cardiorespiratory physiological changes which occur in obesity will lead you down a path of frustration and danger. By focusing on the simple things such as the importance of positioning and applying some of the recommendations I have highlighted in this post the effectiveness to which you can intubate and then ventilate obese patients can be greatly improved. In the end, the 280kg patient I encountered on my night shift eventually found himself in ICU but not until I had the pleasure of battling these challenges, and learning a heap in the process. On a more positive note, the case did provide me with my first experience of using an inflatable air mattress to transfer an obese patient and the utilisation of this equipment is the final, but probably most entertaining, recommendation I have to share when it comes to looking after morbidly obese patients. [Ed – I’ve had to use this a couple of times including once for a peri-arrest 180kg patient on the floor of our triage – it was an absolute game changer].

References

Schetz M, De Jong A, Deane AM, Druml W, Hemelaar P, Pelosi P, Pickkers P, Reintam-Blaser A, Roberts J, Sakr Y, Jaber S. Obesity in the critically ill: a narrative review. Intensive Care Med. 2019 Jun;45(6):757-769. doi: 10.1007/s00134-019-05594-1. Epub 2019 Mar 19. PMID: 30888440.
De Jong A, Chanques G, Jaber S (2017) Mechanical ventilation in obese ICU patients: from intubation to extubation. Crit Care 21:63
Delay JM, Sebbane M, Jung B, Nocca D, Verzilli D, Pouzeratte Y, Kamel ME, Fabre JM, Eledjam JJ, Jaber S (2008) The effectiveness of noninvasive positive pressure ventilation to enhance preoxygenation in morbidly obese patients: a randomized controlled study. Anesth Analg 107:1707–1713
Jaber S, Monnin M, Girard M, Conseil M, Cisse M, Carr J, Mahul M, Delay JM, Belafia F, Chanques G, Molinari N, De Jong A (2016) Apnoeic oxygenation via high‐flow nasal cannula oxygen combined with non‐invasive ventilation preoxygenation for intubation in hypoxaemic patients in the intensive care unit: the single‐centre, blinded, randomised controlled OPTINIV trial. Intensive Care Med 42:1877–1887
Groombridge C, Chin CW, Hanrahan B, Holdgate A. Assessment of Common Preoxygenation Strategies Outside of the Operating Room Environment. Acad Emerg Med. 2016 Mar;23(3):342-6. doi: 10.1111/acem.12889. Epub 2016 Feb 17. PMID: 26728311.
Pelosi P, Croci M, Ravagnan I, Cerisara M, Vicardi P, Lissoni A, Gattinoni L (1997) Respiratory system mechanics in sedated, paralyzed, morbidly obese patients. J Appl Physiol 82:811–818
Pirrone M, Fisher D, Chipman D, Imber DA, Corona J, Mietto C, Kacmarek RM, Berra L (2016) Recruitment maneuvers and positive end‐expiratory pressure titration in morbidly obese ICU patients. Crit Care Med 44:300–307
Piche ME, Poirier P, Lemieux I, Despres JP (2018) Overview of epidemiology and contribution of obesity and body fat distribution to cardiovascular disease: an update. Prog Cardiovasc Dis 61:103–113
Bur A, Hirschl MM, Herkner H, Oschatz E, Kofler J, Woisetschlager C, Lag‐ gner AN (2000) Accuracy of oscillometric blood pressure measurement according to the relation between cuff size and upper‐arm circumference in critically ill patients. Crit Care Med 28:371–376
De Jong A, Molinari N, Sebbane M, Prades A, Futier E, Jung B, Chanques G, Jaber S (2013) Feasibility and effectiveness of prone position in morbidly obese patients with ARDS: a case–control clinical study. Chest 143:1554–1561
Venus K, Munshi L, Fralick M. Prone positioning for patients with hypoxic respiratory failure related to COVID-19. CMAJ. 2020 Nov 23;192(47):E1532-E1537. doi: 10.1503/cmaj.201201. Epub 2020 Nov 11. PMID: 33177104; PMCID: PMC7721267.

NIRAJ MISTRY