Sunday, February 13, 2011

Determinants and limits of the Draeger Narkomed Anesthesia machine in regards to ventilating the morbidly obese patient.

*Correspondence from 2007


Determinants and limits of the Draeger Narkomed Anesthesia machine in regards to ventilating the morbidly obese patient.


The following statements are the author’s own clinical opinion based on evidence based medicine and intensive care ventilation experience. These views do not represent Draeger Medical and do not replace the device operators’ manual, or judgment of the anesthesia provider.


There are several factors which make oxygenating & ventilating the morbidly obese patient a challenge, which can be broken down into three categories; apparatus, pressure, and positioning. Additionally, each category will have a synergistic or additive effect on one another.


The Narkomed anesthesia machine can deliver a max tidal volume of 1400 cc, so from an engineering perspective this machine has no max weight limit.

For example take a 6’-7” man, ideal body weight (IDBW) = 89 kg. So for basal ventilation (no pathological dead space) he would need a minute ventilation of ~ or = 9.0 liters/minute. (1). If we delivered 1400 cc’s this would be 15.7 cc/kg of IDBW.

However, we need to consider the “expiratory time constant (RCe)” (how long it takes the lung to empty), for safety it would be ideal to have a minimum expiratory phase that is equal to 3 RCe, to prevent dynamic hyperinflation.

Example: 6’-7” male, Basal ventilation rate of 9.0 l/m, average lung compliance of 60-100 cc/cmH20. Target VE = 9.0 liters, Vt = 12 cc/kg

This would give us a max frequency setting of 8 breaths/minute on a standard 1:2, I:E ratio to allow for 3 RCe. (2).

Considering this the max VE would be a little over 9.0 l/m.
Conversely, obese patients have much lower compliances, which decrease the RCe so one can set a higher frequency to obtain a greater VE. Additionally, the I:E ratio can be manipulated to provide a longer expiratory phase, allowing the frequency to be increased.

Actual tidal volume delivered

Utilizing the Narkomed the volume, which reaches the patient, will be reduced due to the compliance of the breathing system, volume calculation, and fresh gas flow. The difference between set and delivered tidal volume can be substantial and will vary throughout the procedure as inspiratory pressures change.

-Circuit compliance:

The Narkomed does not compensate for volume of compressed gas in the circuit. A standard anesthesia circuit compliance is ~ 2 cc/cmH20, so if my set Vt is 900 cc & I’m generating a peak pressure of 40 cmH20 my actual delivered Vt is 820 cc, if I have a set frequency of 12 bpm I’m losing ~ 1.0 L/m in this scenario.

-Volume Calculation:

The flow (volume) sensor is located at the expiratory valve of the circle system, which measures both exhaled gas & the gas, which was compressed in the circuit tubing during inspiration. The exhaled flow sensor will therefore, tend to overestimate the patients exhaled volume.

-Fresh Gas Flows:

Fresh gas flow is often decreased after induction, which will reduce delivered tidal volume unless there is a compensatory increase in set Vt.


To prevent ventilator induced lung injury it is recommend that plateau pressures should be < or = 30 cmH2O. During volume ventilation, increases in resistance & decreases in compliance increase plateau pressures. During general anesthesia the increased weight of the chest and weight of the abdominal viscera pressing on the relaxed diaphragm decrease compliance. Additionally, during pneumoperitoneum (PPM) associated with bariatric and laparoscopic surgery this further reduces lung compliance, El-Dawlatly showed that plateau pressures increased significantly and dynamic compliances decreased by a mean of 10cc/cmH2O during PPM with 15 mmHg (3).

Example: 6’-7” ♂, Ideal VE = 9.0 l/m Settings: Vt 900 (~10cc/kg), F 10 bpm

Compliance before PPM = 40cc/cmH20, during PPM = 30cc/cmH20

Delivering a Vt of 900 cc with a compliance of 40 would generate a plateau pressure of ~ 22.5 cmH20 (safe zone)

During PPM- 900cc with a compliance of 30 = ~ plateau pressure of 30 cmH20 (still safe zone) (4).

Considering this the max tidal volume we could safely deliver during PPM for the morbidly obese patient would be 900cc. However, not all obese patients have starting compliances this low and if needed due to increased plateau pressures we could ventilate down to 5cc/kg/idbw and make up VE with increasing frequency, if the patient doesn’t have a obstructive pulmonary pathology.

Another limitation of Narkomed machine is they only provide Volume Controlled ventilation with a constant/square flow pattern. When comparing the constant flow pattern to the descending flow of Pressure Control ventilation the descending flow ventilates at a lower peak inspiratory pressure, improves the distribution of gas, reduces dead space, & increases oxygenation due to increasing mean airway pressure.


It has been previously mentioned the adverse effects of placing the obese patient in a supine position during general anesthesia. To lessen impairment of pulmonary gas exchange researches (5,6, & 7) have utilized the reverse trendelenburg position and recruitment maneuvers. This is could be a limitation on the Narkomed considering the recruitment maneuver you use (PEEP > 20cmH2O).


After reviewing the literature and knowing the limitations of the Narkomed I feel that weight is not a issue at all if the patient is under 6’-7”, no obstructive pulmonary history, and has no metabolic issues (sepsis, or metabolic acidosis). Conversely, for patients with a body mass index greater than 60kg/m2 I would definitely consider using the Fabius if the patient had additional risk factors for prolonged mechanical ventilation after bariatric surgery (8). I believe the primary factor is the anesthesia providers’ comfort & knowledge base of the anesthesia ventilators capabilities.


1. Radford EP Jr. Ventilation Standards for use in Artificial Respiration. New England Journal of Medicine 1954;251:877-83.

2. Otis AB, Fenn WO, Rahn H. Mechanics of Breathing in Man. Journal of Applied Physiology 1950;2:592-607.

3. El-Dawlatly AA, Al-Dohayan A, Abdel-Meguid ME, El-Bakry A, Manaa EM. The effects of pneumoperitoneum on respiratory mechanics during general anesthesia for bariatric surgery. Obese Surgery 2004 Feb; 14(2):212-5.

4. Marini JJ, Crooke PS, Truwit JD. Determinants and limits of pressure-preset ventilation: a mathematical model of pressure control. Journal of Applied Physiology 1989;67:1081-92.

5. Chalhoub V, Yazigi A, Sleilaty G, Haddad F, Noun R, Madi-Jebara S, Yazbeck P. Effect of vital capacity manoeuvres on arterial oxygenation in morbidly obese

patients undergoing open bariatric surgery. Eur J Anaesthesiology. 2007 Mar;24(3):283-8. Epub 2006 Nov 7.

6. Perilli V, Sollazzi L, Modesti C, Annetta MG, Sacco T, Bocci MG, Tacchino RM,

Proietti R. Comparison of positive end-expiratory pressure with reverse Trendelenburg position in morbidly obese patients undergoing bariatric surgery: effects on hemodynamics and pulmonary gas exchange. Obese Surgery. 2003 Aug;13(4):605-9.

7. Whalen FX, Gajic O, Thompson GB, Kendrick ML, Que FL, Williams BA, Joyner MJ, Hubmayr RD, Warner DO, Sprung J. The effects of the alveolar recruitment maneuver and positive end-expiratory pressure on arterial oxygenation during laparoscopic bariatric surgery. Anesthesia & Analgesia. 2006 Jan;102(1):298-305.

8. Helling TS, Willoughby TL, Maxfield DM, Ryan P. Determinants of the need for intensive care and prolonged mechanical ventilation in patients undergoing bariatric surgery. Obese Surgery. 2004 Sep;14(8):1036-41.