Monday, October 1, 2012

Determinants and Limits of the Bellows: The Katz Study

Image 1: Draeger Narkomed 2c Anesthesia Delivery System.

In a previous post "Determinants and Limits or the Draeger Narkomed Anesthesia Machine in Regards to Ventilating the Morbidly Obese Patient", I tried to calculate the largest patient a bellows system could ventilate safely. 

I concluded that if you wanted to deliver a minute ventilation greater than 9 liters per minute, using conventional ventilator settings (tidal volume of 10 ml/kg/IDBW) that the operator would run into issues. 

I tried to use many mathematical models to calculate the max minute ventilation possible with a older bellows ventilator, however was not successful. 

Fortunately, Katz and colleagues  performed a bench study in 2000 evaluating minute ventilation capabilities in older anesthesia machines [1].

The study evaluated GE anesthesia systems however the results could be applied to other older bellows anesthesia system (e.g. Narkomed). 

Here is a synopsis of the study:


This study aimed to compare the flow and pressure capabilities of the Datex-Ohmeda SmartVent (Ohmeda 7900, Datex-Ohmeda, Madison, WI) to previous Ohmeda (7810 and 7000, Datex-Ohmeda, Madison, WI) anesthesia ventilators. Additionally, the study aimed to determine airway pressure and minute ventilation thresholds for intraoperative use of a critical care ventilator. 

The design implemented three anesthesia ventilators and one critical care ventilator (Siemens Servo 900C, Siemens, Solna, Sweden) to study in a lung model, retrospectively.  The study included 145 mechanically ventilated patients treated for acute respiratory failure who experienced 200 surgical procedures.  

The effect of increasing pressure on mean inspiratory flow was determined by cycling each ventilator through increasing restrictors. Maximum minute ventilation was measured at low compliance (10–30 mL/cm H2O), positive end-expiratory pressure (PEEP) (0–20 cm H2O), and increased airway resistance (;19 and ;36 cm H2O/L/sec) in a mechanical lung model.


It was determined that mean inspiratory flow declined with increasing pressure in all anesthesia ventilators. The SmartVent and the 7810 produced greater mean inspiratory flow than did the 7000 ventilator. As compliance progressively decreased, the Siemens, the SmartVent, and the 7810 ventilators maintained V˙ E compared to the 7000 ventilator. 

The Siemens and the SmartVent maintained V˙ E with PEEP, compared to the 7810 and 7000 ventilators. During increased airway resistance, maximal V˙ E was lower for all ventilators.  The SmartVent met the ventilation requirements in 90% of the patients compared to 67% of patients with the 7000 ventilator.

Applying the Results to the Morbidly Obese Patient

One component of the study evaluated the "Effect of Decreasing Compliance on Minute Ventilation and Inspiratory Duty Cycle During Low Airway Resistance". 

We can take these results and apply them to our obese patient. 

In 1998 Pelosi et. al. published "The Effects of Body Mass on Lung Volumes, Respiratory Mechanics, and gas exchange During General Anesthesia". The researchers revealed that the average total system compliance in the obese patient is ~ 40-45 ml/cmH2O [2]. 

In 2004 El-Dawatly showed that a 15 mmHG pneumoperitoneum decreased compliance an additional 10 ml/cmH2O [3]. 

So lets use a compliance of 30 ml/cmH2O.

The results showed that at a compliance of 30 ml/cmH2O the older bellow systems were able to deliver max  minute ventilation's  of 22.4 lpm & 27 lpm. 

Note- the duty cycle (I:E ratio) had to be changed to 1:1 to obtain these minute ventilation's. 

So, if a patients minute ventilation requirement is < 22 lpm then one could consider using an older anesthesia system, as long as they are comfortable with making changes to the duty cycle. 

[1]. Katz, J. A., Kallet, R. H., Alonso, J. A., & Marks, J. D., (2000). Improved Flow and Pressure Capabilities of the Datex-Ohmeda SmartVent Anesthesia Ventilator. Journal of Clinical Anesthesia, 12, 40-47.

[2]. Pelosi et. al. (1998). The Effects of Body Mass on Lung Volumes, Respiratory Mechanics, and Gas Exchange During General Anesthesia.  Anesthesia and Analgesia. 87: 654-660.