Thorax 2002;57:258-262 doi:10.1136/thorax.57.3.258

Effect of assist negative pressure ventilation by microprocessor based iron lung on breathing effort

1. M Gorini,
2. G Villella,
3. R Ginanni,
4. A Augustynen,
5. D Tozzi,
6. A Corrado

1.  

   Respiratory Intensive Care Unit, Careggi Hospital, Firenze, Italy

1. Correspondence to:
   Dr M Gorini, Via Ragazzi del 99 60, 50141 Firenze, Italy;
   mgorini@qubisoft.it

• Accepted 27 September 2001
• Revised 10 August 2001

Abstract

Background: The lack of patient triggering capability during negative pressure ventilation (NPV) may contribute to poor patient synchrony and induction of upper airway collapse. This study was undertaken to evaluate the performance of a microprocessor based iron lung capable of thermistor triggering.

Methods: The effects of NPV with thermistor triggering were studied in four normal subjects and six patients with an acute exacerbation of chronic obstructive pulmonary disease (COPD) by measuring:
(1) the time delay (TDtr) between the onset of inspiratory airflow and the start of assisted breathing;
(2) the pressure-time product of the diaphragm (PTPdi); and
(3) non-triggering inspiratory efforts (NonTrEf).

In patients the effects of negative extrathoracic end expiratory pressure (NEEP) added to NPV were also evaluated.

Results: With increasing trigger sensitivity the mean (SE) TDtr ranged from 0.29 (0.02) s to 0.21 (0.01) s (mean difference 0.08 s, 95% CI 0.05 to 0.12) in normal subjects and from 0.30 (0.02) s to 0.21 (0.01) s (mean difference 0.09 s, 95% CI 0.06 to 0.12) in patients with COPD; NonTrEf ranged from 8.2 (1.8)% to 1.2 (0.1)% of the total breaths in normal subjects and from 11.8 (2.2)% to 2.5 (0.4)% in patients with COPD. Compared with spontaneous breathing, PTPdi decreased significantly with NPV both in normal subjects and in patients with COPD. NEEP added to NPV resulted in a significant decrease in dynamic intrinsic PEEP, diaphragm effort exerted in the pre-trigger phase, and NonTrEf.

Conclusions: Microprocessor based iron lung capable of thermistor triggering was able to perform assist NPV with acceptable TDtr, significant unloading of the diaphragm, and a low rate of NonTrEf. NEEP added to NPV improved the synchrony between the patient and the ventilator.

Overview

Although clinical studies suggest that negative pressure ventilation (NPV) provided by iron lung can be as effective as invasive mechanical ventilation for the treatment of severe acute respiratory failure in patients with chronic obstructive pulmonary disease (COPD), negative pressure ventilators are actually considered second line choice for non-invasive ventilatory assistance for several reasons, including the fact that, traditionally, NPV is a controlled mechanical ventilation—that is, the device provides a fixed number of breaths per minute irrespective of the patient's own breathing pattern.

If the mechanical and spontaneous respiratory cycles are not matched, however, the patient “fights” the ventilator, resulting in discomfort and excessive respiratory muscle effort.

Airway pressure or flow signals are generally used in positive pressure ventilators to detect inspiratory efforts of patients and to trigger the mechanical breath (assist and assist-control ventilation).

Unlike positive pressure ventilation during NPV, the airway opening is free and, as a consequence, it is not possible to monitor continuously airway pressure and flow and to use these signals to trigger mechanical breath. The lack of patient triggering capability during NPV may contribute not only to poor patient synchrony, but also to induction of upper airway collapse due to the lack of coordinated activation between upper airway muscles and inspiratory muscles.

We have recently shown that a prototype microprocessor based iron lung was able to improve the ventilatory pattern and arterial blood gas tensions and to unload inspiratory muscles in patients with an acute exacerbation of COPD.

This study was undertaken to evaluate the performance of the thermistor triggering system used to deliver assist NPV with this new model of iron lung.