Mechanical Ventilation Introduction

     
       

 

         
       

Respiratory failure is caused by failure to ventilate, characterized by increased arterial carbon dioxide tension, or failure to oxygenate, characterized by decreased arterial oxygen tension.
The treatment for failure to ventilate is to increase the patientís alveolar ventilation, that is the rate and depth of breathing, either by reversing the cause or by using mechanical ventilation; invasively or non-invasively.
Failure to oxygenate may occur as a result of decreased alveolar oxygen tension (due to decreased inspired O2 tension or increased CO2 tension), reduced O2 diffusion capacity (due to interstitial edema or fibrosis, or thickened alveolar walls) or ventilation perfusion mismatch (due to loss of functional residual capacity and alveolar collapse/consolidation). The treatment for failure to oxygenate is restoration and maintenance of lung volumes, using recruitment maneuvers and increased baseline airway pressures (PEEP/CPAP). Imposed ventilatory workload is increased by loss of lung compliance and inspiration/ventilation is usually supported to reduce O2 requirements and increase patient comfort. 

Modes of ventilation describe the primary method of inspiratory assistance. A machine generates and regulates the flow of gas into the lungs, flow continues until a predetermined volume has been delivered or airway pressure generated. Flow reverses, when the machine cycles into the expiratory phase, the message to do this is either a preset time, preset tidal volume or a preset percentage of peak flow. Mechanical breaths may be controlled (the ventilator is active and the patient passive) or assisted (the patient initiates and may or may not participate in the breath).

Large tidal volumes overstretch alveoli and injure the lungs (1). Small tidal volumes increase the contribution to minute ventilation of dead space. The science of mechanical ventilation is to optimize pulmonary gas exchange; the art is to achieve this without damaging the lungs.

Residents are usually baffled by the variety of different ventilatory modes, and the fortitude of their seniors in arguing the relative merits of one over another. In truth, there is little evidence that there is an ideal mode of ventilation. Remember, for any patient, it is the same pressure-volume curve (click here)  that we are pushing the patient up, what differs is the way in which gas flow achieves this. In this tutorial, I have added graphics of screen captures from mechanically ventilated patients, so that you may begin to appreciate this interaction of flow, airway pressure and delivered volume.

Learning Objectives

  • To understand the principles behind delivery of gas in inspiration
  • To differentiate different modes of ventilation
  • To learn simple waveform patterns

References

   (1)    Dreyfuss D, Saumon G. Ventilator-induced lung injury: lessons from experimental studies. Am J Respir Crit Care Med 1998; 157(1):294-323.

         
                   
       

         
     

       
       

Please note: these tutorials are for personal study purposes only.  They are not currently peer reviewed, and no responsibility will be taken for mistakes or inaccuracies. Reproduction of information is forbidden. All material is copyrighted by the GasWorks Group.