MECHANICAL VENTILATORS

INTRODUCTION

Aim of all ventilatory support systems

  • Achieve adequate O2 delivery AND elimination of CO2 with minimal complications

Difference between spontaneous ventilation and mechanical ventilation

Spontaneous ventilation

Mechanical ventilation

Airway pressure

 Negative  Positive

Feedback control

 Continuous/Accurate
(except in disease)		 Limited by machine and operator

Work of breathing

 Minimal in health
Increased in disease		 None if no spontaneous breathing allowed
Can be imposed by circuit design/valves etc if spontaneous ventilation occurs

Active manipulation of gas exchange

 Minimal Possible

Complications

 Minimal High
  • Barotrauma, hypotension (acutely)
  • Atelectasis, infection (chronically)

Physiological variables governing ventilation

  • Tidal volume
  • Respiratory rate
  • Inspiratory time
  • Expiratory time

 

  

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 or expressed as I:E ratio  

SETTINGS

    The essential variables that need to be set are:

  • Tidal Volume - VT
  • Respiratory rate - RR
  • FiO2 - Fractional inspired oxygen concentration (21%-100%)

Control of tidal volume - volume or pressure control?

Ventilators can either control VT either by controlling the:

  • volume of gas that you wish to be given (volume control) or
  • pressure required to produce a given tidal volume (pressure control)  

The advantages and disadvantages of either system will not be discussed here

Tidal volume and Respiratory Rate - how big? how fast?

  • Physiological VT  is around 500ml with a RR of 15 giving a minute volume MV of 7.5L/min
  • Mechanical ventilation requires a larger VT   e.g. 700-1000ml to prevent atelectasis (areas of lung collapse)
  • Provide a MV to achieve a normal pCO2
  • Excessive MV results in barotrauma or hypotension
  • Inadequate MV results in hypercarbia +/- hypoxia

Permissive hypercarbia - small volumes/rate

  • Use when impossible to normalise pCO2  without requiring airway pressures e.g. severe obstructive/restrictive lung disease e.g. severe asthma or ARDS
  • allow pCO2 > 100mHg 
  • extreme respiratory acidosis has minimal physiological consequence
  • contraindicated in head injury
  • excessive pCO2 may cause measurable hypoxaemia by displacing oxygen in alveoli

see also Ventilation in asthma

WEANING

  • Weaning can be thought of gradually handing back the responsibility of breathing from the ventilator to the patient
  • Involves allowing the patient to breathe between machine breaths AND/OR applying some kind of assistance to breaths generated by the patient
  • The simplet method is to remove the patient from the ventilator and spontaneously breath through the endotracheal tube (T-piece trial)
  • More sophisticated methods have also been developed including:
  • IMV (Intermittent Mandatory Ventilation)
  • PSV (Pressure support ventilation).

IMV/SIMV

  • Allows the patient to breathe in between machine breaths or trigger a machine breath.  In Synchronised IMV, machine breathes are withheld when the patient attempts to breathe (preventing 'breath-stacking')
  • Setting the SIMV rate sets the minimum number of full machine breaths (patient initiated or not)
  • Weaning involves gradually reducing the SIMV rate  e.g. 10 and reduced gradually down to 5.

PSV

  • Requires the patient to initiate a breath.  If the patient does not breathe, nothing will happen. 
  • PSV applies a set pressure to the airway at the time the patient begins to breathe thereby assisting the patient's effort (i.e. the machine blows gas at the same time the patient suck it in)
  • Can be combined with SIMV to ensure a minimum number of machine initiated breaths (from the SIMV)
  • BiPAP involves PSV (called the IPAP) via a tight-fitting mask rather than endotracheal tube.   
  • Weaning involves gradually reducing the pressure support e.g 25cm H2O down to 10 cm H2O

SPECIAL FUNCTIONS

There are various other settings which are helpful but not essential to mechanical ventilation. 

PEEP (Positive end-expiratory pressure ventilation)

  • commonly used 
  • prevents lungs collapsing at the end of expiration
  • can improve oxygenation without increasing FiO2 

I:E ratio, Inspiratory TI and expiratory time TE 

  • Increasing TI increases mean airway pressure and oxygenation
  • Excessively short TE may cause dynamic collapse of airways and gas trapping (with decreased elimination of carbon dioxide)
  • For any given RR, there is a balance between the effects of changing the ratio of TI  to TE 
  • Asthmatics have trouble expiring air quickly and require a low I:E ratio (i.e. long expiratory time) so that they have chance to empty their lungs.
 
Disclaimer: Note: The information described here relates solely to operational matters in the Emergency Department.  Every effort has been made to verify the accuracy of the content. However, neither the author or the hospital will take responsibility for errors resulting from its use.  Please refer to your own departmental guidelines and verify all clinical decisions with a reliable source.  

Date Last Reviewed: 05/08/2004