A mechanical ventilator is a Positive or Negative pressure breathing device that can maintain ventilation and oxygen delivery for a prolonged period . Mechanical Ventilation may be required for a variety of reasons including :
- the need to control the patient`s respiration during surgery or treatment of severe head injury
- to oxygenate the blood when the patient`s ventilatory efforts are inadequate
- to rest the respiratory muscles .
Indication for Mechanical Ventilation
- If a patient has a continues decrease in oxygenation (Pao2) or a persistant increase in Paco2 and persistant acidosis ( decrease pH) , mechanical ventilation may require.
- A patient with apnea i.e not reversible also require mechanical ventilation .
- Conditions such as abdominal or thoracic surgery, neuromuscular disorders , drug overdose,multiple trauma , shock , multisystem failure ,coma and inhalation injury all leads to respiratory failure and the need for mechanical ventilation
- Pao2 < 50mm Hg with Fio2 > 0.60
- Pao2 > 50 mm Hg with pH < 7.25
- Vital capacity < 2 times tidal volume
- Negative inspiratory force < 25 cm H2o
- Respiratory rate > 35 / min
Types of ventilators
1. Negative Pressure ventilators
- Negative pressure ventilators are mainly used in chronic respiratory failure associated with neuromuscular conditions such as poliomyelitis, muscular dystrophy and mysthenia gravis .
- They do not require intubation of the airway , they are specially adaptable for home use .
Types of Negative Pressure ventilators
1. Iron lung ( Drinker respirator Tank )
2. Body Wrap ( Pneumowrap ) and Chest cuirass ( Tortoise shell )
2. Positive pressure ventilators
Pressure cycled ventilators
- The pressure cycled ventilators ends inspiration when preset pressure has been reached or the ventilators cycles on delivers a flow of air until it reaches predetermined pressure then cycles off .
- In adults, pressure cycled ventilators are intended only for short term use. The most common type is IPPB machine .
Time Cycled ventilators
- Time cycled ventilators terminate or control inspiration after a preset time .
- The volume of air the patient recieves is regulated by lentgh of inspiration and flow rate of the air
- Time cycled ventilators are used in newborns and infants .
Volume Cycled Ventilators
- Volume cycled ventilators are most common used ventilators
- The volume of air to be delivered with each inspiration is preset. Once the preset volune is delivered to the patient , the ventilation cycles off and exhalation occurs passively.
Noninvasive positive pressure ventillation
- In this positive pressure ventilation can be given via facemask (NIV Mask ) that covers nose and mouth
- This does not require intubation
- The most comfortable mode for the patient is pressure controlled ventilation with pressure support .
- Bilevel positive airway pressure ( bi- PAP ) ventilation offers independent control of inspiratory and expiratory pressure while providing pressure support ventilation .
- Bi- PAP is most common used for patients who require ventilatory assistance at night such as with severe COPD or sleep apnea .
Ventilator Modes
Ventilator mode chart
Modes of ventillation | Description |
Controlled ventilation
| A Pre-set volume of air is delivered to the patient under positive pressure with spontaneous patient respiratory effort . |
Assist/ Controlled Ventilation ( A/C ) | A pre-set volume of air is delivered to the patient at a preset rate , but patient may trigger a ventlator breath with negative inspiratory efforts |
Synchronized Intermittent Manditory Ventillation ( SIMV ) | A pre-set minimum no. Of breath are synchronously delivered to the patient but the patient may also take spontaneous breath of varying volumes . · Note how inspiratory and expiratory pressure differ between spontaneous and ventilator breaths |
Positive End Expiratory Pressure (PEEP ) | Airway presure with varying levels of positive end expiratory presure ( PEEP ) · Note that at end expiration , the airway is not allowed to return to zero · Positive pressure is exerted during the expiratory phase of ventilation which improves oxygenation by enhancing gas exchange and preventing atelectesis |
Continues Positive airway pressure (CPAP ) | This ventilatory mode is used only with spontaneous ventilation . · The patient breaths spontaneously through the ventilation at an elevated baseline pressure through the breathing cycle |
Pressure support (PS ) | The patient breaths spontaneously with presure assistance to each spontaneous inspiration |