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mechanical ventilation. Hashmi. mechanical ventilation method to mechanically assist or replace spontaneous breathing when patients cannot do so on their own currently is positive pressure ventilation

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mechanical ventilation
  • method to mechanically assist or replace spontaneous breathing when patients cannot do so on their own
  • currently is positive pressure ventilation
    • increasing the pressure in the patient's airway and thus forcing additional air into the lungs
  • historically common negative pressure ventilators: "iron lung“
    • creates a negative pressure environment around the patient's chest, thus sucking air into the lungs
  • potential complications
    • pneumothorax, airway injury, alveolar damage, and ventilator-associated pneumonia, etc
iron lung aka Drinker and Shaw tank

developed in 1929

first negative-pressure machines used for long-term ventilation

used in the 20th century largely as a result of the polio epidemic that struck the world in the 1940s

large elongated tank, which encases the patient up to the neck

neck is sealed with a rubber gasket so that the patient's face (and airway) are exposed to the room air

The design of the modern positive-pressure ventilators were mainly based on technical developments by the military during World War II to supply oxygen to fighter pilots in high altitude
  • Positive pressure hrough a tracheostomy/endotracheal tube led to a reduced mortality rate among patients with polio and respiratory paralysis
    • Such ventilators replaced the iron lungs as safe endotracheal tubes with high volume/low pressure cuffs were developed
  • Positive-pressure ventilators work by increasing the patient's airway pressure through an endotracheal or tracheostomy tube
  • positive pressure allows air to flow into the airway until the ventilator breath is terminated
  • then airway pressure drops to zero, and the elastic recoil of the chest wall and lungs push the tidal volume -- the breath -- out through passive exhalation
Common medical indications for use include:
  • Acute lung injury (including ARDS, trauma)
  • Apnea with respiratory arrest, including cases from intoxication
  • Chronic obstructive pulmonary disease (COPD)
  • Acute respiratory acidosis (pCO2 > 50 mmHg and pH < 7.25)
    • which may be due to paralysis of the diaphragm due to Guillain-Barré syndrome, Myasthenia Gravis, spinal cord injury, or the effect of anaesthetic and muscle relaxant drugs
  • Increased work of breathing as evidenced by significant tachypnea, retractions, and other physical signs of respiratory distress
  • Hypoxemia with arterial partial pressure of oxygen (PaO2) with supplemental fraction of inspired oxygen (FiO2) < 55 mm Hg
  • Hypotension including sepsis, shock, congestive heart failure
  • Clinical status of patient
Ventilation delivery
  • Hand-controlled ventilation:
    • Bag valve mask
    • Continuous-flow or Anaesthesia (or T-piece) bag
  • Mechanical ventilator:
    • Transport ventilators
      • small, more rugged, and can be powered pneumatically or via AC or DC power sources.
    • ICU ventilators
    • PAP ventilators
      • non-invasive ventilation. this includes ventilators for use at home, in order to treat sleep apnea
Modes of ventilation
  • volume-cycled
    • pre-set volume of gas with each breath
  • time-cycled
    • limits the length of the inspiratory cycle (I:E ratio)
    • I-time is set as a percentage of total time (usually 33%)
  • pressure-cycled
    • limits the pressure provided with each breath
Breath initiation
  • Assist Control (AC)
    • provides a mechanical breath with either a pre-set tidal volume or pressure
    • same setting every time the patient initiates a breath
  • Synchronized Intermittent Mandatory Ventilation (SIMV)
    • pre-set mechanical breath
    • allows for patients own breathing
  • Controlled Mechanical Ventilation (CMV)
    • ventilator provides a mechanical breath on a preset timing
    • patient respiratory efforts are ignored
    • used in an unconscious patient
tv 500 rr 12

Patient breathing 15

TV: 500 x 12

TV: 500 x 3

Patient breathing 10

TV: 500 x 12


Patient breathing 15

TV: 500 x 12

TV: ??? x 3

Patient breathing 10

TV: 500 x 12

TV: 500 - RR:12
Pressure Support Ventilation (PSV)
    • endotracheal tube airway results in higher resistance to airflow
    • higher work of breathing
    • method to decrease the work of breathing (helps to overcome dead-space)
  • Continuous Positive Airway Pressure (CPAP)
    • continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation, decrease the work of breathing
  • Positive End Expiratory Pressure (PEEP)
    • functionally the same as CPAP
    • refers to the use of an elevated pressure during the expiratory phase
    • volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC)
    • FRC is reduced due to collapse of the unstable alveoli, atelectasis, leading to a decreased surface area for gas exchange and intrapulmonary shunting
    • PEEP can reduce the work of breathing (at low levels) and help preserve FRC
    • PEEP increases intrathoracic pressure, there can be a resulting decrease in venous return and decrease in cardiac output
Non-invasive ventilation
  • Continuous positive airway pressure (CPAP)
    • delivering a stream of compressed air via a hose to a nasal pillow, nose mask or full-face mask
    • splinting the airway (keeping it open under air pressure)
    • unobstructed breathing becomes possible, reducing and/or preventing apneas and hypopneas
    • blows air at a prescribed pressure
  • Bi-level Positive Airway Pressure (BIPAP)
    • Pressures alternate between Inspiratory Positive Airway Pressure (IPAP) and Expiratory Positive Airway Pressure (EPAP)
nasal cannula


1 24%

2 28%

3 32%

4 36%

5 40%

6 44%

venti mask

24% blue

28% yellow

31% white

35% green

40% pink

50% orange

partial rebreather




FIO2: <60%
  • RR: 10-12/min
  • Peep: 5
  • PS: 10
  • TV: 6-8 ml/kg
  • As the amount of tidal volume increases, the pressure required to administer that volume is increased
  • This pressure is known as the peak airway pressure
    • peak airway pressure is persistently above 45 cmH2O for adults, the risk of barotrauma is increased
    • convert to pressure control ventilation
Factors effecting O2




I:E Ratio

Factors effecting CO2



The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index
    • is calculated by dividing the respiratory rate by the tidal volume in liters (RR/TV)
    • A rapid shallow breathing index of less than 100 is considered ideal for extubation
  • other measures such as patient's mental status such be considered

PaO2 of 60 mmHg w/FiO2 of 0.35

Alveolar-arterial PO2 gradient of <350 mmHg

PaO2/FiO2 ratio of >200


Vital capacity >10-15 ml/kg body weight

negative inspiratory pressure < -30 cmH2O

Minute ventilation <10 L/min

Airway occlusion pressure <4-6 cmH2O

Frequency to tidal volume ratio (f/VT) <100 b/min/liter

CROP index >13 ml/breath/min

Integrative index of Jabour et al <4/min

respiratory system compliance

work of breathing

NIF (Negative Inspiratory Force)
    • Amount of force generated by a patient against a closed valve
    • >20cmH2O indicates an adequately strong diaphragm
  • P/F Ratio
    • PaO2/FiO2
    • <200 indicates ARDS
    • <300 indicates ALI
  • RSBI (Rapid Shallow Breathing Index)
    • RR/TV
    • <105 declares a patient can be extubated and maintain themselves
    • indicates patient has a good chance of staying extubated
  • MV (Minute volume or Minute Ventilation)
    • calculated by taking the TV x RR