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Oxygen and Ventilation

Some Terminology

  • Hiflow / NIV                    –  combined gases with greater than 15 litres flow, Non-Invasive Ventilation
  • Humidification
    • Essential                 –  H2O present or supplemented in environmental gases
    • Absolute                  –  H2O available when gas is at maximum saturation
  • NIPPV                                –  Non-Invasive Positive Pressure Ventilation (applied via mask interface)
  • Entrainment                    –  mixing of room air with supplied O2 by the passing flow of gas
  • Interface                           –  any delivery system attached to the human face (mask / nasal / hood)
  • Heater Unit / Pot            –  hot water reservoir to produce water vapour / humidification
  • Washout                           –   to remove something by use of flow
  • Mixer / Blender             –   device to mix O2 and air to achieve desired percentage

NOTE  this subsection “oxygen & ventilation”comprises separate 6 areas that work best if you read them all in order building towards safely caring for invasively ventilated patients and understanding why we do what we do!

BREATHING BASICS to start we need to cover…

Gas partial pressures

Gas Atmospheric air Alveolar air Exhaled air
O2 21%

159 mmHg


104 mmHg


120 mmHg

N 78%

597 mmHg


569 mmHg


566 mmHg

CO2 0.04%

0.3 mmHg


40 mmHg


27 mmHg

H2O 0.5%

4 mmHg


47 mmHg


47 mmHg


Wherever possible we aim to decrease any added oxygen to achieve a SaO2 of ≥94% or as indicated in the patients ‘Treatment Plan’,  excessive oxygen has been shown to be detrimental and cause injury in many processes from stroke to ARDS.   The individual SaO2 target for the patient should always be documented on your ICU care plan

It is a sign of the respiratory systems efficiency that of the 21% O2 we usually breathe in we absorb only a small component and breath out 16% O2

Nitrogen is essential, amongst other things, for normal surfactant and alveoli function and this is one of the reasons we constantly seek to get the patient off 100% O2 to return nitrogen into the gas mixture as soon as safe to do so

Signs of insufficient oxygenation (hypoxia) include;
decreased SaO2, cyanosis, tachypnoea, confusion, panic, sweating, restlessness, tachycardia, arrhythmias, death etc

So the symptoms can show or mimic many other processes – Acute MI, PE, sepsis etc (look up the 4 types of hypoxia on NU2ICU Definitions page)


How we breathe

Moving gas in and out of this…


The Mechanics of Breathing

Inspiration – an active process

  • diaphragm lowers
  • ribs pivot upwards
    • intercostal muscles between ribs contract
  • intra-thoracic pressure lowers
    • intrapleural pressure is normally 4mmHg lower than atmospheric pressure, ‘sucking’ the lungs outwards to the ribcage
  • lung expands
    • as volume increases, pressure decreases  (Boyle’s law)
  • air flows from the higher atmospheric sea level pressure (760mmHg) into the lower pressure of the lungs (758mmHg)


Expiration – a passive process

  • inspiratory muscles relax
    • ribs move downwards
    • diaphragm relaxes and its domes rise
  • surface tension of alveolar fluid causes an inward pull
  • elastic recoil of alveolar basement membranes
  • reverse pressure gradient
    • 762mmHg in lungs, 760mmHg atmospheric
  • gas is pushed out


Control of Breathing

Respiratory center is the reticular formation of the brain stem

  • medullary rhythmicity centre
    • controls basic rhythm of respiration
    • inspiratory (predominantly active) and expiratory (usually inactive in quiet respiration) neurons
    • drives muscles of respiration
  • pneumotaxic area
    • inhibits inspiratory area
  • apneustic area
    • stimulates inspiratory area, prolonging inspiration


Regulation of respiratory center

  • chemical regulation
    • central and peripheral chemoreceptors
    • most important factor is CO2 (and pH)
      • ­ in arterial CO2 causes ­ in acidity of cerebrospinal fluid (CSF)
      • ­ in CSF acidity is detected by pH sensors in medulla
      • medulla ­ rate and depth of breathing
    • cerebral cortex
      • voluntary regulation of breathing
    • inflation reflex
      • stretch receptors in walls of bronchi/bronchioles

Image –


External (pulmonary) respiration

  • exchange of O2 and CO2 between respiratory surfaces and the blood (breathing)

Exchange of O2 and CO2 between alveoli and blood

  • partial pressure of O2 higher in alveoli (105mmHg) than blood (40mmHg) so O2 diffuses into blood
  • partial pressure of CO2 higher in blood (45mmHg) than alveoli (40mmHg), so CO2 moves into alveoli in opposite direction and gets exhaled out


Internal respiration

  • exchange of O2 and CO2 between the blood and cells

Exchange of O2 and CO2 between blood and tissues

  • pressure of O2 higher in blood than tissues so O2 gets release into tissues.
  • pressure of CO2 higher in tissue than in blood so CO2 diffused in opposite direction into blood.

CO2 is a waste product

O2 is used in cellular respiration


  • External respiration

Measured by ABG





  • Internal respiration

Measured by VBG

  • Cellular respiration is the process by which cells use O2 to produce ATP

Image –


Oh’s Intensive Care Manual 7th Edition 2014 by Andrew Bersten, Neil Soni

The ICU Book 4th Edition 2015 by Paul L Marino

Pilbeam’s Mechanical Ventilation: Physiological and Clinical Applications 6th Edition by J M Cairo

ACCCN’s critical care nursing 2nd Edition 2012 by Doug Elliott, Leanne Aitken and Wendy Chaboyer  accessed 3rd April 2018


More reading