top of page

The Basics

Acute respiratory failure (ARF) is the inability of the respiratory system to ventilate and/or oxygenate. 

Type  I  ARF: Hypoxaemia (paO2 < 60mmHg)

Type II  ARF: Hypercapnia (pCO2 > 45mmHg)

Type III ARF: Postoperative respiratory failure caused by atelectasis and other factors

Type IV ARF: Respiratory failure in shock through inadequate O2 delivery and excessive loading of breathing muscles

Global Respiratory Failure (GRF) refers to a state of combined Type I and Type II ARF


Dependent on 

   - Mean Airway Pressure (MAP)

   - FiO2 (21 - 100%)

MAP (mPaw) is the sum of all individual airway pressures divided by time. It is represented by the area under the pressure curve (AUC).

It can be increased by

   - increasing PEEP

   - increasing peak inspiratory pressure (PIP)


   - increasing inspiratory time (Ti)


   - increasing inspiratory flow / upstroke


   - increasing the rate (more time spent
     in inspiration)

Effect of Respiratory Rate (RR) on Mean Airway Pressure (MAP)
Effect of Inspiratory Time (Ti) on Mean Airway Pressure (MAP)
Effect of PEEP on Mean Airway Pressure (MAP)
Effect of Peak Inspiratory Pressure (PIP) on Mean Airway Pressure (MAP)
Effect of Inspiratory Ramp on Mean Airway Pressure (MAP)

Respiratory rate


Inspiratory time (Ti)



Effects on Mean airway pressure (AUC)

The extra area under the curve (AUC) adds to mean airway pressure (MAP) and therefore oxygenation.

alveolar gas equation

Alveolar Gas Equation
Bildschirmfoto 2019-11-10 um

Normal lungs in room air:

Alveolar Gas Equation

alveolo-arterial gradient

Grab an arterial gas and look your current ventilator settings to calculate!

Ventilator Settings: SIMV-VC, RR 20, TV 200mls, MAP 18, FiO2 30%, etCO2 40

ABG: pH 7.40, CO2 40, paO2 90

> pAO2 = 0.3 x (760-47) - (40/0.8) = 163.9mmHg

>> A-a Gradient = 163.9 - 90 = 73.9mmHg

EXAMPLE for A-a gradient calculation

Respiratory Quotient

Describes how much CO2 is produced for every O2 that is consumed. 

Depends on the patient's diet.

Balanced diet           > RQ ~ 0.82

Fat only                     > RQ ~ 0.7

Protein only.             > RQ ~ 0.81

Carbohydrates only > RQ ~ 1.0

The higher the RQ the more CO2 needs to be eliminated by the patient! I your patient is critically unwell, a high CH diet might tip him / her over the edge!

1) Hypoventilation

2) Pulmonary venous desaturation (V/Q-Mismatch)

3) Systemic venous desaturation

3) Diffusion restriction / Diffusion block

Learn more by clicking on the buttons.

Differential for Desaturation in patients without parallel circulation


During conventional ventilation the expiratory minute volume is the product of tidal volume and respiratory rate. The expiratory tidal volume is used in this case as the inspiratory tidal volume can be falsely high in case of leakage.  

Bildschirmfoto 2019-10-06 um
Bildschirmfoto 2019-10-06 um

Dead Space

Numa and Newth shoed in 1985 that there is a relationship between anatomic dead space and age. It can be derived from the following formula:

DStotal = 3.28 - 0.56 [ln(1 + Age)]

Dead space represents the volume of ventilated air that does not participate in gas exchange.


There are four types of dead space:

- Anatomical dead space

- Physiologic dead space

- Alveolar dead space

- Mechanical dead space 


- Physiologic DS = Anatomical DS + Alveolar DS

(which is the volume of air in the respiratory zone that does not take part in gas exchange)

Koebi 3.png
Anatomical Dead Space

Anatomical Dead Space

The anatomical dead space consists of all structures that are part of the respiratory "channel" system but do NOT participate in gas exchange. 

This includes the nasal and oral cavity, the larynx and pharynx, trachea, bronchi, bronchioles down to the terminal bronchioles. 

Alveolar Dead Space

The respiratory zone is comprised of respiratory bronchioles, alveolar duct, alveolar sac, and alveoli. 

Part of the respiratory zone that is ventilated but not perfused is considered alveolar dead space. 

Asset 23.png

Mechanical Dead Space

Mechanical dead space is generated by all equipment (mask, tubing, adaptors,...) that are installed in line with the gas flow. 

Total dead space = Physiological dead space + Mechanical dead space 

Dead Space

Alveolar ventilation

Alveolar ventilation (VA) equals VT minus physiologic dead space (VD) multiplied by RR. From this equation, clinicians can determine that the total volume gas inspired is not being fully utilized in the gas exchange due to the constant anatomical dead space

Screen Shot 2019-11-01 at 2.13.29 pm.png
Screen Shot 2019-11-01 at 2.13.17 pm.png

Bohr equation

Screen Shot 2019-11-01 at 2.26.13 pm.png
Screen Shot 2019-11-01 at 2.26.21 pm.png

Used to calculate the fraction of dead space ventilation. 

The lower the PeCO2 the more dead space ventilation. The extreme of this is cardiac arrest where there is no blood flow to the lungs resulting in 100% dead space. 

bottom of page