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Terminology

Created: 19/2/2005
 
 Tidal Volume
The volume of gas entering or leaving the patient during the inspiratory or expiratory phase time. Normally 7 ml/kg. Measured using spirometry.

 Effective tidal volume
Tidal volume minus dead space volume

 Minute Volume
The sum of all the tidal volumes within one minute.

 Ventilatory rate
The number of cycles per minute

 PEEP
Positive end-expiratory pressure. Used as  an adjunct to ventilation. Produced by maintaining a positive airway pressure during expiration. Usually 5-20cmH20. Results in minimising airway and alveolar collapse and increases complicance, by increasing FRC. Can cause reduced urine output, increased secretion of vasopressin and raised ICP.

 Inspiratory flow time
The period between the beginning and end of inspiratory flow.

 Inspiratory pause time
The period from the end of inspiratory flow to the start of expiratory flow.

 Inspiratory phase time
The period of time between the start of inspiratory flow and the beginning of expiratory flow. The sum of the inspiratory flow and inspiratory pause times.

 Inspiratory: Expiratory phase time ratio
The I:E ratio  is the ratio of the inspiratory phase time to the expiratory phase time. Usually 1:2, allowing recovery from the cardiovascular effects of IPPV during expiration. May be adjusted from 1:1 to 1:4. If expiration is too short there is a risk of air trapping. If expiration time is too long, there may be a resultant increase in dead space. A reversed ratio of up to 4:1 (inverse ratio) may improve oxygenation in respiratory failure, especially in association with PEEP.

 Inspiratory flow rate
The volume of gas per unit time that passes from the patient connection of the breathing system to the patient.

 Expiratory flow rate
The volume of gas per unit of time returned from the patient during the expiratory phase.

 Compliance
The ratio of a change in volume to a change in pressure. It functions as a measure of distensibility. Human lung complaince is about 1.5-2 l/kPa (150-200 ml/cmH2O).
1/total thoracic = 1/chest wall + 1/lung

 Static compliance
Alveolar stretchability (measured at steady state).

 Dynamic compliance
Related to airway resistance during equilibration of gases throughout the lung at end-inspiration or expiration.



Compliance varies within the lung, according to the degree of inflation. The apices are well inflated and therefore require a greater pressure to expand them further.


ArticleDate:20050219
SiteSection: Article
 
   
    
                                            
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