Ventilation Flashcards
What is partial pressure?
The pressure exerted by an individual gas in a mixture of known gases
Expressed in terms of its dry gas concentration
What is fractional concentration ?
The percentage or concentration of a gas in a mixture of known gases
What is minute ventilation ?
VE= VT x f
Volume of air entering the lungs per minute
Expired minute ventilation: volume of expired air per minute, equal to minute ventilation
VT: tidal volume (typically about 500 ml)
f: breathing frequency (typically about 12 breaths/minute)
With typical values:
VE= 0.5 liters x 12 min^-1= 6 L/min
Where is tidal volume distributed?
Between two compartments
1. Dead space (VD) in which gas exchange cannot occur
- Alveolar space where there is gas exchange
Wasted air: does not participates in gas exchange
The VD/VT ratio is usually between 0.25 and 0.35 in healthy individuals
What is dead space (VD)?
Volume of inhaled air that does not participate in gas exchange
Conducting airways - anatomical dead space
Alveoli with no perfusion- (alveolar dead space), value is negligible in healthy individuals
What is the physiological dead space formula?
Anatomical VD + alveolar VD
In healthy individuals Physiological VD = anatomical VD Z
Explain the math behind tidal volume compliance and alveolar ventilation
VT: is distributed in the VA+ VD
VD- volume of air in the dead space
VA- volume of air
Alveolar ventilation-
VA= (VT x f)- (VD x f)
What is alveolar ventilation?
Volume of air delivered to the respiratory zone per minute
The fraction of inspired air that participates in gas exchange
VA= (VT-VD) x breaths/min
VA- alveolar ventilation (ml/min)
VT- tidal volume (ml)
VD- physiological dead spacd(ml)
What 2 ways can dead space be measured?
Anatomical dead space measurement- fowlers method
Physiological dead space measurement- bohrs method
What conditions are dead space in human healthy adults?
Estimated at:
- about 1 ml for each pound of the subjects weight
- Hence wasted ventilation for a healthy 150 pound adult is about 150 ml x 12 /min= 1800 ml/min
Explain the Fowler method
- Inspire 100% O2 after a tidal expiration
- Exhale to RV
- O2 in dead space not used in gas exchange, therefore expired unaltered as pure O2 with no N2
- Gas leaving alveolar region is a mix of O2 and stale air with N2
- Expired N2 volume and concentration are measured
What is the Bohr method?
There is virtually no CO2 in inspired air
- The CO2 in expired air must come from alveoli that are both ventilated and perfused
- Inspired air entering anatomical dead space or unperfused alveoli will exit unchanged
- Clinically it is assumed that the alveolar CO2 and arterial CO2 are the same, so PaCO2 can be used in place of PACO2
VD/VT=1-(PECO2/PaCO2)
Give an example of bohrs method
VD/VT=1-(PECO2/PaCO2)
PaCO2 - arterial PCO2(about 40 mmHg)
PECO2- expired PCO2 (about 28 mmHg)
1-28/40=1-0.7= 0.3
If VT = 500 ml
VD= 0.3 x 500 ml= 150ml
Contrast Fowler and Bohr
Fowler methods between measures anatomical dead space only
Physiological dead space can be measured by the Bohr meth9d
E.g. with pulmonary embolism
- N2 and O2 can come from unperfused alveoli (stale air)
- such stale air will be mixed with expired air throughout alveolar plateau
- hence, Fowler method only measures anatomical dead space, ineffectual for detecting physiological dead space
What does alveolar ventilation depend on?
Alveolar ventilation depends on:
- Depth of breathing (tidal volume)
- Rate of breathing (frequency)
Increasing the depth of breathing is far more effective in elevating alveolar ventilation than is increasing the frequency or rate of breathing
