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
Explain the alveolar ventilation equation based on PA/a CO2
VA= VECO2 x 0.863/ PACO2
VA- alveolar ventilation
VECO2- expired volume of CO2 in 1 minute (200 ml)
Constant has units mm Hg x L/ ml, hence VA has units L/min
PACO2- partial pressure of alveolar CO2
Note: clinically PaCO2 is used to determine PACO2 since PaCO2 is almost never diffusion limited
VA and Pa/ACO2 are…
Inversely proportional
What are the clinical effects of hyperventilation?
Alveolar ventilation is usually evaluated in terms of arterial PCO2
Increased alveolar ventilation beyond that required to meet metabolic needs is termed hyperventilation
Hyperventilation leads to hypocapnia (PaCO2 is below normal)
Decreased alveolar ventilation below that required to meet metabolic rate needs is termed hypoventilation
Hypoventilation leads to decrease in PaO2 and hypercapnia (elevated in PaCO2)
What are the factors determining PAO2?
Factors that determine PAO2 are:
PB: alveolar O2 is increased with increased atmospheric pressure (hyperbaric chamber) and decreased when there is lowered atmospheric pressure (high altitude)
FIO2: is 21% in ambient air, this can be altered in the clinical setting when supplemental O2 is given
Metabolic activity: high metabolic activity uses oxygen and would push PAO2 down
Ventilation: increasing ventilation elevates PAO2
What are the normal values of A-a gradient?
Normal values: 5-15 mmHg
What does a high A-a O2 gradient indicate?
A value of more than 5 is usually associated with hypoxemia
Elevated A-a implies high respiratory effort to achieve oxygenation, I.e. a pulmonary problem
Low oxygenation due to hypoventilation (I.e. extra-pulmonary source of hypoxemia) would not affect A-a gradient
What is hypoventilation?
- decreased alveolar ventilation with increased PaCO2
- From GREEk hypo-means under
What is hyperventilation?
-increased alveolar ventilation with decreased PaCO2
What is hypercapnia?
- increased carbon dioxide in blood (PaCO2 above 48 mmHg)
- Greek ‘kapnos’ means smoke
What is hypopnea?
Decreased ventilation in response to lowered metabolic CO2 production
What is hyperpnea?
Increased ventilation in response to increased metabolic CO2 production (e.g. exercise), which should purge CO2
What is Eupnea?
- normal breathing
- from Greek eu- well
And pnoia-breath
What is tachypnea?
Increased frequency of breathing (‘swift’ breath). Ventilation may or may not change depending on tidal volume
What is Dyspnea?
Shortness of breath/ labored breathing (‘ill’ breath)
What is Apnea?
-temporary cessation of breathing (‘a’ prefix= without)
