alveolar ventilation and gas equation

Question 1

describe the point of alveolar gas equation in determining gas exchange

Answer 1

PAO2 (ideal) = PIO2 - (PACO2)/R

• when calculated, can be used to PA-PaO2
• if the difference between the 2 is more than 10mmhG that means the ARTERIAL BLOOD (supposed to be oxygenated), is inadequately oxygenated

= indicates impaired gas exchange

Question 2

explain the time constant and describe different scenarios

Answer 2

time constant = filling time of the lungs

time constant = compliance x resistance

1. if COMPLIANCE decreases: time constant unchanged, but lungs filled INCOMPLETELY
2. if compliance same but RESISTANCE increases = reduced time constant
3. if both compliance reduce and resistance increase: fills slowly AND incompletely

= relates to DEGREE OF FILLING and SPEED of filling

https://derangedphysiology.com/main/cicm-primary-exam/required-reading/respiratory-system/Chapter%20034/time-constants

Question 3

what are the 3 factors that determines the alveolar gas equation

Answer 3

1. metabolism (VO2, VCO2)
2. inspired gas partial pressure (0 Co2, 21kPa 150mmHg O2)
3. alveolar ventilation rate (hypo/hyperventilation)

Question 4

what is the difference between hyperpnoea and hyperventilatoin

Answer 4

1. hyperpnoea: increased ventilation matches the increased metabolic demand
2. hyperventilation does not match increased metabolic demand

Question 5

what is the ventilation capacity
its estimated value and
what are some factors that affects it x5

Answer 5

ventilatory capacity: is the maximum volum of air moved in/out of the lungs per minute

estimated: 15-20X (160-200L) of normal ventilation, older people’s is about 30% less than younger people

factors affecting:

1. lung compliance and its factors
2. airway resistance and its factors
3. STRENGTH of respiratory muscles: diaphragm and intercostal muscles (vital capacity)
4. pathologies
5. lung SIZE, age, gender

Question 6

what is the % used of the ventilatory capacity in normal breathing

Answer 6

5%

Question 7

what is ventilatory reserve? roughly how many % left of ventilation capacity?

Answer 7

Ventilatory reserve = ventilation capacity - ventilation at MAXIMAL exercise

maximal exercise:
respiratory frequency can increase to 70breaths per minute
tidal volume can increase to 50% of vital capacity: ie 2L

ventilatory reserve: 30% of ventilation capacity

cardiac: 5-6X = limiting factor