respiratory ventilation physiology

Introduction

respiratory ventilation is generally closely controlled to adjust breathing rate and depth to keep oxygen, carbon dioxide, and, blood and CSF pH stable
- inadequate ventilation results in hypercapnia and respiratory acidosis
- excessive ventilation (hyperventilation) results in low carbon dioxide levels, resp_alkalosis and electrolyte shifts causing low phosphate levels, hypokalaemia, etc

this estimates an alveolar gas partial pressure of oxygen given a specified inspired oxygen and a blood carbon dioxide as a marker of ventilation
this equation is often used to ascertain the A-a gradient which is the calculated PAO₂ - actual arterial PaO₂ (normal for a healthy adult is 5-10mmHg)
where:
- PAO₂: Alveolar partial pressure of oxygen (mmHg)
- FiO₂: Fraction of inspired oxygen (typically 0.21 for room air)
- P_atm: Atmospheric barometric pressure (typically 760 mmHg at sea level)
- PH₂O: Water vapor pressure (typically 47 mmHg at body temperature)
- PaCO₂: Arterial partial pressure of carbon dioxide
- RQ: Respiratory quotient (ratio of CO₂ produced to O₂ consumed, typically 0.8)

PAO₂ = (FiO₂ x (P_atm - PH₂O) ) - (PaCO₂/RQ) which approximates to = 150 - (PaCO₂/0.8) for room air

A-a gradient = calculated PAO₂ from the alveolar gas equation - actual arterial PaO₂

normal ranges vary with age and FiO2:
- in room air, generally increases by 1mmHg for every 10 yrs of age
- for a healthy adult is 5-10mmHg in room air (may be 14mmHg in the elderly)
- on 100% inspired oxygen, for a healthy adult is about 31mmHg (may be 56mmHg in the elderly)

hypoxia with normal A-a gradient suggests either:
- raised PaCO₂:
  - hypoventilation with hypercapnia
- normal or low PaCO₂
  - low inspired oxygen pressures
    - high altitude or hypoxic environment
hypoxia with raised A-a gradient suggests either:
- V/Q mismatch eg. pulmonary embolism (PE), asthma
- right to left shunt eg. intracardiac, pulmonary
- increased oxygen extraction
  - elevated metabolic activity
    - physical exercise
    - hyperthyroidism
  - decreased oxygen delivery
    - anaemia
    - low blood flow to tissues eg. the shocked patient
- diffusion defect (less common)

medulla oblongata:
- contains the dorsal respiratory group (DRG) for inspiration and the ventral respiratory group (VRG) for rhythm and forced breathing.
pons:
- fine-tunes the transition between breathing in and out via the apneustic and pneumotaxic centers

Central chemoreceptors:
- located in the medulla; sense changes in brain fluid pH caused by carbon dioxide
Peripheral chemoreceptors:
- located in the carotid and aortic bodies; detect large drops in blood oxygen, high carbon dioxide, and acid levels
- the oxygen response is dependent upon local dopamine release which is also dependent upon the MECP2 gene, and this is defective in various conditions such as:
  - Rett syndrome (mutation of MECP2 gene which is on the X chromosome) - these patients have periods of apnea which can be followed by gasping and hyperventilating, and then apnea again, in an extreme cycle of unstable breathing¹⁾
Mechanoreceptors:
- found in the lungs and joints; protect against over-inflation (Hering-Breuer reflex) and signal movement during exercise

Respiratory Muscles:
- signals travel down the spinal cord via the phrenic nerve to contract the diaphragm and via intercostal nerves to the intercostal muscles.
Higher Brain Centers:
- the cerebral cortex allows voluntary overrides for speaking, singing, or holding breath, while the hypothalamus and limbic system adjust breathing for emotions and pain.

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