CO₂ in arterial blood crosses the blood brain barrier in CSF, CO₂ combines with H₂O produces H⁺ + HCO₃^–.

           •    H⁺ acts in central chemoreceptors causing hyperventilation via vagus nerve stimulation.

           •   CSF pH causes hyperventilation which decreases CO₂ in blood.

Peripheral chemoreceptors

           •   They are located at bifurcation of common carotid arteries above and below the aortic arch. They act through three different mechanisms.

           •   arterial pO₂ (<60 mm Hg) activation of chemoreceptors hyperventilation.

           •   arterial pO₂ partial inactivation of chemoreceptors normal ventilation.

           •   arterial H⁺ activation of carotid bodies at bifurcation hyperventilation to correct pH.

Lung stretch receptors

           •   Located in smooth muscles of airways.

           •   Stimulated by lung distention slows inspiration (Hering- Breurer reflex).

Irritant receptors

           •   Located between epithelial cells of airways.

           •   Stimulated by particulates.

           •   Cause reflex bronchoconstriction coughing and RR.

Juxtacapillary receptors

           •   Located in alveolar walls next to capillaries.

           •   Any change in size of these capillaries causes activation of these receptors, e.g., pulmonary edema.

           •   Cause RR.

Gas Exchange

Hypoxemia

           •   PO₂ caused by inspired O₂, hypoventilation, diffusion defects, ventilation to perfusion () mismatch, and R to L shunt.

           •   The degree of hypoxemia can be calculated by using A-a gradient, which is the difference between alveolar oxygen pressure (PAO₂) and arterial oxygen pressure (PaO₂).

A-a gradient = PAO₂ – PaO₂

 

Equation:

PAO₂ = PiO₂ – PACO₂/R

PiO₂= FiO₂ (Patm – 47mmHg) 0.21(760 – 47) 0.21(713) 150 mmHg

PAO₂ = 150 mmHg – (PACO₂ x 1.25) mmHg

Where PiO₂ is inspired oxygen pressure, PACO₂ is alveolar CO₂ pressure (PACO₂ = PaCO₂), R is respiratory exchange ratio (R ≈ 0.8) and Patm is the atmospheric pressure (760 mmHg at sea level).

PaO₂ is taken from the ABG result

 

           •   Normal A-a gradient is <10 mmHg

           •   Abnormal A-a gradient is >10mmHg

           •   An abnormal gradient indicates that gas exchange in the alveolar-capillary unit is abnormal.

           •   Hypoxemia with normal gradient: inspired O₂, hypoventilation.

           •   Hypoxemia with abnormal gradient:

                       Diffusion impairment– increased diffusion distances such as thick basement membranes

                        mismatch – reduced alveolar ventilation in relationship to perfusion. Diseases include emphysema, bronchitis, asthma, interstitial lung disease.

                       Shunts – Alveoli collapse or alveolar filling with exudates, fluid or blood or true anatomic shunt. Examples include ARDS, CHF, hemorrhage, atelectasis, lobar pneumonia.

           •   To differentiate between mismatch and shunt, place patient on supplemental oxygen. With mismatch the PaO₂ increases; with R to L shunt there is no change in PaO₂. Patients with diffusion defects usually have hypoxemia during periods of increased cardiac output.

Hypoxemic Respiratory failure (PaO₂ < 55 mm Hg or SaO₂ <88%):

           •   Decreased inspired PO₂

                       Low PiO₂ at high altitudes

                       Low PiO₂ in fires

           •   Diffusion limitation

           •   Ventilation perfusion abnormalities

                       Emphysema

                       Bronchitis

                       Asthma

                       Interstitial lung disease

           •   Intra-cardiac or intra-pulmonary anatomic shunts

                       Pulmonary embolism

                       Pneumonia

                       Heart Failure

                       ARDS

                       Atelectasis

                       Hemorrhage

Hypercapnia

           •    PCO₂ secondary to inadequate alveolar ventilation

           •   Causes

                       Decreased minute ventilation associated with neuromuscular disease, normal A-a O₂ gradient.

                       Abnormal relation associated with parenchymal lung disease, abnormal A-a O₂ gradient.

                       Rarely increased CO₂ production with a fixed minute ventilation.

Hypercapneic respiratory failure (PaCO₂ > 50 mmHg):

           •   COPD

           •   Asthma

           •   CNS infection or neoplasm

           •   Airway obstruction

           •   Paralysis of the diaphragm due to neurologic disorders

           •   Effect of anesthetic and muscle relaxant drugs

Lung Mechanics

Airway Resistance

Can be calculated by Ohm’s law or Poiseuille’s law.

Ohm’s law- R= /

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