Respiratory (series Iii)

• The volume of air in the conducting airways ( nose, pharynx, trachea, bronchi, bronchioles where there is no gas exchange is called the anatomical dead space  ~154 ml in 70 kg man) (usually estimated ~ 2.2 ml per kg body weight

• The volume of air in the conducting airways ( nose, pharynx, trachea, bronchi, bronchioles where there is no gas exchange is called the anatomical /anatomic dead space

B

B

• Alveolar dead space • Volume of air in alveoli that are ventilated but not perfused & does not take part in gas exchange

B

 Normally negligible

B

• Physiological dead space (VD)

B

= VANA + VALV

• Normal VALV is negligible VD ~ VANA

B

Partial pressure of gases Atmospheric air (dry) ~ 21% O2, 0.03% CO2, 79% N2 The pressure of each gas in a gas mixture is its partial pressure (P) - Dalton’s law

Inspired air (dry) Expired air (dry) 10

• Normal partial pressure values for an adult, resting, breathing room air at sea-level • PO2 (partial pressure of O2) in dry inspired air at sea level = 21/100 (20.94%) x Patm = 0.21 x 760 mm Hg

• Tracheal air PO2? (PO2 inspired air in trachea / airways) • Air is warmed to body temp. & saturated with water vapour as it passes thro’ airways

• PO2 inspired air in trachea / airways : = 21/100 x (Patm - 47) = 0.21 x (760-47) = 149 mm Hg H2O vapour pressure at 37o C

• High altitude →  ↓ Patm • % of O2 in air  ~ 21% • PO2 of tracheal air ↓ (0.21 (Patm- 47 mmHg)

• PAO2 (PO2 of alveolar air) • PACO2 (PCO2 of alveolar air)

Note: A refers to alveolar a refers to arterial

Alveolar gas equation PAO2 = PIO2 – PACO2) R

PIO2=0.21/100 x (Patm – 47) • R = respiratory exchange ratio (VCO2/VO2) • Resting : ~ 0.8 (200 ml/min/250 ml/min) • PACO2 = PaCO2

• Gases dissolved in blood exert partial pressures • PO2 of mixed venous blood (in pulmonarry artery)  (PvO2) : 40 mmHg • PCO2 mixed venous blood  (PvCO2) : 46 mmHg

• PaO2 (PO2 of arterial blood) : ~ 100 mm Hg (young adults)

• PaCO2 (PCO2 of arterial blood ): = 40 mm Hg

Note: a refers to arterial NOT arteriole or atrial

Normal VA (~ 4L/min) → normal: • PAO2 • PACO2 • PaO2 • PaCO2

VA = (TV – VANA) X f

Hypoventilation • VA inadequate to meet the metabolic needs for O2 delivery & CO2 removal

 ↑ PACO2  ↑ PaCO2 (hypercapnia) → acidosis  ↓PAO2  PaO2 ↓ (hypoxaemia)

Hyperventilation • VA in excess of metabolic needs for CO2 removal)

 ↓ PaCO2 (hypocapnia) → alkalosis  ↓ PACO2  ↑ PAO2  ↑ PaO2 Hyperventilation & hypo. mostly associated with changes in PaCO2

Some causes of HYPOVENTILATION •

Inhibition of respiratory centres in brainstem eg. drug overdose, brain damage

•

Paralysis/ weakness of respiratory muscles

•

Lung diseases: airway obstruction, ↓ CL

•

Reduced chest wall mobility

Some causes of HYPERVENTILATION 2. High altitude (hypocapnia at high altitude) 3. Anxiety, hysteria 4. Brain infection 5. Fever 6. Metabolic acidosis 7. Hormones: adrenaline, progesterone, thyroxine

v. thin alveolarcapillary membrane (alveolar wall & capillary wall – one cell thickness

Note: O2 diffuses from alveolar air into pulmonary capillary blood NOT arterial blood)

Resting: 250 ml O2/min (VO2) used/min & 200 ml CO2/min (VCO2) is produced

•O2 diffusing capacity (DLO2) refers to the volume of O2 that diffuses thro’ the alveolar-capillary membrane each min. for a partial pressure difference of 1 mmHg • Resting condition DLO2 ~ 25ml/min/mmHg • Exercise: DLO2 ↑

Factors affecting DLO2

Factors affecting DLO2

Exercise → ↑ CO causes recruitment & distension of pulmonary capillaries → ↑ S.A for diffusion →↑ vel. blood flow → ↑ partial P gradient bet. alveolar air & pulm. capillary blood

↑ DLO2

↑ CO → recruitment & distension of pulmonary capillaries --

-

Factors affecting DLO2

Hb conc.

DLO2 ↓ in anaemia

Mechanisms or physiological causes of hypoxemia (↓↓PaO2)

Diffusion impairment at the alveolar-capillary membrane →

↓↓ DLO2

Hypoxaemia (PaO2 ↓↓)

Hypoxia (lack of O2 at the tissues)

Mechanisms or physiological causes of hypoxemia (↓↓PaO2)

•

Diffusion impairment at the alveolar-capillary membrane

(↓↓ DLO2) ∀

↓ PO2 of inspired air eg.  At high altitude →  Low PAO2 →  ↓ PaO2

Mechanisms or physiological causes of hypoxemia (↓↓ PaO2) •

Diffusion impairment at the alveolar-capillary membrane

(↓

↓ DLO 2 ) ∀

↓ PO2 of inspired air eg.  At high altitude →  Low PAO2 →  ↓ PaO2

•

Alveolar hypoventilation e.g.  Depression of respiratory centre  Airway obstruction→  Low PAO2 →  ↓ PaO2

Mechanisms or physiological causes of hypoxemia (↓↓ PaO2) •

Diffusion impairment at the alveolar-capillary membrane (↓ ↓ DLO2)

∀

↓ PO2 of inspired air eg.  At high altitude →  Low PAO2 →  ↓ PaO2

•

Alveolar hypoventilation e.g.  Depression of respiratory centre  Airway obstruction→  Low PAO2 →  ↓ PaO2 Right-to-left shunt (mixing of oxygenated & deoxygenated blood)

•

Mechanisms or physiological causes of hypoxemia (↓↓ PaO2) •

Diffusion impairment at the alveolarcapillary membrane (↓↓ DLO2)

∀

↓ PO2 of inspired air eg.  At high altitude →  Low PAO2 →  ↓ PaO2

•

Alveolar hypoventilation e.g.  Depression of respiratory centre  Airway obstruction→  Low PAO2 →  ↓ PaO2 Right-to-left shunt (mixing of oxygenated & deoxygenated blood)

• •

V/Q imbalance/mismatch - most common physiological cause of hypoxaemia