L4: Hypoxaemia and Hypercapnia

Compare hypoxia to hypoxaemia

• Hypoxia: Reduced availability of O2 at tissues, impairing normal metabolism and potentially leading to cellular death. ​

• Hypoxaemia: Low concentration of O2 in arterial blood (PaO2)

List the common causes of tissue hypoxia

1. Cytopathic: Mitochondrial dysfunction (e.g., septic shock). ​

2. Anaemic: Reduced O2 carrying capacity (e.g., anaemia, carbon monoxide poisoning). ​

3. Stagnant: Low cardiac output or tissue perfusion. ​

4. Hypoxemic: Low PaO2 leading to reduced O2 delivery to tissues

Explain the steps by which the body corrects hypoxia

• Hypoxia induces hypoxia-inducible factor 1α production. ​

• Stimulates EPO mRNA and EPO synthesis. ​

• Increases proerythroblasts and erythrocytes to improve oxygen delivery

List the mechanisms by which hypoxaemia can occur

1. Hypoventilation or low PiO2 (e.g., altitude, airway resistance). ​

2. Diffusion impairment (e.g., membrane thickness, reduced surface area). ​

3. Right-to-left shunt (e.g., cardiac or intrapulmonary shunts). ​

4. Ventilation/perfusion (V/Q) mismatch (e.g., airway obstruction, vascular issues)

Describe how decreases in PiO2 and hypoventilation can lead to hypoxaemia

• Low PiO2: Caused by hypobaric conditions (e.g., altitude) or reduced O2 concentration in inhaled air. ​

• Hypoventilation: Caused by increased airway resistance or decreased pulmonary compliance (e.g., fibrosis)

Describe how a diffusional impairment can lead to hypoxaemia

• Mechanisms: Reduced gas exchange due to increased membrane thickness (e.g., oedema, fibrosis), decreased surface area (e.g., emphysema), or reduced pressure difference. ​

• Impact: Slower oxygen diffusion into pulmonary capillaries, leading to lower PaO2.

Describe how a right-to-left shunt leads to hypoxaemia

• Blood bypasses oxygenation in the lungs, mixing deoxygenated blood with oxygenated blood. ​

• Can be physiological (bronchial circulation) or pathological (e.g., septal defect). ​

  • Severity depends on the proportion of cardiac output shunted

Describe how a ventilation defect impacts oxygen saturation

• Low V/Q ratio: Reduced ventilation due to airway obstruction or lung inflammation. ​

• Results in lower oxygen saturation in affected alveoli.

Describe the mechanisms by which horses experience arterial hypoxaemia and hypercapnia during high-intensity exercise and quantify their relative contributions.

• Mechanisms: ​

  1. Right-to-left vascular shunts (~1%).

  2. V/Q mismatch (25-40%).

  3. Diffusion limitations and/or alveolar hypoventilation (60-75%). ​

• Impact: PaO2 drops from 92-99 mmHg at rest to <70 mmHg at max speed

List factors which should contribute to improved diffusion during exercise.

1. Increased surface area for gas exchange via dilation and recruitment of pulmonary vascular sections (50-60% increase). ​

2. Low PvO2 widens the alveolar-arterial O2 gradient.

Explain the impact of increased cardiac output during exercise.

• 8-fold increase in cardiac output reduces capillary transit time. ​

• Decreased time for O2 equilibration leads to diffusion impairment.

Describe hypercapnia, and list the mechanisms by which it can occur

• Definition: Excess CO2 in the blood. ​

• Mechanisms: ​

  1. Decreased alveolar ventilation (inverse relationship with PaCO2). ​

  2. Severe V/Q mismatch (low V/Q ratio). ​

  3. Increased CO2 production without ventilatory compensation.