Oxygen therapy
Oxygen therapy can be normobaric or hyyperbaric. During oxygen therapy, ventilation and airway maintenance should be adequate so that oxygen reaches the lung for gas exchange. Reserve of oxygen in the body is 1.5 litres, which lasts for about 6 minutes in circulatory arrest assuming a consumption of 250 ml/min. Hb contains 800 ml and alveoli contains about 400 ml of oxygen. Pre-oxygenation prior to induction of anaesthesia leads to denitration and increase in the alveolar oxygen content, enabling tolerance of longer period of apnea.
Circulatory gradient of oxygen: Oxygen partial pressure decreases gradually from the alveoli to the blood and finally to the mitochondria.
Pasteur point: Critical PO2 below which mitochondrial oxygen transport cannot occur (less than 1 mm Hg).
Global oxygen delivery (oxygen flux): Total amount of oxygen delivered to the tissue.
Oxygen extraction ratio: how much oxygen is extracted at tissue level – normal extraction is about 20-30%. It can increase in situations of oxygen deficit.
Oxygen dissociation curve: Shift to right of the oxygen dissociation curve means oxygen affinity to Hb is decreased so that oxygen delivery to tissue is increased. The opposite occurs in a shift to left of the oxygen dissociation curve.
Hypoventilation: decreases arterial PO2 and increases arterial PCO2.
Ventilation perfusion mismatch: either normally perfused, but poorly ventilated or normally ventilated and poorly perfused.
Right to left shunts: Physiological right to left shunt contributes about 2-3% desaturation. Pathological shunts can occur in lung diseases, inctracardiac shunts and extracardia shunts like patent ductus arterious.
Diffusion impairment: No limitation in normal subjects. Exercise induced diffusion abnormality in patients with lung disease.
Role of oxygen therapy in different types of hypoxia:
Supplemental oxygen is useful in hypoxic hypoxia. Oxygen therapy may be useful in anemic hypoxia along with correction of the abnormality. Supplemental oxygen is useful along with other modalities of treatment in stagnant hypoxia also. But it is not useful in histotoxic hypoxia.
Oxygen therapy systems: Low flow system and high flow systems are available.
Low flow systems: Nasal prongs and catheters, face masks, mask with reservoir bags. Usually they are used to deliver 2-4 litres of O2 per minute. 3-4% of FiO2 increase can be expected with one litre per minute flow rate. Low flow systems are inexpensive and easy to use.
Face masks can supply FiO2 of 0.4 – 0.6. Addition of reservoir to face mask increases reservoir capacity to 750 – 1250 ml and increases the FiO2 levels to 0.6 – 0.8. Partial rebreathing and non-rebreathing masks are also available, depending on the type of valves. In non-rebreathing mask, if the bag collapses completely during inspiration, the oxygen flow rate is inadequate.
High flow systems (Venturi device)
Delivers constant FiO2. There is colour coding of masks for the FiO2 achievable and the oxygen flow rate recommended.
Monitoring oxygen therapy
Patient clinical status – symptomatic improvement
Measurement of ABG and SPO2
Oxygen therapy in special situations
COPD: Hypercapnia is a problem which can occur due to loss of hypoxic drive for ventilation. Never withold oxygen therapy for fear of abolishing hypoxic drive.
Oxygen toxicity
Can occur with 100% oxygen for 12 hrs, 80% oxygen for 24 hrs or 60% oxygen for 36 hrs.
All these are in those with normal lung – can occur with lower levels in those with lung disease. A safe level for those with normal lung is 50%. If there is no response with these safe levels, addition of other modalities like CPAP have to be considered.
Home oxygen therapy devices:
Compressed cylinders, oxygen concentrator and liquid oxygen cylinders are the devices available for home oxygen therapy.
Indication for long term oxygen therapy: Hypoxemia at rest (PaO2 less than 55 mm Hg), borderline hypoxemia or hypoxemia with exercise.
