Oxygen saturation
Oxygen delivery depends on:
- Cardiac output
- Haemoglobin concentration
- Arterial oxygen saturation
Arterial oxygen saturation (SaO2) depends on arterial oxygen tension (PaO2)
The oxygen dissociation curve
- A = arterio-venous oxygen content difference
- Note that due to the non-linear nature of the curve B is greater than A
Pulse oximetry
- Arterial oxygen saturation can be measured non-invasively using pulse oximetry
- Consists of two light emitting diodes of two different wavelengths
- Frequencies are in the red (660 nm) and infrared (940 nm) spectrum
- Monitor has one photodetector
- Absorption spectrum of haemoglobin at the two frequencies depends on degree of oxygenation
- Allows calculation of oxygen saturation
- Arterial component of circulation is targeted by restrict analysis to signal that is pulsatile
- Reading unreliable if:
- Intense vasoconstriction
- Jaundice
- Methaemoglobinaemia
Respiratory failure
- Hypoxaemic failure (type 1) = Reduced PaCO2 + reduced PaO2
- Ventilatory failure (type 2) = Increased PaCO2 + reduced PaO2
Pathophysiology of respiratory failure
Failure of oxygenation (hypoxaemia, type I failure)
- Low inspired oxygen partial pressure
- Alveolar hypoventilation
- Diffusion impairment
- Ventilation to perfusion mismatch
- Right-to-left shunt
Failure of ventilation (hypercapnia, type II failure)
- Abnormalities of central respiratory drive
- Neuromuscular dysfunction
- Abnormalities of the chest wall
- Abnormalities of the airway
- Abnormalities of the lung
Causes of respiratory failure
|
Ventilatory failure
|
Hypoxaemic failure |
| Deranged mechanics |
Collapse |
| COPD |
Consolidation |
| Chest trauma |
Contusion |
| Respiratory depression |
Pulmonary oedema |
| Drugs |
Pulmonary embolus |
| Trauma |
|
| Raised intracranial pressure |
|
| Spinal cord lesions |
|
| Cervical spine trauma |
|
| Motor neurone disease |
|
| Peripheral neuropathy |
|
| Myasthenia gravis |
|
| Muscle relaxants |
|
Artificial ventilation
Indications for tracheal intubation
- Facilitation of mechanical ventilation
- Protection from aspiration
- Facilitation of tracheobronchial suction
- Relief of upper airway obstruction
Indications for mechanical ventilation
- Support in respiratory failure
- Coma (head injury, drug overdose)
- Control of intracranial pressure
- Reduction of metabolic demands
- Allow muscle relaxation and facilitate surgery
- Postoperative ventilation
Modes of ventilation
- Most ventilators are volume/time-cycled with a pressure limit
- Deliver preset tidal volume irrespective of lung compliance
- Pressure limit reduces risk of over-inflation
- Possible modes in which they can be used are:
- Controlled mechanical ventilation
- Assisted controlled or triggered ventilation
- Intermittent mandatory ventilation
- Pressure support
Ventilator variables
- Variables on a ventilator that can be preset or altered include:
- Tidal volume
- Ventilation rate
- Inspiratory to expiratory ratio
- Flow waveform
- Partial pressure of inspired oxygen
- Pressure limit
- Positive end expiratory pressure (PEEP)
- Positive airway pressure (CPAP)
Benefits of ventilation
- Eliminates carbon dioxide
- Improves oxygenation by:
- Reducing respiratory work and oxygen consumption
- Administering a higher inspired oxygen content (FiO2)
- Preventing or reversing atelectasis
Complications of ventilation
- Problems associated with endotracheal tube
- Trauma
- Obstruction
- Misplacement
- Disconnection
- Barotrauma
- Pneumothorax
- Surgical emphysema
- Impaired venous return
- Sodium and water retention
- Bronchopneumonia
Bibliography
Marshal R P. Current strategies for mechanical ventilation in acute lung injury. Hosp Med
2000; 61: 678-679.
Shelly M P, Nightingale P. Respiratory support. Br Med J 1999; 318: 1674-1677.
Tobin M J. Advances in mechanical ventilation. N Eng J Med 2001; 344:
1986-1996.
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