HYPOXIA Patients (Definition, Causes and Prevention)
It is the deficiency of an adequate supply of oxygen to the body tissues or cells.
It is the total lack of oxygen to tissues.
Hypoventilation is inadequate breathing leading to an increase of Carbon dioxide (hypercapnia) and hypoxaemia.
Apnoea means cessation of breathing in expiration.
CLASSIFICATION OF HYPOXIA
- Hypoxic Hypoxia: reduced oxygen entering the blood.
- Hypaemic/anaemic hypoxia: reduced capacity of blood to carry oxygen.
- Stagnant hypoxia: poor oxygenation due to circulation
- Histotoxic hypoxia: inability of cells to use oxygen.
Postoperative causes (usually hypoxic hypoxia)
- CNS depression, e.g. post-anaesthesia
- Airway obstruction, e.g. aspiration of blood or vomit, laryngeal oedema.
- Poor ventilation, e.g. abdominal pain, mechanical disruption to ventilation
- Loss of functioning lung, e.g. V/Q mismatch (pulmonary embolism, pneumothorax, collapse/consolidation)
- Central respiratory drive depression, e.g. opiates, benzodiazepines, CVA, head injury, encephalitis.
- Airway obstruction, e.g. facial fractures, aspiration of blood or vomit, thyroid disease or head and neck malignancy.
- Neuromuscular disorders (MS, myasthenia gravis)
- Sleep apnoea (obstructive, central or mixed)
- Chest wall deformities
- Carboxyhaemoglobinaemia, methaemoglobinaemia
KEY DETAILS ABOUT HYPOXIA
- 80% of patients following upper abdominal surgery are hypoxic during the first 48 hours postoperatively. Have a high index of suspicion and treat prophylactically.
- Adequate analgesia is more important than the sedative effects of opiates – ensure good analgesia in all postoperative patients.
- Ensure the dynamics of respiration are adequate – upright position, abdominal support, humidified oxygen
- Acutely confused (elderly) patients on a surgical ward are hypoxic until proven otherwise.
- Pulse oximetry saturations 85% equate to an arterial Po2 8kPa and are unreliable in patients with poor peripheral perfusion.
In the Unconscious Patient
- Central Cyanosis
- Abnormal Respirations
In the Conscious Patient
- Central Cyanosis
- Anxiety, restlessness and confusion
- Tachycardia, dysrhythmias (AF) and hypotension
- Pulse oximetry saturations: monitors the percentage of haemoglobin that is saturated with oxygen – gives a guide to arterial oxygenation.
Very useful for patient monitoring
- Arterial blood gases (Pco2 Po2 pH base excess): respiratory acidosis, metabolic acidosis later.
- Chest X-ray: collapse/pneumothorax/consolidation.
- ECG: AF.
- Triple airway manoeuvre (mouth opening, head extension and jaw thrust), suction secretions, clear oropharynx.
- Consider endotracheal intubation in CNS depression/exhausted patients (rising Pco2), neuromuscular failure.
- Consider surgical airway (cricothyroidotomy/minitracheostomy) in facial trauma, upper obstruction.
- Position patient – upright
- Adequate analgesia
- Supplemental oxygen – mask/bag/ventilation
- Support respiratory physiology – physiotherapy, humidified gases, encouraging coughing, bronchodilators.
- Maintain cardiac output
- Ensure adequate fluid resuscitation
Determine and treat the cause.
EFFECT ON CELLS
Hypoxia causes the production of transcription factors (hypoxia – inducible factors; HIFs).
These are made up of alpha and beta subunits.
In normally oxygenated tissues, the subunits are rapidly destroyed.
However, in hypoxic cells, the factors dimerize with the beta subunit
The dimmers activate genes that produce angiogenic factors and erythropoietin
CAUSES OF HYPOXIA
Carbon Monoxide Intoxication
Hypoxia Secondary to High Altitude
Hypoxia Secondary to Right-to-Left Extrapulmonary Shunting
Specific Organ Hypoxia
Increased Oxygen Requirements
Improper Oxygen Utilization
CAUSES OF HYPOXIA
- Hypoxic Hypoxia
- Inadequate oxygenation of blood in the lungs due to extrinsic causes
- Deficiency of oxygen in the atmosphere – Altitude; Mines
- Hypoventilation – Neuromuscular disorders; fatigue and depression of RC
TYPES OF HYPOXIA
There are four types of hypoxia such as
- Hypoxic Hypoxia
- Anaemic Hypoxia
- Stagnant/Ischemic Hypoxia
- Histotoxic Hypoxia
Hypoxia/anoxia: it denotes a partial or complete lack of oxygen respectively, in one or more tissues of the body, including the blood stream.
Asphyxia: it is the state in which pulmonary or placental gas exchange is affected leading to progressive hypoxemia, which is severe enough to be associated with acidosis.
Ischemia: it is a reduction in or cessation of blood flow that arises from systemic hypotension, cardiac arrest, or occlusive vascular disease.
It is due to reduced oxygen tension in arterial blood (Supply Problem)
- Low oxygen tension in the inhaled air.
- Leaking mask, inadequate oxygen regulator function, and faulty hose connections.
- Impaired gas exchange in the lungs. E.g. Chronic Bronchitis and Emphysema
- Gross Ventilation/Perfusion Mismatch, as occur in high G forces
It is due to decreased oxygen carrying capacity of the blood (Transport Problem)
Causes: CO POISONING, CHEMICALS/SULPHA DRUGS AND HAEMORHAGE/HEMOLYSIS AND ANAEMIA
It occurs when blood circulation through tissue is reduced. (Distribution Problem)
Causes: High G Forces, Syncope (Fainting), Heart Failure and Shock
It is due to inability of the tissues to make use of the oxygen supplied to them (Utilization Problem)
Cyanide Poisoning – Cytochrome Oxidase
Alcohol and Barbiturate
Oxygen Toxicity – 100 % Oxygen for 8 to 10 hours
Inhibition of certain enzymes (Cytochrome Oxidase) Hampering Oxygen Metabolism
- Decreased cardiac output/sluggish blood flow due to heart failure, hemorrhage, circulatory shock and venous obstruction.
- Blood remains in tissues for longer time, so tissue extracts increased oxygen from blood – more arteriovenous difference of oxygen concentration.
- So PC02 increases, it facilitates unloading of oxygen from hemoglobin (shifts the oxy-hemoglobin association dissociation curve to right).
Three factors that develop the sensation of dyspnea:
- Abnormality of respiratory gases in body fluids (mainly hypercapnia and partly hypoxia)
- Work of ventilation by respiratory muscles
- State of Mind (Neurogenic/emotional dyspnea)
CAUSES OF HYPOXIA IN FLIGHT
- Ascent to altitude without supplemental oxygen.
- Breathing a gas mixture poor in oxygen
- Exposure of high G Forces
- Failure of oxygen equipment
- Loss of cabin pressurization
FACTORS INFLUENCING HYPOXIA
- Rate of ascent
- Duration of exposure to altitude
- Individual tolerance
- Physical fitness
- Psychological stresses
- Physical Activity
TIME OF USEFUL CONSCIOUSNESS
- Time interval between the reduction of oxygen tension in the inspired gas and the individual’s effective performance
- It is time available to the individual to take remedial action following the onset of hypoxia
SIGNS AND SYMPTOMS OF HYPOXIA
- Indifferent Stage
- Compensatory Stage
- Disturbance Stage
- Critical Stage
SIGNS AND SYMPTOMS OF HYPOXIA
- Indifferent Stage – (0 to 10,000 feet breathing air, 40,000 feet on 100% oxygen)
- No symptoms usually
- Fatigue on long exposure
- Night vision is impaired
- Short term memory starts deteriorating at an altitude of 5000 feet
- Compensatory Stage: (10 to 15,000 feet breathing air, 40000 to 42000 feet on 100% oxygen)
- Little or no symptoms at rest
- When exposure is prolonged or physical activity is undertaken then headache, dizziness are experienced.
- Respiratory and CVS (Compensatory Response come into Play)
- Disturbance Stage: (15 to 20,000 feet breathing air, 42-45000 feet on 100% oxygen)
- Sign and symptoms develop even at rest
- Loss of will power
- Impairment of judgment
- Thinking is slowed
- Calculation unreliable
- Reaction time slows down
- Euphoric and Elated
- Impairment of muscular coordination
- Visual acuity decreased
- Tough and pain sensation decreased, hearing also decreased but it is last to go
- Critical Stage: (Above 20,000 feet breathing air, Over 45000 feet on 100% oxygen)
- All symptoms become severe even at rest
- Mental functions decline rapidly
- Unconsciousness generally comes on with little or no warning
- Unconsciousness occurs when oxygen tension falls below 30 mmHg
- If exposure is prolonged, death can occur due to cardiopulmonary failure
ALTITUDE TIME OF USEFUL CONSCIOUSNESS
22,000 10 MINUTES
25,000 5 MINUTES
28,000 2.5 TO 3 MINUTES
30,000 1.5 MINUTES
35,000 0.5 TO ONE MINUTE
40,000 15 SECONDS
65,000 9 SECONDS
PREVENTION OF HYPOXIA
- Increasing concentration/pressure of oxygen
- Cabin pressurization
- Price check before lowering canopy
P = PRESSURE
R = REGULATOR
I = INDICATOR
C = CONNECTIONS
E = EMERGENCY OXYGEN SYSTEM
- Periodic check of following systems
Regulator – all functions
Cabin Pressurization Systems
Pressure Suits (Anti G Suit)
- All Switches up – On, 100% Oxygen
- Mask On
- Check Regulator and Connections
- Control Breathing
- Notify Aircraft Commander