HYPOXIA

 

Introduction

Hypoxia - deficiency of oxygen in the tissues

Hypoxaemia - low partial pressure of oxygen in the arterial blood

 

ATP ADP

Adenosine triphosphate, ATP

Adenosine diphosphate, ADP

Phosphate units

Adenosine unit

 

Mitochondria

Mitochondrion

100 several thousand

Liver cells 1 700

Cellular respiration

Oxidative phosphorylation

Self replicating

37 genes

 

Hypoxic damage

Aerobic

Anaerobic

Lactic acid

Low cellular pH

Damage to NDA, organelles, cell membranes

Na+ and water swell cells

Injury to lysosomal cell membranes

 

CNS 1 - 4 minute

Myocardial tissue 5 minutes

Kidney 10 20 minutes

Liver 10 minutes.

Skeletal muscles 2 hours

 

Oxygen chain

Oxygen in air (20.84%)

Action of chest wall and diaphragm

Patent respiratory passages

Elastic alveoli

Surfactant

Wall of alveoli and capillary

RBC, haemoglobin

Adequate venous and arterial circulation

Capillary walls

Tissue fluid

Tissue cells

Mitochondria

 

Classification of hypoxia is by its cause

Extrinsic

Pulmonary

Anaemic

Stagnant

Histotic

 

 

 

 

 

1. Extrinsic hypoxia

Low oxygen in atmosphere

HACE

HAPE

Adaptation to altitude

More RBC production

Mitochondria increase

Inadequate oxygenation of lungs for extrinsic reasons

Hypoventilation

Restricted chest and abdomen movement

Chest wall injuries

Flail segment

Pain

Opiate overdose

Neuromuscular blockade

Myasthenia gravis

Guillain-Barre syndrome

Elapid toxicity

Upper airway obstruction

Unconscious patients

Stridor

Complete

Vomiting

Inflammatory swelling

Epiglottitis

Sleep apnoea

 

2. Pulmonary hypoxia

Lower airway obstruction

Increased airway resistance

Alveolar problems

Infections

Reduced membrane transport of oxygen

Pulmonary oedema

Lung collapse

 

3. Anaemic hypoxia

Blood loss

Anaemia

Iron deficiency

Megaloblastic

Pernicious

B12, Foliate deficiency

Chronic haemorrhage

Haemolytic anaemia

Sickle cell

Aplastic

Carbon monoxide

 

4. Stagnant hypoxia

Venous to arterial shunts

Atrial septal defect (foramen ovale)

Patent ductus arteriosus

Sluggish circulation

Congestive cardiac failure

Shock

Localised circulatory deficiency

Ischaemia and infarct

Localised pressure effects

Tissue oedema

 

5. Histotic hypoxia

Poisoning

Vitamin deficiency

Carbon monoxide

Alcohol

Venous blood more oxygenated than usual

 

 

Physiological response to hypoxia

 

Compensatory

Consequences

 

Oxygen delivery from lungs to tissues

Oxygen flux = cardiac output x arterial oxygen saturation x haemoglobin concentration x 1.39 (the volume of oxygen in mls carried by 1g of haemoglobin)

 

Peripheral chemoreceptors

Communicate with the medulla oblongata via the glossopharyngeal and vagus nerves -- induces increased sympathetic outflow -- increased cardiac output, vasoconstriction, increased stimulation of diaphragm and intercostal muscles

 

Central chemoreceptors

Medulla oblongata, CO2 increase only

 

Cardiovascular response

Increase in heart rate and stroke volume -- tachycardia

Peripheral vasoconstriction -- pallor

Increase in BP -- hypertension

Possible sweating

Attempted cardiovascular compensation

 

Respiratory response

Tachypnoea attempt to increase SaO2

Increased respiratory volumes

CO2 increase stimulates medulla oblongata

 

Red cell response

Increases haemoglobin

Erythropoietin mechanism

Altitude

EPO

Polycythaemia

COPD

Congestive heart failure

Renal failure

 

Increased number of capillaries in many tissues

Increased myoglobin

Intracellular changes

 

Consequences

 

Cyanosis

Low PaO2 85%, 8 KPa (60mmHg) and less,

5g reduced haemoglobin per 100 mls of blood

Caused by the presence of deoxyhaemoglobin in the tissues

Peripheral often reduced circulation

Central (tongue) hypoxia

 

 

 

Cerebral hypoxia

> 85% SaO2 mental impairment

> 75% SaO2 severe mental impairment

> 65% SaO2 patient usually unconscious

 

Mild

Depressed mental function, impaired judgement, irritability, restlessness, confusion,

lethargy, excitement, headache, nausea, dizziness, vomiting, fatigue

 

Moderate

Dulled pain sensitivity

 

Severe

Possible convulsions, coma death.

 

Sudden drop in pO2 say to < 20 mm Hg (16,000 m) causes loss of consciousness in about 20 seconds, death in 4-5 minutes.

 

Skeletal muscle

Reduced work capacity of muscle

Extreme fatigue

SOBE

 

Cardiac

Possible bradycardia

Eventually cardiac dysrhythmias and ventricular fibrillation

 

Cerebral oedema

Vasogenic oedema resolves by slow diffusion

Cytotoxic cellular swelling

Ischaemia

Review after 48 hours

 

 

Treatment of hypoxia

 

A and B and oxygen first

Give oxygen at 92% SaO2

Breathlessness is not a criteria

 

Assessment

Recognition

Oxygen saturations, the 5th vital sign

Respiratory rate and depth

Mental state

GCS

Pallor

Cyanosis

Heart rate

Blood pressure

Blood gas analysis

Haematology

ECG

 

Correction of underlying cause

Clear airway

Treat chest injuries

Treat lung disease

Correct anaemia

Improve circulation

Give vitamin Bs

 

Improve blood oxygen saturations

Target saturations in illness 94 98%

Carbon dioxide retainers 88 92%

Adjunct airways

Upright posture

Oxygen therapy

Aids to ventilation

Spontaneous breathing - CPAP

Ventilated - PEEP

 

Improve cardiac output

Treat cause of cardiovascular problem

Intravenous fluids for shock

Management of CCF

 

Improve haemoglobin levels

Blood transfusions

Blood forming foods

Iron

B12

Folic acid,

EPO

 

 

Indications for oxygen therapy

 

High concentrations

Give 15 l / min until target saturations are reached, then maintain

Cardiac arrest or resuscitation

Shock

Sepsis

Major trauma

Major head injury

Near-drowning

Anaphylaxis

Major pulmonary haemorrhage

Carbon monoxide poisoning

 

Moderate concentrations

Undiagnosed hypoxaemia

Acute asthma

Pneumonia

Lung cancer

Postoperative breathlessness

Acute heart failure

Pulmonary embolism

Pleural effusions

Pneumothorax

Interstitial lung disease

Severe anaemia

Sickle cell crisis

 

Low concentration

COPD

Exacerbation of cystic fibrosis

Chronic neuromuscular disorders

Morbid obesity

 

Respiratory drive

Hypoxic drive

Hypercapnia drive

 

 

 

Questions

 

Where in the cell is oxygen used

 

Recite a useful quote about hypoxia

 

List 3 causes of stagnant hypoxia

 

List 4 causes of pulmonary hypoxia

 

Why may oxygen saturation reading be unreliable in a burns patient

 

What 2 factors can cause tachypnoea

 

Give 3 causes of anaemic hypoxia

 

What causes cyanosis

 

Give 3 cardiovascular responses to significant hypoxia

 

What hormone will be released in response to chronic hypoxia