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RESPIRATORY FAILURE
Definition and Classification:
• Defined as failure of gas exchange due to inadequate function of one or more of the
essential components of the respiratory system.
• Classified as hypoxemic type I (PaO2 < 60 mmHg, or <8 kpa), hypercarbic type II
(PaCO2 > 45 mmHg or >6.6 kpa), or combined.
• Also classified in terms of acuity—acute respiratory failure diagnosis is made on the
basis of decrease in PaO2 by 10–15 mmHg from baseline or increase in PaCO2
associated with a pH ≤ 7.30. reflects a sudden catastrophic deterioration, chronic
respiratory failure reflects long-standing respiratory insufficiency, and acute or chronic
respiratory failure is an acute deterioration in a patient with chronic respiratory failure,
usually due to chronic obstructive lung disease. The normal (Pao2) is between 11 and 13
kPa (83 and 98 mmHg). The (Paco2) is 4.8-6.0 kPa (36-45 mmHg
).
Respiratory failure: underlying causes and blood gas abnormalities
Type I
Type II
Hypoxia (PaO
2
< 8.0 kPa (60
mmHg)) Normal or lowPaCO
2
(<
6.6 kPa (50 mmHg))
Hypoxia (PaO
2
< 8.0 kPa (60 mmHg))
RaisedPaCO
2
(> 6.6 kPa (50 mmHg))
Acute
Chronic
Acute
Chronic
H
+
→ or ↑
→
↑
→ or ↑
Bicarbonate →
→
→
↑
Causes
Acute asthma
Emphysema
Acute severe asthma COPD
Pulmonary
oedema
Lung fibrosis
Acute
exacerbation
COPD
Sleep apnoea
Pneumonia
Lymphangitis
carcinomatosa
Upper
airway
obstruction
Kyphoscoliosis
Lobar collapse
Right-to-left
shunts
Acute
neuropathies
/paralysis
Myopathies/muscular
dystrophy
Pneumothorax
Brain-stem lesion Narcotic drugs
Ankylosing spondylitis
Pulmonary
embolus
Primary
alveolar
hypoventilation
ARDS
Flail chest injury
Management
Prompt diagnosis and management of the underlying cause is crucial to the
management of patients with respiratory failure. Occasionally, rapid reversal of the
precipitating event-e.g. tracheostomy for laryngeal obstruction, fixation of ribs in a flail
chest injury, reversal of narcotic poisons, nebulised bronchodilators in acute severe
asthma or tube drainage of a tension pneumothorax-will restore good gas exchange. In
acute left ventricular failure, in massive pulmonary embolism and when pulmonary
infarction or pneumonia is the cause of pleural pain, treatment with opiates is entirely
appropriate, but these drugs depress respiratory drive and should never be used in
asthma or COPD, except immediately prior to and during assisted mechanical
ventilation.
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Oxygen therapy
Oxygen therapy improves hypoxaemia by increasing alveolar PO
2
in poorly ventilated
lung units. Normally, high-flow oxygen (35-60%) is appropriate treatment in respiratory
failure (e.g. severe asthma, pulmonary oedema or pneumonia) because respiratory drive
is high. A small percentage of patients with severe chronic COPD and type II
respiratory failure develop abnormal tolerance of raised CO
2
and may become
dependent on hypoxic drive to breathe. In these patients only, lower concentrations of
oxygen (24-28%) may be needed to avoid precipitating worsening respiratory
depression.
Toxic effects of oxygen
100% oxygen is both irritant and toxic if inhaled for more than a few hours. Premature
infants develop retrolental fibroplasia and blindness if exposed to excessive
concentrations. In adults, pulmonary oxygen toxicity (manifested by pulmonary oedema
and free radical damage leading ultimately to fibrosis) would not be expected to occur
unless the patient had been treated with inappropriately high concentrations of oxygen
for more than 24 hours.
Monitoring of response to therapy
In patients with acute respiratory failure, close monitoring is essential and arterial blood
gases taken on presentation should be repeated within 20 minutes to establish that
treatment has achieved acceptable PaO2 levels. If hypoxia persists despite appropriate
oxygen therapy, progressive hypercapnia (PaCO2 > 6.6 kPa (50 mmHg)) with acute
respiratory acidosis develops or the patient becomes exhausted, an early decision
should be made about whether it is appropriate to support ventilation temporarily by
means of non-invasive ventilation or formal intubation and mechanical ventilation.Very
ill patients may require immediate ventilatory support on presentation.
chronic and 'acute on chronic' type II respiratory failure
:
The most common cause of chronic type II respiratory failure is COPD. Here CO
2
retention may occur on a chronic basis, the acidaemia being corrected by renal retention
of bicarbonate, which results in the plasma pH remaining within the normal range. This
'compensated' pattern, which is also seen in some patients with chronic neuromuscular
disease or kyphoscoliosis, is maintained until there is a further pulmonary insult, such
as an exacerbation of COPD which precipitates an episode of 'acute on chronic'
respiratory failure. The further acute increase in PaCO
2
results in acidaemia and
worsening hypercapnia, and may lead to drowsiness and eventually to coma. The
principal aim of treatment in acute on chronic type II respiratory failure is to achieve a
safe PaO
2
(> 7.0 kPa (52 mmHg)) without increasing PaCO
2
and acidosis, while
identifying and treating the precipitating condition. These patients usually have severe
pre-existing lung disease, and only a small insult may be required to tip the balance
towards severe respiratory failure. Moreover, in contrast to acute severe asthma, a
patient with 'acute on chronic' type II respiratory failure due to COPD may not feel
overtly distressed despite being critically ill with severe hypoxaemia, hypercapnia and
acidaemia.
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Assessment and management of 'acute on chronic' type ii respiratory failure
Initial assessment
N.B. Patient may not appear distressed despite being critically ill
·
Conscious level (response to commands, ability to cough)
·
CO
2
retention (warm periphery, bounding pulses, flapping tremor)
·
Airways obstruction (wheeze, prolonged expiration, hyperinflation, intercostal
indrawing, pursed lips)
·
Cor pulmonale (peripheral oedema, raised JVP, hepatomegaly, ascites)
·
Background functional status and quality of life
·
Signs of precipitating cause
Investigations
1-Arterial blood gases (severity of hypoxaemia, hypercapnia, acidaemia, bicarbonate)
2-Chest X-ray
Management
·
Maintenance of airway
·
Treatment of specific precipitating cause
·
Frequent physiotherapy ± pharyngeal suction
·
Nebulised bronchodilators
·
Controlled oxygen therapy
o
Start with 24% controlled-flow mask
o
Aim for a PaO
2
> 7 kPa (52 mmHg) (a PaO2 < 5 (37 mmHg) is dangerous)
·
Antibiotics
·
Diuretics
Progress
If PaCO
2
continues to rise or patient cannot achieve a safe PaO
2
without severe
hypercapnia and acidaemia, respiratory stimulants (e.g. doxapram) or mechanical
ventilatory support may be required. Doxapram (1.5-4 mg/min) by slow intravenous
infusion should only be used as a respiratory stimulant where non-invasive ventilation
is not available or is poorly tolerated, or in those with reduced respiratory drive. Even in
these circumstances this agent provides only minor and transient improvements in
arterial blood gas parameters.
mechanically assisted ventilation
Patients with initially severe respiratory failure (type I or type II) or those who fail to
improve despite optimal medical therapy may require mechanical ventilation.
lung transplantation
Lung transplantation is now an established treatment for carefully selected patients with
advanced lung disease unresponsive to medical treatment. Single-lung transplantation
may be used for older patients with emphysema and patients with intrapulmonary
restrictive disorders such as lung fibrosis. It is contraindicated in patients with chronic
bilateral pulmonary infection, such as cystic fibrosis and bronchiectasis, where bilateral
lung transplantation is the favoured option. Combined transplantation of the heart and
lungs remains necessary for the treatment of patients with advanced congenital heart
disease such as Eisenmenger's syndrome and is preferred by some surgeons for the
treatment of primary pulmonary hypertension unresponsive to prostanoid therapy.
Although prognosis is good with modern immunosuppressive drugs, the availability of
lung transplants remains limited due to shortage of donor lungs.
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Some indications for lung transplantation
Parenchymatous lung disease
·
Cystic fibrosis
·
Emphysema
·
Pulmonary fibrosis
·
Langerhans cell histiocytosis
·
Lymphangioleiomyomatosis
·
Obliterative bronchiolitis
Pulmonary vascular disease
·
Primary pulmonary hypertension
·
Thromboembolic pulmonary hypertension
·
Veno-occlusive disease
·
Eisenmenger's syndrome