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Chapter 3: Respiratory failure, heart disease and embolism 129

Table 3.17 Types of assisted ventilation
Type of ventilation Description Comments
Non-invasive
Continuous positive airway pressure Constant airway pressure via a Unsuitable for Type II respiratory failure
(CPAP) close-fitting nasal or face mask (worsens CO 2 retention). Patients
need to be co-operative.
Non invasive positive pressure Additional supported respiration by Useful for avoiding intubation in
ventilation (NIPPV), Bilevel positive mask is provided. In BiPAP exacerbations of COPD, patients
airway pressure (BiPAP) inspiratory and expiratory pressures need to be co-operative.
are set separately to increase Administered by tight fitting face or
inspired lung volume, and to nasal mask.
maintain some positive
end-expiratory pressure to prevent
alveoli from collapsing.
Invasive
Intermittent positive pressure The inspiratory and expiratory
ventilation (IPPV) pressures and timing of ventilation
can all be altered to give variations.
Continuous positive pressure CPAP can be given via an Used in the weaning of patients from
ventilation endotracheal tube. ventilation towards normal
breathing.

Pulmonary embolism Pathophysiology
Following a pulmonary embolus there is a reduction in
Definition
the perfusion of the lung supplied by the blocked vessel.
Thrombus within the pulmonary arteries causing lack
Ventilation perfusion mismatch occurs, leading to hy-
of lung perfusion. Thrombus within the systemic veins
poxaemia. Production of surfactant also stops if perfu-
or uncommonly from the heart embolises to the lungs.
sion is interrupted for a number of hours after which the
alveoli collapse. Infarct is rare (only occurring in around
Prevalence 10% of cases) as the lung is also supplied by the bronchial
Common. circulation,butthereisanincreaseinpulmonaryarterial
pressure.
Aetiology
The causes of thrombosis can be considered according Clinical features
to Virkhow’s triad: The result of a pulmonary embolism depends on the size
Disruptioninbloodflowparticularlystasis:Prolonged and number of the emboli.

bed rest, air travel, pelvic and lower limb fractures, Small emboli may be silent or present with symptoms
pelvic or abdominal surgery, pregnancy and child- such as dyspnoea on exertion, haemoptysis, pleuritic
birth. Right sided cardiac thrombosis may occur in pain or rarely cardiac arrhythmias.
atrial fibrillation, septal or right ventricular infarcts. Medium-sized emboli typically present with sudden
Abnormalities of the vessel wall: Following direct onset pleuritic pain and dyspnoea. There may be a dry

trauma to the vein in leg trauma. cough or haemoptysis.
Abnormalities in the blood such as hypercoagulable A large embolus may present with syncope, sudden

states – antithrombin III, protein C and S deficiencies onset of severe central chest pain, shock, loss of con-
and Factor V Leiden (a single point mutation in the sciousness or sudden death. Signs include hypoten-
Factor V gene, which causes resistance to activated sion, a loud pulmonary component of the second
protein C), oral contraceptives, malignant disease and heart sound, tachycardia with third and fourth heart
smoking. sounds heard as a gallop rhythm. There is a right