Intermittent positive pressure ventilation (IPPV)

Overview:

• This is achieved by intermittently inflating the lungs with a positive pressure delivered by a ventilator and applied via an endotracheal tube or a tracheostomy
• A number of refinements and modifications of IPPV have been introduced with:
• Positive end-expiratory pressure (PEEP)
• Intermittent mandatory ventilation (IMV)
• High-frequency jet ventilation (HFJV)

• The rational use of IPPV depends on a clear understanding of its potential beneficial effects as well as its dangers

Beneficial effects of IPPV:

• Improved CO₂ elimination
• Relief from exhaustion:
• Artificial ventilation removes the work of breathing and relieves the extreme exhaustion that may be present in patients with respiratory failure
• Effects on oxygenation:
• In those with severe pulmonary parenchymal disease, the lungs may be very stiff and the work of breathing is therefore greatly increased
• Under these circumstances, the institution of IPPV may significantly reduce total body O₂ consumption

Indications for IPPV:

• Acute respiratory failure
• Acute ventilatory failure
• Other indications:
• Prophylactic postoperative ventilation
• Head injury – to avoid hypoxia and hypercarbia which increase ICP. Hyperventilation reduces ICP
• Trauma – chest injury and lung contusion
• Severe LVF with pulmonary oedema
• Coma with breathing difficulties (e.g. following drug overdose)

Institution of IPPV:

• Requires endotracheal intubation
• Intubating patients in severe respiratory failure is an extremely hazardous undertaking and should only be performed by experienced staff. It extreme emergencies, it may be preferable to ventilate the patient by hand using and oropharyngeal airway, a face mask and a self-inflating bag until experienced help arrives
• The patient is usually hypoxic and hypercarbic, with increased sympathetic activity:
• The stimulus of laryngoscopy and intubation can precipitate dangerous arrhythmias and even cardiac arrest
• ECG and O₂ saturation should be monitored
• Patient should be preoxygenated with 100% O₂ before intubation
• In some deeply comatose patients, no sedation will be required but in the majority of patients, a short-acting IV anaesthetic agent followed by muscle relaxation will be necessary

• Endotracheal tubes can now be left safely in place for several weeks and tracheostomy is, therefore, less often performed
• Tracheostomy may be required for the long-term control of excessive bronchial secretions and/or to maintain an airway and protect the lungs in those with impaired pharyngeal and laryngeal reflexes
• Tracheostomy can be performed surgically, the trachea being opened through the second, third and fourth tracheal rings via a small transverse skin incision, or percutaneously using a guidewire and a series of dilators
• Tracheostomy has a mortality rate of up to 3%
• A life-threatening obstruction of the upper airway that cannot be bypassed with an endotracheal tube should have a cricothyroidotomy

Dangers of IPPV:

• Airway complications
• Disconnection, failure of gas or power supply, mechanical faults
• CVS complications:
• The intermittent application of positive pressure impedes venous return and distends alveoli, thereby ‘stretching’ the pulmonary capillaries and causing a rise in pulmonary vascular resistance
• This produces a fall in cardiac output
• Respiratory complications:
• V/Q mismatching
• Collapse of peripheral alveoli
• Secondary pulmonary infection
• Barotrauma:
• Pneumothorax
• Pneumomediastinum
• Pneumoperitoneum
• Subcutaneous emphysema
• GI complications:
• Abdominal distension associated with an ileus
• Unknown cause
• Salt and water retention:
• Increases secretion of ADH
• Decreases secretion of ANP
• The fluid retention is often particularly noticeable in the lungs