Respiratory failure, including ards

Respiratory failure, including ards - The majority of patients admitted to ICU/HDU have respiratory problems either as the primary cause of admission or secondary to pathology elsewhere. Respiratory failure is classified on the basis of blood gas analysis as:

diaphragm, rib cage, pulmonary arteries, brain, and spinal cord in the body. Figure B shows the major conditions that cause respiratory failure.

Respiratory failure

• type 1: hypoxaemia (PaO2 < 8 kPa (< 60 mmHg) when breathing air) without hypercapnia caused

by a failure of gas exchange due to mismatching of pulmonary ventilation and perfusion

• type 2: hypoxaemia with hypercapnia (PaCO2 > 6.5 kPa (> 49 mmHg)) due to alveolar hypoventilation which occurs when the respiratory muscles cannot perform sufficient work to clear the carbon dioxide produced by the body.

Although this distinction is conceptually useful, it cannot be applied too rigidly in critically ill patients since they may change from type 1 to 2 as their illness progresses; hypercapnia may develop in pneumonia or pulmonary oedema as the patient tires and can no longer sustain the increased work of breathing. Pulmonary problems in critically ill patients can also be classified according to the functional residual capacity (FRC, or the lung volume at the end of expiration). Examples of low FRC include lung collapse, pneumonia and pulmonary oedema; examples of high FRC (i.e. over-distended lungs) include asthma, COPD and bronchiolitis.

This allows logical management directed at improving lung compliance and reducing the work of

breathing. The more common causes of acute respiratory failure presenting to ICU/HDU for respiratory support are shown in Box 8.6. The presentation, differential diagnosis and initial treatment of the primary respiratory conditions causing acute respiratory failure are covered

in Chapter 19. The assessment of respiratory failure in the critically ill patient should be guided by several important principles: The patient’s appearance (tachypnoea, 

• difficulty speaking in complete sentences, laboured breathing, exhaustion, agitation or increasing

obtundation) is more important than measurement of blood gases in deciding when it is appropriate

to provide mechanical respiratory support or intubation.

• Adequate supplemental oxygen to maintain SpO2 > 94% should be provided. If the inspired oxygen concentration required exceeds 60%, refer to the critical care team.

• Measurement of SpO2 and ABGs is essential in monitoring progress.

• Restless patients dependent on supplementary oxygen or with deteriorating conscious level are at risk. If they remove the mask or vomit, the resulting hypoxaemia or aspiration may be catastrophic.

• An attempt should be made to reduce the work of breathing, e.g. by treating bronchoconstriction or

using CPAP (see below)