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By Root 1806 0

between the PaO2 and the SaO2 is represented as the oxygen dissociation curve. Oxygen uptake in the lungs is shown by the upper flat part of the curve. When the PaO2 is between 8.0 and 13.3 kPa (60–100 mmHg), the haemoglobin is 90% or more saturated with oxygen. At this point of the curve, large changes in the PaO2 lead to small changes in the SaO2 of haemoglobin, because the haemoglobin is almost completely saturated. Release of oxygen to the tissues is shown by the lower part of the curve. There is easy removal of oxygen from the haemoglobin for use by the cells. It is at this part of the curve that small changes in the PaO2 cause major changes in the SaO2. This is important clinically (Carpenter 1991, Guyton and Hall 2006, Hess 2000).

A patient’s oxygen level must be kept at 8.5 kPa or above (60 mmHg). Below this level, desaturation can occur at a rapid rate, resulting in tissue hypoxia and cell death (Marieb et al. 2010).

Oxygen consumption

At rest the normal oxygen consumption is approximately 200–250 mL/min. As the available oxygen per minute in a normal man is about 700 mL, this means there is an oxygen reserve of 450–500 mL/min. Factors that increase the consumption of oxygen include fever, sepsis, shivering, restlessness and increased metabolism (Bersten et al. 2009). It is difficult to say at which absolute level oxygen therapy is necessary, as each situation should be judged by the requirements for oxygen and the availability of oxygen. Therefore, all the above information needs to be taken into account together with the measurement of the arterial blood gases.

Generally, additional oxygen will be required when the PaO2 has fallen to 8.5 kPa (60 mmHg) or less (Bersten et al. 2009).

Carbon dioxide excretion

The second function of the respiratory system is to excrete carbonic acid from the lungs during expiration. The normal level of carbon dioxide in the blood is 3.5–5.3 kPa. Carbon dioxide has a direct effect on the respiratory centre in the brain. As the carbon dioxide level rises and diffuses from the blood into the cerebrospinal fluid (CSF), it is hydrated and carbonic acid is formed. The acid then dissociates and hydrogen ions are liberated and as there are no proteins in the CSF to buffer the hydrogen ions, the pH of the CSF falls, which excites the central chemoreceptors and the respiratory rate is increased (Marieb et al. 2010).

Evidence-based approaches

Respiratory assessment

Once information about the person’s past medical history has been obtained, one of the most reliable and important assessments is to closely observe and talk to the patient. During this time a patient’s smoking status should be ascertained and, if appropriate, their smoking habits and the benefits of stopping should be discussed. For smoking cessation, brief interventions typically take about 5–10 minutes and may include one or more of the following.

Simple opportunistic advice to stop.

Assessment of patient’s desire to stop.

Offer of pharmacotherapy and/or behavioural support.

Provision of self-help material and referral to more intensive support such as the NHS Stop Smoking Services.

Normal respiration is effortless and almost unconscious and the person can eat, drink and speak in full sentences without appearing breathless. Essential first steps in respiratory assessment are therefore to observe the person’s breathing for the following.

Ease and comfort.

Rate.

Pattern.

Position the patient has adopted; for example, does the patient need to sit at 90° upright to breathe effectively?

Rate and ease of breathing during speaking or movement.

General colour and appearance: is there any evidence of greyness, cyanosis, pallor, sweating?

Additional audible breath sounds: wheezing or stridor?

Having rapidly made this assessment, other essential assessments are a chest X-ray and arterial blood gas, and a computed tomography (CT) scan or ventilation/perfusion (V/Q) scan may also be necessary (see Chapter 12).

Having made a comprehensive assessment, the immediate cause of respiratory insufficiency should be corrected