Respiratory rate is the number of times per minute that a person breathes. There is some disagreement over what constitutes a ‘normal’ rate or Eupnoea in adults, with normal rates varying from 10 breaths per minute to 25 breaths per minute.^1,2,4,5 It is agreed however that breathing rates rise in the young and in the elderly,⁶ as can be seen below. The ratio of respiratory rate to pulse rate is suggested to be approximately 1:5,⁶ with some authors suggesting a ratio of 1:4.⁷ As there is little consensus as to what constitutes a normal rate, it is therefore difficult to ascertain what is abnormal rate, therefore consideration of respiratory rate should be taken as an overall picture using other observations of the patient, such as level of consciousness or colour, as a guide.

Tachypnoea

Tachypnoea is an abnormally fast respiratory rate based upon norm values (>20 breaths per minute),⁸ and can be one of the first indications of respiratory distress. There can be numerous causes of tachypnoea such as anxiety, fever, exercise, hypoxia and pain.^1,4,9 The British Thoracic Society (2007) suggest that respiratory rate is a major indicator in severity of respiratory illness, with respiratory rates of over 25 breaths per minute indicative of acute severe exacerbation of conditions such as asthma (in adults).¹⁰

Bradypnoea

Bradypnoea is an abnormally slow respiratory rate (<12 breaths per minute),⁸ that can indicate a severe deterioration of a patients condition.¹⁰ Causes include fatigue, hypothermia, central nervous system depression and certain drugs such as opiates.¹¹

Measuring respiratory rate

There is little evidence of how to measure respiratory rate, however consensus suggests that rate should be counted over a minute with the patient at rest and ideally without the patients knowledge it is being counted so as not to make them conscious of their respiratory rate.^2,5 There are a variety of methods that can be used to identify respiratory rate as seen below:

• Direct vision of chest movement.
  
• Use of specialist oxygen masks that incorporate respiratory rate indicators. The accuracy of these has been validated within the Emergency Department; however large scale studies have not been undertaken.¹²
  
• Counting of respiratory rate during auscultation.

Other methods have been suggested such as the visualisation of condensation on the inside of an oxygen mask; however the use of such techniques has not been validated and cannot be recommended.

Respiratory depth is the volume of air inhaled and exhaled from the lungs with each respiration. The volume of air moved in a normal breath is termed tidal volume, in the average adult tidal volume is considered to be 500 mL.¹ This is typically measured with a spirometer, however a spirometry is not commonly utilised in prehospital care, and therefore the estimation of respiratory depth is undertaken by reviewing chest expansion or through the use of ventilatory methods discussed in other chapters such as the bag-valve-mask. However the assessment of the depth of respiration is important when considering the respiratory status of any patient.

Whilst there is no scientific evidence to support the visualisation and inspection of the respiratory system, it is an inherent consideration for all patients with respiratory disease and related injury as visual indicators can emphasise the level of effort required for the patient to breathe, highlight the presence of injury, and provide clues in the formulation of a diagnosis. A thorough inspection of the airways, the neck (larynx and trachea), and the chest (for signs such as bruising, deformity, accessory muscles and equality of chest movement) can aid the practitioner and should form a part of every respiratory assessment. It is recommended that a thorough understanding of the underlying anatomy, physiology and pathophysiology of illness and injury be achieved so that the presence of and importance of visual indicators can be fully understood.

Healthy lungs are stretchy and distensible; this is known as lung compliance. Lung compliance is determined by two factors, the distensibility of the lung tissue and alveolar surface tension.¹ However since the lungs are contained within the thoracic cage the compliance of the thoracic wall is also a key factor in lung compliance. Within the prehospital setting it is not possible to measure lung compliance, however thoracic wall compliance can be considered using a simple technique. In the healthy adult both sides of the thorax should expand symmetrically. Failure of the chest wall to expand either unilaterally or bilaterally may indicate disorders such as fibrosis, lung collapse or pleural effusion.

Assessing chest compliance

There is little evidence regarding the use of chest compliance as a measurement of respiratory function, however it can be a useful tool in the assessment of respiratory illness or injury. The method described below is a simple technique to assess the equality and depth of respiration.

Step-by-step procedure for assessing chest compliance

The normal respiratory rhythm has regular cycles co-ordinated by the central nervous system, with the expiratory phase slightly longer than the inspiratory phase. However, variations exist in the presence of underlying disorders. It is therefore important to note the rhythm of breathing during the measurement of respiratory rate.

Definitions

There is a variety of recognised breathing patterns that have clinical significance and should be noted if present.

• Apnoea is a loss of all respiration,¹ therefore it is an indication of critical illness or injury and death.
  
• Hypopnoea is a severe reduction of respiratory rate and depth that differs from bradypnoea due to the severity.
  
• Biot’s breathing is an irregular breathing pattern that is differentiated from Cheynes–Stokes breathing (below) by the equal depth of respiration. It is commonly a result of central nervous system disorders.⁶
  
• Cheyne–Stokes breathing is a periodic breathing pattern in which there is a gradual increase in depth of breathing, followed by diminished respiratory effort, often associated with a brief period of apnoea.¹³ This is often seen in patients with severe illness and most noticeably seen in cardiac failure, narcotic poisoning and neurological disorders.
  
• Kussmauls breathing is described as an increase in the depth and rate of breathing often resulting in a sighing pattern. This is commonly a result of metabolic acidosis secondary to conditions such as renal failure, salicylate poisoning and diabetic ketoacidosis.¹⁴ Kussmauls breathing is a variation of hyperventilation that is also an increase in respiratory rate and depth.¹

Measuring respiratory rhythm

There are no set guidelines or evidence for the observation of respiratory patterns; however consensus of opinion suggests that observing breathing for a period of one minute is best practice to allow for any changes to be noted and evaluated.^6,13

Oxygen is transported to the tissues in the blood, with approximately 3% transported in the blood plasma and 97% transported on haemoglobin creating oxyhaemoglobin.^1,15 Oxygen saturation is defined as the ratio of oxyhaemoglobin to the total concentration of haemoglobin.¹⁵ Disease and injury processes can alter the ability of blood to carry or receive oxygen. Therefore the ability to measure oxygen levels in the blood can be a useful indicator of respiratory function. Oxygen saturations can be measured invasively via arterial blood gases (SaO₂) and non-invasively via pulse oximetry (SpO₂). Prior to the introduction of methods such as pulse oximetry detection of hypoxaemia (lack of oxygen in the blood) was reliant upon observation methods such as cyanosis at the lips (circumoral cyanosis). However this is considered to be a late sign with high levels of subjectivity due to eyesight differences and experience of the examiner.¹⁶ The ability to undertake arterial blood gas sampling for SaO₂ levels is impractical in the prehospital; setting, therefore pulse oximetry is the routinely utilised method.

Pulse oximetry

The pulse oximeter measures the pulse rate and the saturation of haemoglobin in arterial blood. Through the use of two light sources (red and infrared) and a sensor, light absorption is measured. Pulse oximeters utilise the physiological difference in light absorption between oxygenated haemoglobin and reduced (deoxygenated) haemoglobin to provide a ratio reading displayed as a percentage of total haemoglobin.

There are a wide variety of products available within healthcare settings, providing pulse oximetry probes that are designed for differing areas of the body including the finger, forehead and the ear-lobe. Each practice area will have a preferred pulse oximeter manufacturer and probe configuration, however the principles remain the same regardless of make and model.

Indications for pulse oximetry

There are numerous reasons for the undertaking of pulse oximetry; examples of these are shown below:

• To assess respiratory function in respiratory illness or injury.
  
• To review the efficacy of respiratory interventions such as oxygen therapy.
  
• To gain a baseline measurement of oxygen saturation prior to interventions such as sedation and anaesthesia.

Limitations of pulse oximetry

The use of a pulse oximeter is not without limitations and sources of error. A variety of potential limitations have been identified and call into question measurements achieved through pulse oximetry:

• Nail polish – there has been growing concern over the validity of SpO₂ measurements achieved in patients who are wearing nail polish. Common practice as a result has been to remove nail polish prior to measurement¹⁷ which obviously has time and expense implications. However recent large-scale studies have noted that whilst small differences (<1.6%) occur with dark colour nail polishes (brown and blue), that these changes are not clinically significant.^17,18 However it is noted that a difference in measurement can be caused by nail polish.
  
• Carbon monoxide poisoning – due to the affinity of carbon monoxide (CO) and haemoglobin, the presence of CO will result in the formation of carboxyhaemoglobin. However pulse oximeters are unable to detect the difference between oxyhaemoglobin and carboxyhaemoglobin, therefore in the presence of carbon monoxide (for example in smoke inhalation) pulse oximeters can provide a falsely elevated reading. In these situations SpO₂ should not be relied upon.^2,16,19
  
• Poor peripheral circulation – as the pulse oximeter is reliant upon pulse waves, poor perfusion due to conditions such as cold or hypotension can result in inadequate readings.¹⁶ Pulse oximetry has been found to be reliable with systolic blood pressures of > 80 mmHg; however blood pressure readings below this level can lead to inaccurate or unreliable detection of the pulse waves and subsequent erroneous readings.^20,21 This is supported by findings in critically ill patients in two studies reviewing centrally sited SpO₂ probes (ear lobe/forehead) versus peripheral fingertip probes, with greater reliability and sensitivity found in more centrally sited probes.^22,23
  
• Motion artefact – the excessive motion of digits from tremor, seizure or shivering can interfere with signal detection or interpretation. Therefore the placement of the probe is vital, alongside calming of the patient wherever possible.^16,24
  
• High intensity lighting – this can lead to false readings due to the infiltration of light into the probe. This can be limited by correct application of the probe and by reducing bright light sources.¹⁶
  
• Age, sex and dark skin have not been shown to interfere with SpO₂ monitoring in previous studies²⁵ however a recent study suggests that dark skin can decrease the accuracy of pulse oximetry at low levels of oxygen saturation (<80%). Further research is required to clarify this situation before practice can be changed.
  
• High bilirubin levels in hepatitis and cirrhosis of the liver and some blood dyes used in angiography may reduce the accuracy of monitoring as they alter the colour of the blood.²⁶
  
• Oximeters require a steady pulse signal, therefore conditions that affect the consistency of a pulse may reduce accuracy. One common example is the presence of cardiac arrhythmias such as atrial fibrillation.

Technique of pulse oximetry

A standardised approach to pulse oximetry can aid accuracy of results, and a step-by-step procedure is given below.

A step-by-step approach to pulse oximetry