This is the first explainer in a series of eight, outlining reversible causes of cardiac arrest.
Hypoxaemia is the reduction in the values for partial pressure of oxygen dissolved in arterial blood (PaO2) and arterial oxygen saturation (SaO2) (Pruitt, 2004). Pa02 is best measured by arterial blood gas (ABG) analysis, while SaO2 can be routinely assessed using a non-invasive test called pulse oximetry.
During nursing assessment, hypoxaemia should be considered if a patient has a respiratory rate greater than 24 per minute, and arterial oxygen saturations (SaO2) below 94 per cent on room air (Rittayamai, Tscheikuna, Praphruetkit and Kijpinyochai, 2015). Clinically, however, hypoxaemia is defined as: PaO2 < 8 kilopascals (kPa), or 60 millimetres of mercury (mmHg), on ABG.
The degree of hypoxaemia is reflected in the PaO2 value:
|Mild hypoxaemia||Moderate hypoxaemia||Severe hypoxaemia|
|60 to 79 mmHg||40 to 59 mmHg||less than 40 mmHg|
When hypoxaemia and acute dyspnoea occur together – such as in acute pulmonary oedema (APO), pneumonia, or during exacerbation of chronic obstructive airway diseases (COAD) – it constitutes a medical emergency. If the hypoxaemia is not reversed by administering oxygen it can precipitate cardiac arrest (Rittayamai et al., 2015).
Prolonged or severe hypoxaemia causes tissue to become hypoxic (see ‘Understand COPD and the Hypoxic Drive to Breathe‘), resulting in anaerobic metabolism and altering the patient’s acid-base balance (Pruitt, 2004). This can precipitate cardiac arrest.
Exposing a patient to high concentrations of oxygen for long periods of time can result in life-threatening oxygen toxicity (Lynes, 2003), therefore provide sufficient oxygen to manage the patient safely and prevent deterioration, whilst ensuring that excessive amounts are not delivered.
Select the correct oxygen delivery device for your patient. Non-invasive options range from nasal cannulae, which provide low concentration oxygen typically between 24 and 35 per cent , to high flow devices (such as Venturi masks), which can deliver concentrations of oxygen ranging from 24 to 60 per cent (Lynes, 2003). Non-invasive Positive Pressure Ventilation (NIPPV), including Continuous Positive Airway Pressure (CPAP) or Bilevel Positive Airway Pressure (BiPAP) systems, may also be used. These provide high-level ventilatory support using a face mask, as apposed to intubation which bypasses the upper airway (Baudouin, Blumenthal, Cooper et.al, 2002).
Firstly, ensure that the patient’s airway is open and clear. Mechanical causes of airway obstruction – such as foreign bodies (food, mucous plugs etc.) – should be excluded, and addressed if able, before proceeding.
Ensure that your oxygen source is connected properly. Give the maximal feasible inspired oxygen concentration via a bag valve mask (BVM) connected to a face mask or airway adjunct such as an endotracheal tube (ETT) or laryngeal mask airway (LMA). Even without supplementary oxygen, this system ventilates the patient’s lungs with ambient air (21 per cent oxygen), but the oxygen concentration increases to about 85 per cent by using a reservoir system and attaching oxygen at a flow of 10 litres per minute (lpm) (Soar, Nolan, Böttiger et al., 2015) (review lung sounds).
Assess for adequate ventilation by observing the patient for bilateral chest rise and fall. Chest auscultation can indicate air entry, but waveform capnography with end-tidal CO2 (ETCO2) monitoring is the gold standard for assessing ventilation (also read How to Perform a Chest Pain Assessment).
Early intubation is no longer advocated as the preferred method of managing an airway during cardiac arrest (Australian Resuscitation Council and New Zealand Resuscitation Council Whakahauora Aotearoa, 2016), but exceptions are high-risk patients for whom laryngoscopy and intubation may become more difficult over time (includes those with airway burns, laryngeal oedema secondary to anaphylaxis, or severe facial trauma). Other reasons to consider intubation include protection from aspiration, facilitation of tracheobronchial suction or upper airway obstruction (Gunning, 2003).
After return of spontaneous circulation (ROSC), as soon as arterial blood oxygen saturation can be monitored reliably using ABGS and pulse oximetry, the inspired oxygen concentration level should be titrated to maintain SaO2 between 94 and 98 per cent (Soar, Nolan, Böttiger et al., 2015).
Even in non-monitored settings, cardiac arrest is rarely a sudden, unpredictable event, as most patients demonstrate slow but progressive physiological deterioration. Unfortunately, this is often unnoticed or mismanaged, and results in poorer outcomes.
Early, effective recognition and response to signs of hypoxaemia will prevent some cardiac arrests, deaths and unanticipated ICU admissions (Soar, Nolan, Böttiger et al., 2015).