RESPONSE

Arterial Blood Gas Sampling (ABG)

ABG paper print out

Arterial blood gas syringe and needle

An ABG is obtained from an artery – usually the radial artery using a blood gas syringe and needle. Details of this procedure can be found in the University of Leeds clinical eBook (http://clinicalskills.leeds.ac.uk/). Interpreting the results of an ABG is important to determine the cause and severity of acute patient deterioration.”

There are four key components to analysing an ABG

1. What is the pH?
2. What is the PaO₂?
3. What is the PaCO₂? - need to take into account FiO₂
4. What is the bicarbonate (HCO₃^-) and base excess?

pH

The normal range of pH is 7.35-7.45. Acidosis is defined as a pH <7.35 and can be either metabolic or respiratory in origin. Alkalosis is defined as a pH >7.45 and can be either metabolic or respiratory in origin. Sometimes an acidosis or alkalosis can be due to more than one process and is termed ‘ mixed’. This text will be restricted to describing the simple acid/base disturbances which are secondary to one process.

Respiratory Acidosis

A respiratory acidosis is characterised by a pH <7.35 with an elevated level of carbon dioxide (CO₂). Carbon dioxide  is converted into acid in the blood stream and so raised levels will result in an acidosis. This can occur acutely due to respiratory depression secondary to drugs or neurological damage, respiratory muscle weakness, chest injury or acute airways obstruction.

Respiratory Alkalosis

Low levels of CO₂ are found in conditions where the patient is hyperventilating (e.g. pulmonary embolism, neurological insult, anxiety) and will result in an alkalosis. A respiratory alkalosis is characterised by a pH >7.45 with a reduced level of CO₂.

Metabolic Acidosis

A metabolic acidosis is characterised by a pH <7.35 with a reduced bicarbonate (HCO₃^- ) level. Bicarbonate acts like a base in the blood and mops up excess hydrogen ions (H⁺) resulting in a reduced bicarbonate level.  There are four major causes of a metabolic acidosis including ketoacidosis, lactic acidosis, poisonings or advanced acute or chronic kidney disease.

Metabolic Alkalosis

A metabolic alkalosis is characterised by a pH >7.45 with a high bicarbonate level and can occur in association with fluid depletion (for example vomiting) or hyperaldosteronism excess.

Compensation

When an acid/base disturbance occurs the body will try to compensate for it to try and normalise the pH. Compensation occurs over a variable period of time depending on the cause. Full compensation has occurred when the pH has returned to within normal limits. Partial compensation implies that the pH is still abnormal but a change has occurred to try and normalise it.
Respiratory compensation happens quickly (minutes to hours) by increasing or decreasing the respiratory rate and therefore the amount of CO₂ removed or retained. Metabolic compensation occurs via the kidneys by increasing the regeneration of HCO₃^- or removal of H⁺ and may take hours to days to occur.

Portable chest X-ray

This is an adequate portable plain AP chest X-ray. Bones and soft tissues look normal. ECG lines are visible attached externally to the patient. The trachea is central, lung fields are clear, costophrenic angles are visible. The cardiac shadow does not look enlarged (even considering it is an AP film). There is no free air below the diaphragm and a normal gastric bubble is visible below the left hemi-diaphragm.

A plain chest X-ray (CXR) is a valuable investigation in the acutely ill patient. If the patient is unstable, do not transfer them to the radiology department but request a portable X-ray on the ward. CXR signs tend to lag behind clinical signs so it is not a substitute for thorough clinical examination. It is important to have a system when assessing a chest X-ray. One example that is commonly used is based on an ABCDE mnemonic.

• Confirm the identity of the patient, date and time
• Is the CXR adequate (penetration, rotation, all of chest included on film)?
• Is it a PA (film placed in front of chest) or an AP (film placed behind the chest) film?
• Airway – Is the trachea central or deviated to one side?
• Breathing – assess lung markings throughout the lung fields including costo-phrenic angles
• Cardiac – any evidence of cardiomegaly? (Cannot comment on AP film)
• Diaphragms – any evidence of free air below?
• Everything else – including bones, soft tissues, lines, pacemakers, breast tissue etc.

OXYGEN THERAPY

The British Thoracic Society Guidelines published in 2017 outlines emergency oxygen use in adult patients, https://www.brit-thoracic.org.uk/quality-improvement/guidelines/emergency-oxygen/. These were updated in 2019.

The underlying principle is that hypoxia is dangerous and needs to be treated assertively. A reservoir bag mask with 15 L/min O₂ running is the treatment of choice in an acutely ill -patient.

In an improving patient with previously normal, healthy lungs, O₂ therapy should be titrated to target saturations of 94-98% via pulse oximetry using a variety of O₂ delivery devices.

There are a subset of patients who are at risk of type 2 respiratory failure and CO₂ retention, especially if exposed to uncontrolled O₂ therapy. This can occur in a percentage of patients with Chronic Obstructive Pulmonary Disease (COPD). This leads to a respiratory acidosis. There are several theories as to why this happens which includes the impairment of hypoxia-induced respiratory drive.

This subset of patients requires controlled O2 which can be given most effectively via a Venturi mask which provides a fixed concentration of O₂ at a given flow rate. Venturi masks come in different colours denoting the different inspired O₂ concentration (24%, 28%, 35%, 40%, 60%). At 24%, the hole through which the jet of O₂ is delivered is much smaller than at 60%. Forcing a jet of O₂ through a small hole causes a low-pressure area within the mask, which causes surrounding air to be dragged in, thus diluting the O₂ causing a lower inspired O₂ concentration for the patient. As the percentage increases, less air is drawn into the mask causing a higher inspired O₂ concentration. The Venturi masks are flow dependent, and it is essential to use the recommended flow rate to get the required percentage of inspired oxygen.

Venturi attachments for face masks - the different colours denote different inspired oxygen concentrations. These are flow dependent and the O₂ flows are normally printed on them.

Venturi mask on mannequin.

Hypoxia can rapidly lead to hypoxic brain injury, respiratory and cardiac arrest and death within minutes. Therefore never allow a patient to become hypoxic in order to attempt to control rising CO₂ levels. Treat hypoxia in a controlled manner and if the CO₂ continues to rise, get help.

In the subset of patients who are at risk of type 2 respiratory failure and CO₂ retention, especially if exposed to uncontrolled O₂ therapy, it is important to titrate O₂ saturations to 88-92%, using a venture mask. Once within the target O₂ saturation range using pulse oximetry, arterial blood gas sampling must be used to ensure CO₂ levels are within normal limits.

Patients with Chronic Obstructive Pulmonary Disease (COPD) have chronically raised CO₂ levels resulting in a chronic respiratory acidosis, and rely on a hypoxic drive mechanism to maintain their ventilation. This may be indicated by a compensated respiratory acidosis, as reflected by a normal pH, high PCO₂ and high HCO₃^-.

It is essential to monitor the pH, and if the patient is acidaemic with rising CO₂, non-invasive ventilation and/or escalation of care may be required.