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Procedural Skills (SLO6)
Question 109 of 141
A 59 year old patient presents to ED complaining of chest pain and breathlessness. You are supervising a medical student performing an arterial blood gas. Regarding arterial blood gas sampling, which of the following statements is true?
Answer:
- The radial artery is best palpated between the distal radius and the tendon of the flexor carpi radialis
- ABG uses: Detection and quantification of the levels of abnormal haemoglobins (e.g. carboxyhaemoglobin and methaemoglobin)
- Anticoagulants or coagulopathy are relative contraindications to ABG sampling.
- In the modified Allen's test, a pink color should return to the palm, usually within six seconds, indicating that the ulnar artery is patent and the superficial palmar arch is intact. Although the timing of return of circulation to the palm varies considerably, the test is generally considered abnormal if ten seconds or more elapses before color returns to the hand.
- Air or thrombus embolism is a potential rare complication of ABG sampling.
Arterial Blood Gas Sampling
Procedural Skills (SLO6)
Last Updated: 1st March 2022
Indications
- Identification and monitoring of acid-base disturbances
- Measurement of the partial pressures of oxygen (PaO2) and carbon dioxide (PaCO2)
- Assessment of the response to therapeutic interventions (e.g. insulin in patients with diabetic ketoacidosis)
- Detection and quantification of the levels of abnormal haemoglobins (e.g. carboxyhaemoglobin and methaemoglobin)
- Procurement of a blood sample in an acute emergency setting when venous sampling is not feasible (most tests can be performed from an arterial sample)
Contraindications
- Absolute
- An abnormal modified Allen's test
- Local infection, thrombus, or distorted anatomy at the puncture site (e.g. previous surgical interventions, congenital or acquired malformations, burns, aneurysm, stent)
- Arteriovenous fistula or vascular graft at the puncture site
- Severe peripheral vascular disease of the artery selected for sampling
- Active Raynaud's syndrome (particularly sampling at the radial site)
- Relative
- Anticoagulants or coagulopathy
- Use of thrombolytic agents
- Thrombocytopaenia
- History of Raynaud's disease
- Evidence of poor peripheral perfusion
Site selection
Common sites include the radial, femoral, brachial, dorsalis pedis, or axillary artery. There is no evidence that any site is superior to the others. However, the radial artery is used most often because it is accessible and more comfortable for the patient than the alternative sites.
- The radial artery is best palpated between the distal radius and the tendon of the flexor carpi radialis when the wrist is extended.
- The femoral artery is best palpated just below the mid-inguinal point, when the lower extremity is extended and the patient is lying supine.
Assessing collateral circulation
One of the risks associated with arterial puncture is ischaemia distal to the puncture site. Although rarely performed in practice, identifying collateral flow to the region supplied by the artery can be used by clinicians prior to puncture.
Modified Allen's Test:
- The patient's hand is initially held high with the fist clenched.
- Both the radial and ulnar arteries are compressed firmly by the two thumbs of the investigator (this allows the blood to drain from the hand).
- The hand is then lowered and the fist is opened (the palm will appear white).
- The pressure is released from the ulnar artery while occlusion is maintained on the radial artery.
- A pink color should return to the palm, usually within six seconds, indicating that the ulnar artery is patent and the superficial palmar arch is intact. Although the timing of return of circulation to the palm varies considerably, the test is generally considered abnormal if ten seconds or more elapses before color returns to the hand.
Complications
- Common
- Local pain and paraesthesia
- Bruising
- Local minor bleeding
- Less common
- Vasovagal response
- Local haematoma from moderate or major bleeding
- Artery vasospasm
- Rare
- Infection at the puncture site
- Arterial occlusion from a local haematoma
- Air or thrombus embolism
- Local anaesthetic anaphylactic reaction
- Local nerve injury
- Needlestick injury to health care personnel
- Pseudoaneurysm formation
- Vessel laceration
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- Biochemistry
- Blood Gases
- Haematology
| Biochemistry | Normal Value |
|---|---|
| Sodium | 135 – 145 mmol/l |
| Potassium | 3.0 – 4.5 mmol/l |
| Urea | 2.5 – 7.5 mmol/l |
| Glucose | 3.5 – 5.0 mmol/l |
| Creatinine | 35 – 135 μmol/l |
| Alanine Aminotransferase (ALT) | 5 – 35 U/l |
| Gamma-glutamyl Transferase (GGT) | < 65 U/l |
| Alkaline Phosphatase (ALP) | 30 – 135 U/l |
| Aspartate Aminotransferase (AST) | < 40 U/l |
| Total Protein | 60 – 80 g/l |
| Albumin | 35 – 50 g/l |
| Globulin | 2.4 – 3.5 g/dl |
| Amylase | < 70 U/l |
| Total Bilirubin | 3 – 17 μmol/l |
| Calcium | 2.1 – 2.5 mmol/l |
| Chloride | 95 – 105 mmol/l |
| Phosphate | 0.8 – 1.4 mmol/l |
| Haematology | Normal Value |
|---|---|
| Haemoglobin | 11.5 – 16.6 g/dl |
| White Blood Cells | 4.0 – 11.0 x 109/l |
| Platelets | 150 – 450 x 109/l |
| MCV | 80 – 96 fl |
| MCHC | 32 – 36 g/dl |
| Neutrophils | 2.0 – 7.5 x 109/l |
| Lymphocytes | 1.5 – 4.0 x 109/l |
| Monocytes | 0.3 – 1.0 x 109/l |
| Eosinophils | 0.1 – 0.5 x 109/l |
| Basophils | < 0.2 x 109/l |
| Reticulocytes | < 2% |
| Haematocrit | 0.35 – 0.49 |
| Red Cell Distribution Width | 11 – 15% |
| Blood Gases | Normal Value |
|---|---|
| pH | 7.35 – 7.45 |
| pO2 | 11 – 14 kPa |
| pCO2 | 4.5 – 6.0 kPa |
| Base Excess | -2 – +2 mmol/l |
| Bicarbonate | 24 – 30 mmol/l |
| Lactate | < 2 mmol/l |
