Capillary blood gas (CBG) analysis is a valuable diagnostic tool used to assess a patient’s respiratory and metabolic status with minimal invasiveness. This test is particularly common in neonatal and pediatric care, where it provides critical insights into pH, carbon dioxide (PCO2), and other blood gas parameters.
In this article, we’ll explore the procedure, equipment, interpretation, and limitations of capillary blood gas testing, highlighting its importance in modern healthcare.
What is a Capillary Blood Gas?
A capillary blood gas (CBG) is a diagnostic test used to measure the levels of oxygen (O2), carbon dioxide (CO2), and the acidity (pH) of blood obtained from capillary vessels. This test is often performed by collecting a small sample of blood from a superficial site, such as a finger, heel, or earlobe, using a lancet.
The CBG test serves as an alternative to arterial blood gas (ABG) analysis when arterial sampling is not feasible or necessary.
How is a Capillary Blood Gas Performed?
A capillary blood gas (CBG) test involves collecting a small blood sample from a superficial site, typically in neonates and young children, to evaluate their respiratory and metabolic status.
The procedure is minimally invasive and requires pricking the skin with a small lancet to access capillary blood.
Preferred Puncture Site
For neonates, the lateral area of the heel is the most commonly used site for capillary blood sampling. This area is highly vascularized, ensuring a sufficient and reliable blood sample with minimal discomfort.
The medial or posterior portions of the heel are avoided to reduce the risk of injury to the underlying bone or tissue.
Alternative Puncture Sites
If a heel puncture is not feasible or unsuccessful, other sites may be used, including:
- Fingertip
- Big toe
- Earlobe
Note: These alternatives are typically considered in cases where the heel is inaccessible or has poor perfusion. However, the heel remains the most effective site in neonates due to its superior vascularization and ease of access.
Key Considerations for CBG Collection:
- Warm the Site: Prewarming the puncture site improves blood flow, ensuring an adequate sample.
- Proper Technique: Use a sterile lancet and avoid excessive squeezing, as this can hemolyze the sample or dilute it with interstitial fluid.
- Avoid Overuse: Repeated punctures in the same area should be minimized to prevent tissue damage or infection.
Note: By following proper techniques and site selection guidelines, the capillary blood gas test provides a reliable alternative to arterial sampling for assessing blood gas levels and acid-base balance.
Capillary Blood Gas Equipment
Collecting a capillary blood gas (CBG) sample requires specific equipment to ensure the process is safe, accurate, and efficient.
Here is a list of the essential tools and supplies needed for the procedure:
Essential Equipment
- Lancet: Used to create a small puncture in the skin for blood collection.
- Pre-heparinized capillary tubes: Prevents blood clotting and maintains sample integrity.
- Small metal stirrer bar: Helps mix the blood sample to prevent clotting.
- Magnet: Used to manipulate the stirrer bar inside the capillary tube.
- Clay or wax sealant: Seals the capillary tube after blood collection to prevent spillage or contamination.
Supportive Supplies
- Gauze: For applying pressure to the puncture site after blood collection.
- Bandages: To cover the puncture site and protect it after the procedure.
- Ice: To store the sample temporarily if immediate analysis isn’t possible.
- Gloves: Ensures proper hygiene and protects both the patient and the healthcare provider.
- Skin antiseptic: Cleans the puncture site to prevent infection.
- Warming pads: Prepares the site by increasing blood flow for easier sample collection.
- Sharps container: For the safe disposal of the lancet and other sharp tools.
- Patient label: Ensures proper identification and prevents mix-ups during analysis.
Once all the necessary equipment is gathered and prepared, you are ready to collect the capillary blood sample. This procedure is typically performed by respiratory therapists or other trained medical professionals, ensuring both safety and accuracy during collection.
Having the correct tools and adhering to protocol are crucial for obtaining reliable results and maintaining patient comfort throughout the process.
Capillary Blood Gas Procedure
Collecting a capillary blood gas (CBG) sample involves several critical steps to ensure accuracy and patient safety.
Here are the step-by-step instructions:
- Verify the Doctor’s Order: Confirm the physician’s request for a capillary blood gas test and review any specific instructions.
- Prepare the Equipment: Gather all necessary supplies, including pre-heparinized capillary tubes, lancet, antiseptic, warming pads, and other essential items.
- Perform Hand Hygiene and Wear PPE: Wash your hands thoroughly and don appropriate personal protective equipment (PPE), such as gloves.
- Select the Puncture Site: Identify the lateral area of the heel as the preferred puncture site. If the heel is unavailable, consider alternative sites like the fingertip or earlobe.
- Warm the Puncture Site: Apply a warming pad or compress to the selected site, heating it to 42°C for at least 10 minutes. This improves blood flow and ensures an adequate sample.
- Clean the Skin: Disinfect the puncture site with an antiseptic solution and allow it to dry completely to minimize the risk of infection.
- Perform the Puncture: Use a sterile lancet to create a puncture no deeper than 2.5 mm in the heel. This depth prevents injury to underlying tissue or bone.
- Discard the First Drop of Blood: Wipe away the initial drop of blood to remove potential contaminants or tissue fluid.
- Collect the Blood Sample: Allow blood to flow freely into the pre-heparinized capillary tube without squeezing or milking the puncture site, as this can dilute the sample with interstitial fluid.
- Seal the Collection Tube: Once filled, seal the capillary tube securely using clay or wax to prevent leakage or contamination.
- Provide Post-Procedural Care: Apply sterile gauze to the puncture site and secure it with a bandage or tape.
- Mix the Sample: Use a magnet to move the stirrer bar inside the capillary tube, ensuring proper mixing to prevent clotting.
- Analyze or Store the Sample: Analyze the sample immediately if possible. If a delay is unavoidable, place the sample on ice to preserve its integrity.
- Document the Procedure: Record the procedure details, including the puncture site, any observations, and the test results, in the patient’s medical record.
Note: Once the sample is analyzed, the physician will review the results to determine the patient’s respiratory or metabolic status and plan the appropriate course of treatment. Proper adherence to each step ensures reliable results and patient safety.
Capillary Blood Gas Interpretation
Interpreting a capillary blood gas (CBG) sample involves evaluating key values to assess an infant’s respiratory status. While capillary blood gas analysis is a practical tool for measuring certain parameters, it has limitations that must be considered during interpretation.
Here are the key parameters for interpretation:
pH
- High pH: Indicates the patient is hyperventilating and exhaling excessive carbon dioxide, leading to respiratory alkalosis.
- Low pH: Suggests hypoventilation, where the patient is retaining carbon dioxide, resulting in respiratory acidosis.
Carbon Dioxide (PCO2)
- PCO2 is a reliable indicator of the patient’s ventilation status in capillary blood samples, correlating well with arterial blood gas values.
- Elevated PCO2 indicates hypoventilation, while low PCO2 reflects hyperventilation.
Capillary Oxygen Content (PO2)
- Limited Reliability: The oxygen concentration in capillary blood does not accurately represent arterial oxygen levels. Capillary PO2 values are lower because oxygen is extracted by tissues before the blood reaches the capillary bed.
- Supplemental Monitoring: Due to this limitation, pulse oximetry should be used concurrently to monitor the patient’s oxygen saturation and ensure adequate oxygenation.
Practical Considerations
- Respiratory Alkalosis Example: If the pH is high and PCO2 is low, the patient is likely hyperventilating, blowing off too much CO2.
- Respiratory Acidosis Example: If the pH is low and PCO2 is high, the patient may be experiencing hypoventilation and CO2 retention.
Note: By focusing on pH and PCO2, practitioners can reliably assess the patient’s respiratory status. However, for oxygenation evaluation, pulse oximetry is essential to complement the CBG results, ensuring a complete understanding of the patient’s condition. This combined approach enhances diagnostic accuracy and informs effective treatment decisions.
Capillary Blood Gas Practice Questions
1. Which arterial sampling sites are used for newborns?
The radial artery is the preferred site. Avoid the brachial and femoral arteries. The dorsalis pedis or posterior tibial arteries are acceptable alternative sites.
2. What equipment is required for an arterial puncture?
A 1 ml pre-heparinized tuberculin syringe, 25-gauge needle or pre-heparinized 25-gauge butterfly needle infusion kit, exam gloves, povidone iodine and alcohol wipes, sterile gauze, needle capping and protection device, eye and splash shield, and a patient label.
3. How much heparin should you push out before performing an arterial stick?
All of it.
4. How do you insert the needle for an arterial stick?
Bevel up, inserted at a 45-degree angle.
5. How long is pressure applied after an arterial stick?
At least 5 minutes or until bleeding stops.
6. What are the advantages of a capillary sample?
Used when an ABG is indicated but peripheral access is unavailable; less hazardous and easier to obtain; useful for assessing pH and CO2 disturbances.
7. What are the disadvantages of a capillary sample?
PO2 values are unreliable, cannot be performed on patients with calluses or those who have begun to walk, and should not be performed on inflamed, swollen, cyanotic, or poorly perfused areas. Samples also clot easily.
8. What should you do with air bubbles in a sample?
Expel all air bubbles completely to avoid inaccurate results.
9. What should you do with samples that are not analyzed immediately?
Place the sample on ice.
10. How long can you wait to analyze an arterial blood sample?
It is best to analyze the sample within one hour.
11. What can be prevented by alternating arms for arterial sticks and holding pressure for 5 minutes?
Hematomas
12. What are the contraindications for an arterial puncture?
Lack of collateral circulation, extremities with previously lost temporary blood flow, distal to or through a surgical shunt, infection at the site, or evidence of peripheral vascular disease.
13. What are the complications of an arterial puncture?
Hematoma formation (treat with pressure and switch sites), scarring, artery laceration (due to small size), and nerve damage (brachial, femoral, posterior tibial nerves).
14. What is a capillary blood gas used for?
It serves as an alternative to an ABG.
15. What best correlates with a capillary gas?
Arterial pH and CO2 correlate well with capillary gas results, but PaO2 does not.
16. What are three factors that affect low accuracy in capillary gas samples?
Hypotension (most significant), hypothermia, and hypovolemia.
17. What are the puncture sites for a capillary blood gas?
The posterolateral foot, palmar or fleshy surface of the distal fingers and toes, and earlobes.
18. Which puncture site for a capillary gas has a higher risk of nerve damage?
Fingers and toes
19. What are the contraindications for capillary blood gas sampling?
If accurate oxygenation assessment is needed, neonates <24 hours old, decreased peripheral blood flow, polycythemia, and areas that are edematous, inflamed, infected, or calloused.
20. What are the complications of capillary blood gas sampling?
Inaccurate results leading to mismanagement, burns from over-warming the heel, infection, scarring, inflammation, nerve and artery damage, bruising, hematoma, and bleeding.
Access our quiz, which includes sample TMC practice questions and detailed explanations to help you master the key concepts of neonatal/pediatric care.
21. The umbilical cord has one floppy vein and how many arteries?
Two
22. What is the tip placement for umbilical artery catheterization?
High: T6-T8; Low: L3-L4
23. What is the umbilical artery catheter connected to?
A fluid pressure transducing system.
24. What are the complications of an arterial catheter?
Infection after 72 hours, thrombosis (especially in children under 5), hemorrhage (may require a transfusion), ischemia (pallor, decreased pulses, poor cap refill), and air embolism (even a small amount can cause brain embolism).
25. What does the placement of an arterial line allow?
Direct measurement of arterial blood pressure.
26. Blood pressure varies with age and the infant’s blood pressure is approximately what?
Approximately 60/30 mmHg
27. The MAP is often an indication of what?
The left ventricular afterload and resistance against the left ventricle.
28. What are the benefits of continuous invasive blood gas monitoring?
Provides an alternative for patients requiring frequent blood gas analysis, designed for small patients with limited blood volumes, and is compatible with an arterial or umbilical catheter.
29. What are the indications for a central venous catheter?
Cardiovascular instability, intravascular volume disturbances (e.g., dehydration, hemorrhage, increased ICP), and administration of drugs, fluids, or nutritional support.
30. CVP monitoring measures right atrial pressure, helping to assess fluid volume and what else? It helps secure a long-term venous access site in chronically ill children.
31. What will cause CVP measurements to rise?
Heart problems such as tricuspid and pulmonic stenosis.
32. What are the monitoring sites of the central venous catheter?
Umbilical and subclavian veins are the most common. Other sites include the external and internal jugular veins, brachial vein, and saphenous vein.
33. What are the complications of central venous monitoring?
Catheter sepsis after 72 hours (risk of fungal infection), pulmonary embolism, cardiac dysrhythmias (from the catheter slipping into the RV), and perforation of the trachea or accidental entry into the jugular vein.
34. What does the central venous catheter measure?
Right atrial pressure (RAP)
35. A decreased CVP indicates what?
Hypovolemia and/or shock
36. An increased CVP indicates what?
Hypervolemia and/or left ventricular (LV) failure.
37. Pulmonary artery catheters are also known as what?
Swan-Ganz catheters
38. What is the pulmonary artery catheter used for?
It measures left ventricular function, guides fluid management, aids in diagnosing pulmonary disease and cardiac dysfunction, and monitors cardiac output (CO) and mixed venous oxygen saturation (SvO2).
39. What are the complications of a pulmonary artery catheter?
Bleeding, pneumothorax, heart valve damage, heart chamber perforation, and cardiac arrhythmias (e.g., PVCs).
40. What does the pulmonary artery pressure measure?
Right atrial pressure (RAP), pulmonary artery pressure (PAP), pulmonary capillary wedge pressure (PCWP), and cardiac output (CO).
41. What is the normal cardiac output for a baby?
0.6 L/min
42. Fetal hemoglobin causes a shift to which direction of the oxygen dissociation curve?
It shifts to the left, increasing the affinity of hemoglobin (Hb) for oxygen (O2).
43. When does methemoglobin form?
When hemoglobin is oxidized to the ferric (Fe³⁺) state.
44. What causes a baby to develop methemoglobinemia (MetHb)?
Certain medications and exposure to nitric oxide.
45. Carbon monoxide poisoning causes Hb to combine with what?
Carbon monoxide (CO) instead of oxygen (O2).
46. What is the CaO2 equation?
(Hb × 1.34 × SaO2) + (PaO2 × 0.003).
47. What is the most accurate way to detect changes in oxygen levels of the blood?
Arterial blood gas (ABG) analysis
48. What method for obtaining ABG should be tried initially in a neonate?
Umbilical artery catheterization
49. The high placement of an umbilical artery catheter should be at which landmark on an x-ray?
Between T6 and T8
50. What is the most important advantage of continuous in-line blood gas sampling compared with umbilical blood gas sampling in neonates?
It decreases the amount of blood wasted.
Access neonatal/pediatric practice questions (and correct answers) to prepare for your final exam.
51. What is the cardiac index?
Cardiac output divided by body surface area (CO/BSA).
52. What is a common factor that can reduce PVR?
Nitric oxide
53. Which measurement requires a Swan-Ganz catheter?
Pulmonary capillary wedge pressure (PCWP)
54. What are the complications associated with a dwelling vascular catheter?
Infection and air embolism
55. What are the arterial sample sites for neonates?
The radial artery and the umbilical artery.
56. What is the capillary sampling site for neonates?
The lateral portion of the heel.
57. What is the umbilical artery catheter placement?
It is inserted through the umbilicus into one of the two umbilical arteries. A heparinized isotonic saline solution is attached to prevent clotting. It is secured by suturing or using umbilical tape.
58. What are the advantages of an ABG sample?
Provides reliable values for pH, oxygen, and CO2; arterial lines allow easy and quick access.
59. What are the disadvantages of an ABG sample?
Difficult to obtain, risk of arterial damage, potential for infection or clotting, and infant stimulation can affect the values.
60. What are the complications of an ABG sample?
Improper technique, instrument (analyzer) issues, and inadequate post-puncture care.
61. What are the indications for transcutaneous monitoring?
Monitor arterial oxygenation and ventilation adequacy, evaluate responses to diagnostic and therapeutic interventions, useful for continuous monitoring, and helpful when direct ABG measurement is unavailable.
62. What are the contraindications for transcutaneous monitoring?
Poor skin integrity and adhesive allergy (relative contraindication).
63. What are the hazards of transcutaneous monitoring?
False-negative or false-positive results and potential tissue injury.
64. What are the advantages of transcutaneous monitoring?
Allows continuous and non-invasive monitoring of oxygen (O2) and carbon dioxide (CO2).
65. What are the disadvantages of transcutaneous monitoring?
Risk of burns and erythema, frequent calibration required, equilibration time needed, and oxygen values become less reliable in older infants.
66. What is capnography?
Capnography is used with ventilated patients. It requires a special neonatal adapter to accurately measure smaller volumes. Moisture and secretions can affect readings. ETCO2 values are usually 2-3 torr lower than ABG values.
67. What is a pneumogram and apnea monitor?
A device that measures heart rate and respiratory rate. Electrodes are placed along the mid-axillary line and secured with a flexible belt. The monitor senses changes in impedance as the chest moves and calculates respiration.
68. What are the indications for a pneumogram?
Used for infants at risk of Sudden Infant Death Syndrome (SIDS), bradycardia, or apnea episodes.
69. What is the preferred puncture site for a capillary blood gas?
The heel is the preferred site.
70. How long should you warm the site before puncture?
Warm the site for 10 minutes using a compress, heat lamp, or commercial hot pack.
71. What are the alternative puncture sites if the heel is unavailable?
The earlobe, big toe, and finger.
72. What is used to puncture the skin?
A lancet should be used.
73. What should you do with the first drop of blood?
Wipe away the first drop of blood before collecting the sample.
74. How long do you have to analyze the sample?
Analyze the sample within 10-15 minutes.
75. What parameters should you look at during the analysis of a capillary sample?
The pH and PaCO2 correlate well with arterial samples. The PaO2 does not. Focus on the infant’s ventilatory parameters during capillary blood gas analysis.
Final Thoughts
Capillary blood gas testing is a quick, efficient, and minimally invasive method for assessing respiratory and metabolic status, particularly in vulnerable populations like neonates and children.
While it offers reliable measurements of pH and carbon dioxide, its limitations, such as lower accuracy in oxygen assessment, necessitate the use of complementary tools like pulse oximetry.
By understanding the procedure and proper interpretation of results, healthcare professionals can make informed decisions that contribute to better patient outcomes. Whether in critical care or routine monitoring, CBG testing remains an indispensable part of clinical practice.
Written by:
John Landry is a registered respiratory therapist from Memphis, TN, and has a bachelor's degree in kinesiology. He enjoys using evidence-based research to help others breathe easier and live a healthier life.
References
- Evans DL, Volsko TA, Capellari E, Strickland SL. AARC Clinical Practice Guidelines: Capillary Blood Gas Sampling for Neonatal and Pediatric Patients. Respir Care. 2022.
- Zavorsky GS, Cao J, Mayo NE, Gabbay R, Murias JM. Arterial versus capillary blood gases: a meta-analysis. Respir Physiol Neurobiol. 2007.