Capillary Blood Gas Overview and Practice Questions

Capillary Blood Gas: Overview and Practice Questions

by | Updated: Nov 25, 2023

A capillary blood gas (CBG) is a diagnostic test that involves collecting a blood sample from the capillaries of an infant.

It is used to measure the pH and carbon dioxide concentration in the blood, which can help indicate how well the infant is breathing.

In this article, we will provide an overview of capillary blood gas sampling and how to interpret the results. We provided helpful practice questions on this topic as well.

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What is a Capillary Blood Gas?

A capillary blood gas (CBG) is a test that involves puncturing and collecting a blood sample from an infant. The purpose of this test is to analyze the neonate’s ventilatory status by measuring the pH and carbon dioxide concentration in the blood.

This helps determine how well the infant is breathing and removing carbon dioxide from their body.

How is a Capillary Blood Gas Performed?

A capillary blood gas test is performed by pricking the neonate’s skin with a small needle and collecting a drop of blood from the resulting capillary.

The lateral area of the heel is the preferred puncture site when collecting a capillary blood sample. However, if the heel is not accessible, there are other sites that can be used.

Other Puncture Sites

The other possible puncture sites that can be used to collect a capillary blood sample include:

  • Fingertip
  • Big toe
  • Earlobe

These sites would only be recommended if a lateral heel puncture attempt was unsuccessful. The heel is the most effective site to stick in infants because it is highly vascularised.

Capillary Blood Gas Equipment

The equipment needed to collect a capillary blood sample includes the following:

  • Lancet
  • Pre-heparinized capillary tubes
  • Small metal stirrer bar
  • Magnet
  • Clay or wax sealant
  • Gauze
  • Bandages
  • Ice
  • Gloves
  • Skin antiseptic
  • Warming pads
  • Sharps container
  • Patient label

Once you have gathered all of the necessary equipment, you will be ready to collect the sample. This duty is typically performed by respiratory therapists.

Capillary Blood Gas Procedure

The following steps are involved in the procedure of collecting a capillary blood gas sample:

  1. Check and confirm the doctor’s order.
  2. Obtain all necessary equipment and supplies.
  3. Wash your hands and put on gloves and other necessary personal protective equipment (PPE).
  4. Select the puncture site (e.g., heel).
  5. Warm the heel to 42°C for at least 10 minutes using a compress or commercial hot pack.
  6. Clean the heel with an antiseptic solution.
  7. Use a lancet to puncture the heel (<2.5 mm).
  8. Wipe away the first drop of blood and check for free flow. You do not need to squeeze or milk the puncture.
  9. Fill the collection tube with blood from the puncture site and seal the end of the tube.
  10. Secure sterile gauze over the puncture wound with a bandage or tape.
  11. Mix the sample using a magnet.
  12. Analyze the sample immediately or place it on ice for a delayed analysis.
  13. Document the procedure and findings in the patient’s medical record.

Once the sample has been analyzed, the physician will interpret the results to determine the best course of treatment.

Capillary Blood Gas Interpretation

After collecting and analyzing a capillary blood gas sample, practitioners will look at two primary values:

  1. pH
  2. Carbon dioxide (PCO2)

Both values provide reliable information about the infant’s respiratory status. For example, a high pH indicates that the patient is breathing too fast and is blowing off too much carbon dioxide.

This is a sign of respiratory alkalosis.

On the other hand, a low pH indicates that the patient is not blowing off enough carbon dioxide. This is a sign of respiratory acidosis.

Capillary Oxygen Content

While the capillary pH and PCO2 correlate well with arterial blood, the same is not true for oxygen.

The concentration of oxygen in the capillary blood sample will be lower than that of arterial blood because some oxygen molecules have already been taken up by tissues.

The capillary PO2 is not reliable; therefore, pulse oximetry must be used to monitor the patient’s oxygenation while a capillary blood gas test is performed.

Capillary Blood Gas Practice Questions:

1. Which arterial sampling sites are used for newborns?
The radial artery is the preferred site. You should avoid the brachial and femoral arteries. Also, the dorsalis pedis or posterior tibial are alternative sites.

2. What equipment is required for arterial puncture?
A 1 ml pre-heparinized tuberculin syringe, 25-gauge needle or pre-heparinized 25-gauge butterfly needle infusion kit, correctly fitting exam gloves, povidone iodine and alcohol wipes, sterile gauze, needle capping, and protection device, eye and splash shield, and patient label.

3. How much heparin do you push out before performing arterial stick?
All of it.

4. How do you insert the needle for an arterial stick?
Bevel up and insert at a 45-degree angle.

5. How long is pressure applied after an arterial stick?
At least 5 minutes.

6. What are the advantages of the capillary sample?
Used when ABG indicated but peripheral access not available, less hazardous and easier to obtain, and assess pH and CO2 disturbances.

7. What are the disadvantages of the capillary sample?
PO2 unreliable, not be done on a patient who has begun to walk or has calluses, not be done on inflamed or swollen areas, not be done on cyanotic or poorly perfused areas, and samples clot easily.

8. What should you do with air bubbles?
They should all be expelled completely.

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 an hour.

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11. What will be prevented by alternating arms for the arterial stick and holding pressure for 5 minutes?

12. What are the contraindications for an arterial puncture?
Lack of collateral circulation, extremities previously blanched (temp blood flow lost), distal to or through a surgical shunt, signs of infection at the site, and evidence of peripheral vascular disease.

13. What are the complications for an arterial puncture?
Hematoma formation treatment– pressure and switch sites, scarring, laceration of the artery (due to the artery being so tiny), and nerve damage (brachial, femoral; posterior, tibial).

14. What is a capillary blood gas used for?
It is an alternative to an ABG.

15. What best correlates best with a capillary gas?
The arterial pH and CO2 correlate well with capillary blood gas results. The PaO2, on the other hand, does not correlate well.

16. What are the 3 things that affect low accuracy in capillary gas samples?
Hypotension is the most important as it affects the correlation between arterial and capillary samples. Hypothermia and hypovolemia also affect low accuracy.

17. What are the puncture sites for a capillary blood gas?
Posterolateral foot, palmar or fleshy surface of distal aspects of fingers and toes, and earlobes.

18. Which puncture site for a capillary gas has the higher risk of nerve damage?
Fingers and toes.

19. What are the contraindications for capillary blood gas sampling?
If you need an accurate assessment of oxygenation, 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 reflection of patient’s condition leading to mismanagement of respiratory care, burns from warming heel too much, infection, scarring, inflammatory changes, nerve and artery damage, bruising, hematoma, and bleeding.

21. The umbilical cord has one floppy vein and how many arteries?

22. What is the tip placement for umbilical artery catheterization?
High= T6-T8 and low= L3-L4.

23. What is the umbilical artery catheter connected to?
The fluid pressure transducing system.

24. What are the complications of an arterial catheter?
Infection after 72 hours, thrombosis less than 5 years old, blood volume, hemorrhage (blood transfusion may be given), ischemia (pallor, decreased pulses, poor cap refill), and air embolism (smallest amount of air can cause embolism to brain).

25. What does the placement of an arterial line allow?
The direct measurement of arterial blood pressure.

26. Blood pressure varies with age and the infant’s blood pressure is approximately what?

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?
Alternative for patients requiring frequent blood gas analysis, designed for small patients with small blood volumes, and in line with an arterial or umbilical catheter.

29. What are the indications for a central venous catheter?
Cardiovascular instability, intravascular volume disturbance (dehydration, hemorrhage, increase ICP), and administration of drugs, fluids, nutritional support.

30. CVP monitoring measure the right atrial pressure helping to assess the fluid volume and also helps with what?
It helps to secure a long-term venous site in a chronically ill child.

31. What will cause the 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 most common, external and internal jugular, brachial, and saphenous.

33. What are the complications of central venous monitoring?
Catheter sepsis in over 72 hours (fungal issue), pulmonary embolism, cardiac dysrhythmias (due to catheter slipping into RV), and perforation of the trachea (catheter may slip into jugular vein).

34. What does the central venous catheter measure?
Right atrial pressure.

35. A decreased CVP indicates what?
Hypovolemia and/or shock.

36. An increased CVP indicates what?
Hypervolemia and/or LV failure.

37. Pulmonary artery catheters are also known as what?
Swan Ganz catheter.
38. What is the pulmonary artery catheter used for?
It measures left ventricular function, guide fluid management, aid in diagnosing pulmonary disease and cardiac dysfunction, and CO and mixed venous O2.

39. What are the complications of a pulmonary artery catheter?
Bleeding, pneumothorax, heart valve damage, heart chamber perforation, and cardiac arrhythmias (PVC’s).

40. What does the pulmonary artery pressure measure?
Right atrial pressure (RAP), Pulmonary artery pressure (PAP), Pulmonary capillary wedge pressure (PCWP), and cardiac output.

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 and consequently an increase affinity of HB for O2.

43. When does methemoglobin form?
When HB is oxidized to the ferric (iron) state.

44. What causes a baby to develop methemoglobinemia (MetHb)?
Medications and nitric oxide.

45. Carbon monoxide poisoning causes Hb to combine with what?
CO, not oxygen.

46. What is the CaO2 equation?
(Hb x 1.34 x SaO2) + (PaO2 x 0.003).

47. What is the most accurate way to detect changes in oxygen levels of the blood?

48. What method for obtaining ABG should be tried initially in a neonate?
Umbilical artery catheter.

49. The high placement of umbilical artery catheter should be which landmark on X-ray?

50. What is the most important advantage of continuous in-line blood gas sampling compared with umbilical blood gas sample in neonates?
Decreased amount of blood wasted.

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51. What is the cardiac index?
Cardiac output/BSA.

52. What is a common factor that can reduce PVR?
Nitric oxide.

53. Which measurement requires a Swan-Ganz catheter?
Pulmonary 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?
Radial artery and umbilical artery.

56. What is the capillary sampling site for neonates?
The lateral portion of heal.

57. What is the umbilical artery catheter placement?
It is inserted through umbilicus into 1 of the 2 Umbilical arteries. Heparinized isotonic saline solution is attached to prevent clotting. It is secured by suturing or umbilical tape.

58. What are the advantages of an ABG sample?
It provides reliable values for pH, oxygen, and CO2, and arterial lines make for easy and quick access.

59. What are the disadvantages of an ABG sample?
They are hard to obtain, can damage the artery, infection/ clotting, and stimulation of the infant will affect the values.

60. What are the complications of an ABG sample?
Technique, instrument (analyzer), and post puncture care.

61. What are indications for transcutaneous monitoring?
Monitor adequacy of arterial oxygenation and ventilation. Evaluate response to diagnostic & therapeutic interventions. It is useful for continuous monitoring. Last, it is useful when direct measurement of ABG is not available.

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 positive results and tissue injury.

64. What are the advantages of transcutaneous monitoring?
Continuous monitoring of O2 or CO2 and non-invasive.

65. What are the disadvantages of transcutaneous monitoring?
Burns and erythema, frequent calibration, equilibration time, and oxygen values become less reliable on older infants.

66. What is capnography?
Used with ventilated patients, special neonatal adapter needed, smaller ETCO2 adapters, accurately measure smaller volumes, moisture and secretions affect readings, and ETCO2 values usually 2-3 torr lower than ABG values.

67. What is a pneumogram and apnea monitor?
It measures heart rate and respiratory rate. Electrodes placed in mid-axillary line and held in place with a flexible belt. Monitor senses the changing impedance as chest moves and calculates respiration.

68. What are the indications for a pneumogram?
It is used with babies at risk for Sudden Infant Death Syndrome (SID), bradycardia, or apnea spells.

69. What is the preferred puncture site for a capillary blood gas?
The heel is preferred.

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.

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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?
You should wipe away the first drop of blood before collecting a sample.

74. How long do you have to analyze the sample?
The sample should be analyzed 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 an arterial sample. The PaO2, however, does not. Only look at the infant’s ventilatory parameters during a capillary blood gas analysis.

Final Thoughts

A capillary blood gas test is a valuable diagnostic tool that can provide information about an infant’s respiratory status. It involves puncturing the infant’s heel with a lancet in order to collect a small sample of blood.

The pH and PCO2 are the two primary values that are interpreted in order to determine the best course of treatment. The capillary PO2 is not reliable and should not be used to make clinical decisions.

Respiratory therapists are involved in this entire process, which is why this is an important topic.

Be sure to read our guide on arterial blood gas (ABG) interpretation to learn about analyzing breathing in adults. Thanks for reading!

John Landry, BS, RRT

Written by:

John Landry, BS, RRT

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.


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