Obtaining blood samples in newborns or infants can be a challenging task. Venous access in the patients can be difficult or limited. The vessel size and volume status of these patients can prove to be a challenge for the most skilled Emergency Physician.1 Capillary blood sampling is the most frequently used method to obtain blood samples from infants. This chapter reviews the basic principles and techniques of heel stick blood sampling.
ANATOMY AND PATHOPHYSIOLOGY
The selection site for capillary sampling is usually based on age and weight of the patient. The heel is the most common location for capillary blood sampling because the junction between arterial and venous blood supply is located just below the dermis.2 The depth of this capillary bed in newborns and infants is consistently between 0.35 and 1.6 mm and significantly increases with weight gain.3 A thick callus can form as the child ambulates and can interfere with adequate capillary blood collection at this site. Heel blood sampling is recommended in children weighing up to 10 kg. This equates to an approximate age of 6 months. Finger prick can be an alternative modality in patients over 6 months of age and weighing greater than 10 kg. The fingers or ear lobes are recommended in adults for capillary blood sampling. The third or fourth digits of the nondominant hand are the recommended site as they are less likely to have calluses. It is suggested to draw samples from the medial or lateral aspects of the heels and fingers (Figure 60-1).2 These areas avoid the blood vessels, bone, nerves, and tendons.
The topographic anatomy of the sole of an infant’s foot. The arrows represent the areas devoid of structures to perform the heel stick.
This technique is useful for small samples of blood or repeated small-volume blood samples. Commonly drawn samples include hemoglobin, hematocrit, blood glucose, and electrolytes.2 This modality is ideal for premies, newborns, and infants. Heel sticks are often used up to 12 months of age. This is a source of blood when other sources (e.g., arterial lines, central venous access, and intraosseous access) are not available or appropriate.
A heel stick is not recommended during poor peripheral perfusion or states of compromised blood flow to an extremity. Blood samples should not be collected through local infections, significant edema, or hematomas. Capillary blood gas collection is not the recommended specimen if a precise PaO2 is needed.2 Heel sticks are not the collection method of choice for blood cultures due to the likelihood of bacterial contamination.