Blood Sampling of the Dorsal Pedal Artery

Arterial and venous blood gas samples can be used to assess acid-base status, electrolyte levels, and respiratory function. Arterial samples are the standard for measuring oxygenation (partial pressure of oxygen of arterial blood gas [PaO₂]) and ventilation (partial pressure of carbon dioxide of arterial blood gas [PaCO₂]).¹

Samples for arterial blood gases can be obtained from several sites, including the dorsal pedal, femoral, lingual, and coccygeal arteries. The dorsal pedal artery is the preferred sampling site because it is easy to access and a pressure bandage to minimize bleeding can be applied following sample collection.²

Arterial blood samples should be collected in a heparinized syringe to prevent the sample from clotting. Purpose-made heparinized blood gas syringes that allow for automatic syringe filling with the arterial pulse are available (Figure 1). Standard syringes can be used and filled with heparin; however, heparin should be maintained at <4% of the total blood volume to avoid excessive sample dilution.³

FIGURE 1 A purpose-madeheparinized blood gas syringe

Following collection with a narrow-bore needle, air should be expelled from the syringe, and the sample should be capped immediately to avoid exposure to atmospheric air and subsequent sample contamination (Figure 2). Air bubbles should also be removed from the sample to reduce analytical error.

FIGURE 2 A sample capped to avoid air exposure and contamination

Complications of arterial sampling are rare but include pain, hemorrhage, hematoma formation, thrombosis, and infection at the sampling site.¹ Arterial sampling is contraindicated in patients with thrombocytopenia or coagulopathy.

Step-by-Step: Blood Sampling of the Dorsal Pedal Artery⁴

What You Will Need

• Examination gloves

• Hair clippers

• Diluted chlorhexidine solution (2% or 5%)

• Small-bore needle (25 or 27 gauge)

• Heparinized blood gas syringe

• Pressure bandage materials

Step 1: Restrain the Patient

While wearing examination gloves, restrain the patient in lateral recumbency with the pelvic limb used for sampling placed on the dependent side.

Author Insight

Sedation is typically not necessary; however, butorphanol can be considered if deemed safe in a patient that is potentially hypoxemic.

A team member should hold the distal thoracic limbs with one hand and place their forearm over the patient’s neck to control the head, ensuring normal breathing is not restricted. The other hand should grasp the patient’s Achilles tendon above the hock to restrain the leg that will be used for sampling.

Step 2: Palpate the Pulse

Palpate the pulse on the dorsal medial aspect of the dependent limb.

Step 3: Prepare the Site

Clip the hair, and use a diluted chlorhexidine solution to aseptically prepare the arterial sampling site.

Step 4: Insert the Needle

Palpate and localize the arterial pulse using a finger on your nondominant hand. Hold a heparinized blood gas syringe between the thumb and first fingers of your dominant hand (similar to holding a pen), and advance the needle into the vessel at a 45-degree angle directly below the palpated arterial pulse.

Author Insight

The force of the arterial pulse should fill the syringe passively.

Step 5: Remove the Needle

After an adequate sample is obtained, remove the needle, and apply direct pressure with a pressure bandage over the arterial puncture site for 5 minutes. Closely monitor the site for bleeding.

Step 6: Secure the Sample

Ensure all air is expressed from the syringe immediately after sample collection by holding the syringe vertically with the needle pointing up and tapping gently with a finger. Quickly cap the sample to avoid exposure to atmospheric air.

Author Insight

Samples should be analyzed immediately after collection. If immediate analysis is not possible, samples should be capped and stored on ice.⁵

Interpretation of Arterial Blood Gas Results

Hypoxemia is defined as PaO₂ <80 mm Hg or oxygen saturation (SpO₂) <95%. Hypoventilation is defined as PaCO₂ >45 mm Hg or partial pressure of carbon dioxide in venous blood (PvCO₂) >55 mm Hg.⁶

The ratio of PaO₂ to fraction of inspired oxygen (FiO₂; P:F ratio) and the alveolar-arterial (A-a) gradient can assess oxygenation. Only arterial samples can be used to assess oxygenation. Conversely, both arterial and venous samples can be used to assess ventilation. PaCO₂ from arterial samples is ≈3 to 6 mm Hg lower than in venous samples (PvCO₂).⁷

P:F ratio

The P:F ratio can help determine the adequacy of oxygenation (which is particularly useful for patients receiving oxygen supplementation) and oxygen supplementation requirements.⁶

In general, PaO₂ should be ≈5 times FiO₂. Room air is 21% oxygen (0.21). A patient without lung disease breathing room air would therefore have a PaO₂ of ≈100 mm Hg.

A normal P:F ratio is ≈500. For example, in a healthy patient breathing room air, the P:F ratio is 476: 100/0.21 = 476 mm Hg.

P:F ratio ≤200 mm Hg suggests significant pulmonary disease, defined as Veterinary Acute Respiratory Distress Syndrome (ie, VetARDS); P:F ratio ≤300 mm Hg is defined as Veterinary Acute Lung Injury (VetALI). For example, in a patient with pulmonary disease and PaO₂ of 78 mm Hg breathing 30% oxygen via nasal cannula, the P:F ratio is 260: 78/0.3 = 260 mm Hg, which is consistent with VetALI.

The P:F ratio is most useful for evaluation of oxygenation in patients receiving supplemental oxygen. The A-a gradient or 120 rule (see The 120 Rule) should be used to evaluate oxygenation in patients breathing room air.

Alveolar-Arterial Gradient

A-a gradient is a measure of the difference between the concentration of oxygen in alveoli (P_AO₂) and the concentration of oxygen in arterial blood (P_aO₂).⁶ This gradient uses the following formula to help establish whether hypoxemia is due to ventilation failure (ie, hypoventilation) or pulmonary disease⁸:

P_AO₂ − P_aO₂

To calculate the A-a gradient, P_AO₂ must first be calculated using the alveolar gas equation:

P_AO₂ = [FiO₂ × (Pb-PH₂0)] – P_aCO₂/RQ

FiO₂ is expressed as a decimal; Pb (atmospheric pressure at sea level) is 760 mm Hg; PH₂0 (partial pressure of water) is 47 mm Hg; and RQ (respiratory quotient) is generally 0.8 but can vary based on diet and metabolism. For patients living at sea level and breathing room air, the formula can be simplified to:

P_AO₂ = 150 – (P_aCO₂/0.8)

For patients breathing room air, a normal A-a gradient should be <10 mm Hg; values >20 mm Hg indicate decreased oxygenation efficiency.

The 120 Rule

P_aCO₂ + P_aO₂ (ie, the 120 rule) can be used to assess lung function in patients breathing room air and assesses oxygenation alongside ventilation, as a change in one value can affect the other.⁶

In healthy patients, P_aCO₂ added to P_aO₂ should equal at least 120. For example, a patient with a normal P_aCO₂ of 40 mm Hg and a minimum normal P_aO₂ of 80 mm Hg will have a P_aCO₂ + P_aO₂ value of120.

The 120 rule can also help determine whether hypoxemia is due to hypoventilation alone or both hypoventilation and lung dysfunction. For example, if a patient has an increased PaCO₂ of 65 mm Hg and a PaO₂ of 55 mm Hg, then P_aCO₂ + P_aO₂ = 120, which indicates hypoxemia is due to hypoventilation alone. If the patient instead has a P_aCO₂ of 65 mm Hg and a PaO₂ of 35 mm Hg, then P_aCO₂ + P_aO₂ = 100, which indicates hypoxemia is due to lung dysfunction in addition to hypoventilation.

Example of an Arterial Blood Gas

The patient in Table 1 is breathing room air; FiO₂ is 21%.

The simplified formula for P_AO₂ is 150 − (21.6/0.8), which equals P_AO₂ of 123. The A-a gradient is thus 123 − 48.9 = 74.1. This result indicates severe venous admixture or markedly decreased oxygen deficiency. Using the 120 rule, the P_aCO₂ + P_aO₂ value is 70.5, which is markedly below 120 and indicates severe lung dysfunction.