Arterial catheters and blood gases: What do all these numbers mean? (Proceedings)
The use of arterial blood gas analysis has become commonplace in many veterinary practices for the assessment both qualitative and quantitative assessment of metabolic and respiratory acid-base problems, including the interrelationships between ventilation, oxygenation, and metabolic conditions. Blood gas analysis is a useful adjunct to clinical patient assessment in determining appropriate therapy for specific and complex conditions. There are two reliable means to procure an arterial sample, whether by a one time draw or by placement of an arterial catheter either can get you the necessary sample. The placement of arterial catheters, once learned, is a useful, invaluable skill for veterinary technicians to possess.
The advantage of analyzing arterial over venous blood samples is that you can access both gas exchange and metabolic status. When using venous samples the data is only reliable for metabolic status. The more clinical applications for blood gas analysis would be ventilation status and acid-base status by differentiating hypo-ventilation from other causes of hypoxemia, helping to determine the need for supportive therapy and to monitor the response to therapy.
Sample drawing techniqueMost times a one or three milliliter heparinized syringe is used along with a 25ga. needle. Artery location is up to the person drawing the sample but commonly used sites are the femoral artery, the dorsal pedal artery and the lateral coccygeal artery (tail). NOTE: When "sticking" an awake patient a moderate amount of discomfort and pain are felt by the patient, to avoid this you can apply 4% lidocaine crème to the area, wait a few minutes and proceed as the area will be effectively numb. Once a site is chosen the artery should be palpated. It is best to secure the artery between two fingers and advance the needle into the artery. Arteries are thick walled and loosely attached to the adjacent tissues; therefore the needle must be place above the artery and advance with a short, jabbing motion to achieve entry. Many times arteries will constrict as you attempt to enter them, hence some training is involved before you become proficient at this skill. Once in the artery, many times the plunger on the syringe will pulse back and fill the syringe. The sample should be a brighter red (than venous samples) unless the patient is extremely compromised.
After removing the needle from the site, pressure should be applied to the site for 5 minutes to prevent the formation of a hematoma. Any air that is entrapped in the sample syringe should be immediately removed, the syringe should be capped with a rubber stopper (needle discarded) to prevent false readings due to room air exposure. If the sample is not to be immediately analyzed it should be place on ice.
Placement of arterial catheters
As stated, once a technician becomes proficient at placing arterial catheters, their use can be invaluable. The most common site for an arterial catheter is the dorsal pedal artery as it allows mobility in the awake patient. Although this site is at times difficult for catheter placement in short legged animals as you tend to hit the carpal joint with the catheter and are unable to feed it past that point. Typically, a smaller arterial than venous catheter is placed with 22-20 gauge catheters being the most commonly used. With larger patients 2 inch catheters give you more stability in transport. However, it is not always possible due to disease progression causing contracted arteries. Other common places are the lateral coccygeal (tail) artery for either awake or anesthetized patients or the lingual artery as a last resort in anesthetized patients. Caution should be used when placing tail catheters for contamination from feces. Femoral catheter placement, while not impossible, causes it's own set of problems if the catheter becomes dislodged as bleeding from this site is a bigger issue than the other sites. Caution should be used when dealing with any arterial catheter and the real possibility of significant blood loss via disconnection.
Monitoring blood pressure
Placement of an arterial catheter will allow us to most accurately monitor blood pressure. This is the most precise form of BP monitoring as it measures beat-by-beat inside of the artery as opposed to non-invasive devices which tend to give non-reliable readings through fur and tissue. The values that are collected via this method are: systolic pressure defined as the peak pressure in the arteries, which occurs near the beginning of the cardiac cycle, the diastolic pressure is the lowest pressure (at the resting phase of the cardiac cycle), the mean arterial pressure (MAP) and the pulse pressure which reflects the difference between the maximum and minimum pressures measured. The main concern of these 4 measurements in clinical practice is the MAP or mean arterial pressure which in small animals should be between 60-70 mmHg in order to profuse the bodies organs sufficiently.