Barr: Comparison of two methods of blood sampling and two anesthetics on extended database values


We hope that this study will help validate a method of blood sampling that will cause less stress to our pets as well as limit the risk of anemia, blood contamination and sampling errors. Additionally this study will show whether blood drawn after anesthesia can be analyzed for routine assessment.


Critically ill pets as well as pets undergoing anesthesia generally require multiple blood samples to monitor their disease state and response to therapy. These blood samples may be obtained by directly drawing blood from a vein with a needle or from an indwelling intravenous (IV) catheter. Direct venipuncture can cause repeat trauma to vessels and can cause pain and anxiety in many patients. In order to minimize repeated stress to the patient and trauma to the blood vessel, blood samples may be obtained from indwelling IV catheters. This practice is routinely performed in people (Braun 2015), however there are only a limited number of studies in veterinary medicine comparing blood values obtained from IV catheters to those obtained via direct venipuncture.

When a blood sample is taken from an indwelling IV catheter, approximately 1 or more tablespoons of blood is removed prior to obtaining the sample for analysis in an attempt to clear the catheter of fluids or drugs that might dilute or contaminate the sample. However, there is often a significant amount of blood removed in a pre-sample in order to get more accurate results. With small patients and repeated blood sampling the potential for significant blood loss and subsequent anemia increases (Adlard 2008). One method for addressing this concern is the re-infusion technique of blood sampling whereby after the pre-sample and sample are drawn the blood from the pre-sample is given back to the patient via re-infusion. Some concerns with this method include the potential for re-infusing blood clots, damage to the red blood cells, potential contamination of the blood and the potential for mixing up the lab specimen and reinfusion syringes (Cosca et al 1998, MacGeorte et al 1988, Adlard 2008). Given these concerns a third technique has been proposed for sampling blood from IV catheters. The push-pull or mixing method involves drawing blood into a syringe and then pushing it back into the catheter 3 times without disconnecting the syringe. After the third cycle, the syringe is disconnected. A second syringe is then used to obtain the volume of blood necessary for lab analysis. This method limits blood loss and the potential for contamination of the re-infused blood (Adlard 2008). This method has been validated in adults (MacGeorge et al 1988) and children (Adlard 2008) but to our knowledge has not been examined scientifically in veterinary patients.

Unfortunately the temperament of some patients does not allow for the placement of an IV catheter for blood sampling and medication administration without the use of sedation. Different sedative drugs can affect blood cell count (Dhumeaux et al 2012, Wilson et al 2004). The effects of a newer anesthetic drug in North America, alfaxalone, on blood parameters has not yet been compared to propofol, another commonly used anesthetic. The objective of this study is to compare blood values obtained via direct venipuncture and via an intravenous catheter using the push-pull method. An additional objective is to assess the effect of two commonly used anesthetics on these hematologic values.

This study will be performed within 6 months using 30 healthy client-owned dogs that are undergoing anesthesia for a necessary surgical procedure. The dogs will be randomly assigned into 2 groups, one of which will receive propofol for anesthesia and the other will receive alfaxalone for anesthesia. Both groups will receive pain medication (methadone) prior to anesthesia. An IV catheter will be placed in all dogs prior to anesthesia. A small amount of blood (3ml) will be drawn from this catheter using the push-pull technique. At the same time the same amount of blood will be drawn directly from another vein. After the animal is under anesthesia these 2 small (3 ml) blood samples will again be obtained from all dogs using the previously described techniques. The blood samples will be analyzed for the proportion of red blood cells (PCV), total protein, electrolyte levels (sodium, potassium, chloride, calcium), lactate as well as acid-base status.