An Update on Vascular Fast Ultrasound: Predicting Vascular Volume, Response to Fluid Therapy and Guiding Difficult Peripheral Vascular Catheter
S. Chalhoub; S. Boysen
Faculty of Veterinary Medicine, Veterinary Clinical and Diagnostic Sciences, University of Calgary, Calgary, AB, Canada
The goal of vascular VPOCUS is not an extensive evaluation of the great vessels. Rather, vascular VPOCUS, in conjunction with other clinical, history and VPOCUS findings (cardiac assessment of volume status, contraindication suggestive of volume overload), is used to assess changes in vena cava volume that can help us assess if we should be giving a bolus of fluids.
Caudal Vena Cava Volume Estimation
Emergency and critical care patients are often at risk to develop hypo- and hypervolemia. Unfortunately, predicting which patient can be challenging. Although results are preliminary, evaluating the caudal vena cava (CVC) shows promise in estimating the intravascular volume status in veterinary patients. By placing the probe longitudinally at the subxiphoid site and slowly tilting/fanning the probe to the right of midline the CVC can be seen crossing the diaphragm. The caudal vena cava diameter and the change in the CVC diameter between the expiratory and inspiratory phases of respiration can be detected at this site. The diameter and change in diameter with the respiratory cycle reflect the patient›s volume status. The CVC has a larger diameter at the end of expiration than it does at the end of inspiration. The changes between expiration and inspiration varies but is approximately 25–60%. The opposite is true in hypervolemic patients, or patients with increased right atrial pressures (i.e., pericardial effusion, right sided heart failure, etc.), where the CVC becomes “fatter” than normal, hardly changing (<20%) between expiration and inspiration. This is important because if the vena cava is “fatter” than normal, we should ask ourselves why and if IV fluids would be detrimental.
If the hepatic veins are visualized (often seen at the site, they enter the CVC just caudal to the diaphragm) they are often distended as well in cases with increased right atrial pressures and/or hypervolemia. There are a number of artifacts that can make the CVC appear smaller than normal. This includes 1) pressure artifact, that occurs when too much pressure is placed on the probe when trying to visualize the CVC, 2) increased abdominal pressure which may occur with organ enlargement or significant abdominal effusion and 3) increased respiratory effort which creates greater negative pleural pressure and therefore more collapse of the CVC.
Although these factors are likely to impact euvolemic or hypovolemic patients, they are less likely to change the findings noted in patients with hypervolemia or right atrial pressure increases (FAT CVC). For these reasons the author tends to ask the questions, “if the patient has clinical signs suggestive of hypovolemia (tachycardia, pale mucous membranes, weak pulses etc.) is it likely to be a fluid responder”? If the CVC is consistent with euvolemia or hypovolemia, then a bolus of fluids is administered provided there is no contraindication to giving a bolus (e.g., no increased B lines, normal left atrial:aortic ratio, no cerebral edema etc.). If the patient has a FAT CVC, further work up is required to determine if a fluid bolus is contraindicated. Ultrasound guided vascular access for difficult IV access. The following study (Costantino et al. Ultrasonography-guided peripheral intravenous access versus traditional approaches in patients with difficult intravenous access. Ann Emerg Med. 2005 Nov;46(5):456–61) is just one of many in the human literature demonstrating the value of ultrasound guided vascular access when traditional percutaneous IV access attempts fail.
The summary of this study is as follows:
- If a nurse failed to place an IV catheter after 3 attempts…
- Emergency physicians attempt to place percutaneous IV catheter
- One group using ultrasound guidance
- One group using “blind” traditional techniques
- Success rate was greater for the ultrasonographic group (97%) versus traditional (33%)
- Less time to successful cannulation from first percutaneous puncture (4 minutes versus 15 minutes)
There is emergency veterinary evidence that ultrasound guided peripheral vascular access is also helpful in small animal veterinary patients. Advantageous of ultrasound guided vascular access include the fact the vessel can be assessed for thrombosis prior to attempting placement of an IV catheter. In cases of hematoma or perivascular fluid (edema) the vessel can still be visualized making ultrasound guided access easier.
It should be pointed out that in an emergency unstable patient presenting in shock venous cut downs and automated intraosseous devices (EZIO) are preferred and ultrasound guided catheter placement is generally reserved for difficult IV catheter patients that are cardiovascular stable (Figures 4–6). Either longitudinal (in plane) or transvers (out of plane) vascular access ultrasound guided peripheral IV catheter placement can be used, although the author prefers out of plan/transverse placement, which will be demonstrated in the lab. Ultrasound guided arterial blood gas sampling is also advantageous to help ensure an artery and not a vein is punctured during sampling.
VIN editor: Figures not available at time of publication.