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How to Measure Central Venous Pressure in Standing Adult Horses
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1. Introduction
Central venous pressure (CVP) is the blood pressure within the vena cava (usually measured within the cranial vena cava). It closely corresponds with the pressure in the right atrium and can be used to gain information about cardiac function as well as intravascular blood volume. Central venous pressure affects the end diastolic volume of the right ventricle and therefore influences stroke volume and cardiac output. In equine patients, CVP is used most commonly in the monitoring of fluid therapy, and is particularly important for patients with altered fluid balance (i.e., kidney failure, heart failure). The mean reported value for a standing, adult horse has ranged between 7.5 and 12 cm H2O depending on the point of reference that was used [1,2]. Trends in CVP may be particularly helpful in deciding when to increase or decrease the rate of fluid administration.
The measurement of CVP requires the presence of a catheter tip within the cranial (or caudal) vena cava. While this can easily be achieved with the typical catheters used in neonatal foals, it is more difficult in adult horses. Previous articles have described the use of a 25-in, 16-gauge catheter or the use of sterile tubing passed through a 10- or 12-gauge catheter inserted through the jugular vein [1-4]. Unfortunately, most equine practitioners use 14-gauge, 5.25-in catheters during hospitalization and fluid administration.
The following technique describes a simple, inexpensive approach to measure CVP in adult horses that primarily uses materials stocked by most equine practitioners (ambulatory or in-hospital).
2. Materials and Methods
A pre-placed 14-gauge catheter [ a] with a T-port extension set [ b] is needed, along with the following equipment:
- 14-gauge needle (1 or 1.5 in)
- 3.5-French polypropylene urinary catheter (sterile, ~55 cm) [c]
- Glass or plastic manometer (sterile) [d]
- 60-ml syringe containing heparinized saline
- Two to three sets of 30-in extension tubing (sterile) [e]
- Three-way stop-cock [f]
- Sterile gloves
Restraint
If the horse is quiet, a halter and lead rope is sufficient restraint for the procedure. Otherwise, standing stocks or a nose twitch is recommended.
Technique
Set-Up
- Attach the three-way stop-cock to the bottom of the manometer and one to two extension sets to either end of the stop-cock (Fig. 1).
- Attach the 60-ml syringe to one end of the extension sets and fill the lines and approximately one-half of the manometer with the heparinized saline. The stop-cock should then be turned off to the patient side.
- Wipe off the proximal port of the T-port catheter extension set with alcohol and allow to completely dry.
- Flush the 3.5-French polypropylene urinary catheter with heparinized saline.
Figure 1. Set-up for central venous pressure measurement using a 60-ml syringe, extension tubing, stop-cock, manometer, 14-gauge needle, 3.5-French polypropylene catheter, and a 14-gauge catheter with T-port extension set.
Catheter Insertion
- After applying sterile gloves, insert the 14-gauge needle into the proximal port of the T-port.
- After estimating the length between the insertion needle and the cranial vena cava, insert the 3.5-French polypropylene urinary catheter through the needle to this point.
Measurement
- Place the zero point of the manometer at the level of the point of the shoulder that is approximately the level of the cranial vena cava.
- Attach the free end of the extension tubing to the polypropylene catheter and turn the stop-cock arm toward the syringe.
- Wait for the meniscus of water to adjust to a constant level. Small fluctuations will be seen because of changes in pressure from breathing and myocardial contraction. Multiple measurements can be taken to ensure repeatability.
Removal
- Turn the stop-cock arm toward the patient side and then remove the 14-gauge needle from the T-port.
- Remove the polypropylene catheter.
3. Results
Central venous pressure was measured in three clinically normal horses and four horses with clinical disease (Table 1). Included in these horses were two Quarter horses, two paint horses, two thoroughbred horses, and one Arabian horse.
All measurements were performed in a standing position. Sedation was not required in any case and the procedure was tolerated well by all animals. Five horses were placed in standing stocks, and two horses were restrained only by a handler with a halter and lead rope.
Table 1. Central Venous Pressure Measurement in Seven Horses: Signalment, Clinical Diagnosis, and Measurement | ||||
Horse | Age | Sex | Diagnosis | Central Venous Pressure Measurement (cm H2O) |
1 | 20 | G | Normal | 10 |
2 | 24 | G | Normal | 12 |
3 | 18 | M | Normal | 10 |
4 | 20 | G | Anuric renal failure | 25 |
5 | 10 | G | Uncontrolled hemorrhage | <0 |
6 | 12 | G | Right dorsal colitis | 0 |
7 | 9 | M | Mediastinal lymphoma (40 liter of pleural effusion) | 30 |
4. Discussion
Compared with previously reported techniques, the major advantage of the CVP measurement procedure described here is that it can be used in horses with a standard 14-gauge catheter. This makes the cost of measuring CVP minimal, and it does not require the placement of a new catheter if this measurement becomes necessary after the horse was initially catheterized. It is also a technique that can be performed in a field situation. If sedation is required, xylazine has been reported to have minimal effects on CVP, whereas acepromazine decreased this measurement in a group of normal horses [2].
One limitation of the technique described in this paper is the length of the polypropylene catheters that are available (55 cm). For most horses in our practice, this length has been adequate. Particularly large horses may require the catheter to be placed lower on the neck or may need longer tubing. In a previous study evaluating 74 horses and ponies, 55 cm was the maximum length required [1].
The sensitivity and specificity of CVP to detect alterations in intravascular volume in horses has not been determined. It is extremely important to use all available information (physical exam, history, laboratory work) when making decisions about fluid therapy and not to base treatment decisions on one specific test. In the small number of clinical horses included here, CVP measurements were consistent with expected values based on the conditions affecting each horse. Additional research is needed with a larger number of clinical horses to develop more specific criteria for use of this measurement. In the experience of the authors, CVP values of <5 cm H20 may be consistent with inadequate circulating volume, whereas values >20 cm H20 may indicate the need for more conservative fluid therapy. Evaluating changes in CVP over time or in response to a fluid bolus can also provide useful clinical information.
Based on these results and our experience with this technique, CVP is a simple and inexpensive diagnostic test that can aid in the treatment of critically ill horses. This measurement is particularly important in horses that are showing signs of fluid overload, or horses displaying signs of hypo-perfusion that are not improving with additional fluid boluses.
Footnotes
- Abbocath-T, Abbott Ireland, Sligo, Ireland.
- B. Braun Medical Inc., Bethlehem, PA 18018.
- Tyco Healthcare Group LP, Mansfield, MA 02048.
- Allegiance Healthcare Corporation, McGaw Park, IL 60085.
- Abbott Laboratories, North Chicago, IL 60064.
- Sherwood Medical, St. Louis, MO 63103.
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