Inferior Vena Cava
Draining half of the systemic blood toward the heart and the necessary crossroads of lower extremity thromboses, the inferior vena cava (IVC) has a clear strategic situation. Ultrasound occupies a major place in the search for thromboses, but also in assessing the IVC dimensions, a possible marker of the circulating blood volume, as well as other more marginal applications.
The iliac veins are discussed in Chap. 14.
The Normal Inferior Vena Cava
The inferior vena cava can be separated by the renal veins into supra- and infrarenal portions.
The infrarenal portion analysis is conditioned by gas, frequent in this area. However, the free hand of the operator (and not the probe itself) can drive most gas away by applying gentle pressure. The suprarenal portion is often visible using the liver acoustic window. It makes its way vertically, at the right of the aorta, receives the hepatic veins and opens into the right auricle (see Fig. 4.2, p. 19). A spontaneous echoic flow can sometimes be ob- served. This flow can hesitate, or even be inverted at inspiration (in mechanically ventilated patients), an obvious sign of tricuspid regurgitation. This echoic flow is possibly explained by agglomerated blood cells  and can be massive (Fig. 13.1). Fine analysis of the content of the inferior vena cava is generally possible. Extrinsic obstacles, catheters or caval filters can be observed (Fig. 13.2).
The venous caliper is modified by respiratory and cardiac rhythms. There is usually inspiratory collapse in the spontaneously breathing subject.
These variations in caliper are a sign of venous patency.A compression maneuver is perfectly pos- sible, but the pressure should be brought by the operator’s free hand with spread fingers, with the probe applied between two fingers. A compression by the probe alone would possibly damage the probe, and it can be harmful for the patient. This
Fig. 13.1. Inferior vena cava, longitudinal scan. In this vein, an echoic flow with visible particles goes toward the right cavities. In addition, there is a bulge in the upper portion of the vein (arrows), a frequent variant of the normal (saber profile). Note that a measurement of the vein caliper at this level would yield misleading information in predicting central venous pressure
Fig. 13.2. Catheter (arrow) within the inferior vena cava lumen
maneuver pays off for subjects with favorable mor- photype: the inferior vena cava can be easily col- lapsed. Note that such a maneuver does not affect the instantaneous blood pressure. The infrarenal segment can also be collapsed this way. If gentle pressure does not succeed, it seems wise not to insist.
The technique is the same as for the upper or low- er extremity veins. The only difference is that the static approach should be called a pseudo-static approach, so to speak, as the frequent necessity to drive digestive gas off can alter some parameters.
Thrombosis will give signs:
∑ Static in the static approach:
∑ Endoluminal echoic irregular pattern (Fig. 13.3).
∑ Dynamic in the static approach:
∑ Absence of spontaneous inspiratory changes (see »Normal and Pathological Patterns«
∑ In the dynamic approach:
– Noncompressible vein. This maneuver is redundant and should not be performed if previous approaches have identified a throm- bosis.
Caval Filter and Ultrasound
When local conditions are good, the correct posi- tion of a caval filter and its relations with the renal veins can be accurately assessed (Fig. 13.4).
If transportation of a critically ill patient or irra- diation in a pregnant woman must be avoided, it could be advantageous to insert caval filters at the bedside, using ultrasound guidance. Once the floating infrarenal thrombus is identified, and once the indication is adequate (this would war- rant an entire chapter), one operator inserts the fil- ter while another locates the main landmarks using ultrasound. As for the pilot–bombardier relation in a B25, the two operators should be per- fectly trained since the roles are permanently inversed.
The inferior vena cava can be round or flat- tened; see the next section.
Inferior Vena Cava Diameter and Central Venous Pressure
This long section gives clues for accurate measure- ment of the caliper of the inferior vena cava, which should take only a few seconds.
The accuracy of central venous pressure as a marker of circulating blood volume will not be dis- cussed here. It could warrant another chapter in itself. Recently, this data has been ignored, as it appears old-fashioned to some. A discussion of modern hemodynamics can be read in Chap. 28.
Fig. 13.3. Massive thrombosis of the infrarenal inferior vena cava. Transverse scan of the umbilical area. Anteri- or to the rachis (R) and at the right of the aorta (A), the venous lumen of the inferior vena cava is filled with echoic material, indicating here a recent thrombosis.
Note that this recent thrombus is still soft. Hence, a com- pression maneuver may collapse the venous lumen, with doubtful consequences. Young patient with polytrauma
Fig. 13.4. Caval filter, perfectly identified within the lumen of the suprarenal IVC (arrow). Epigastric trans- verse scan. One can imagine the possibility of inserting this device at the bedside
Our aim is to provide simple noninvasive data to the intensivist who may find it useful . Ultra- sound measurement of the IVC caliper lies between the invasive method of inserting a central venous pressure system and the more invasive trans- esophageal approach.
Circulating blood volume is mainly located (65%) in the venous system. We therefore imagine that a variation in this volume will affect this sec- tor, the IVC being an ultrasound-accessible por- tion. A flattened pattern in the obviously hypov- olemic patients having been regularly observed, we investigated this parameter in 54 ventilated patients (Fig. 13.5). A caliper less than 10 mm was correlated with a central venous pressure under 10 cm H2O with an 84% sensitivity, a 95% speci- ficity, an 89% positive predictive value and a 92%
negative predictive value . Figure 13.5 shows that the relation is better for the small caliper val- ues. Some studies have been conducted in this field [4–7], but most came from cardiologic, non- critical, spontaneously breathing, laterally posi- tioned patients, with measurements made at the hepatic vein level, making any comparison diffi- cult. Only one study dealt with ventilated patients and indicated that a caliper of⭐ 12 mm always pre- dicted a central venous pressure ⭐ 10 mmHg .
Simple requirements are necessary for a both accurate and reliable information.
1. The patient remains supine. Lateral decubitus would squash the IVC by the liver.
2. The IVC should be sought in a longitudinal axis first. A probably frequent mistake is the confu- sion between the IVC and a hepatic vein (see Fig. 4.3, p 20). Several profiles exist:
– A regular profile.
– A saber profile (Fig. 13.1). This frequent find- ing, with a bulge when the IVC receives the hepatic veins, should be recognized and the operator should remain far from this area, whose measurement would give erroneous information.In addition,the venous tissue pro- gressively becomes cardiac tissue in this area.
– An irregular, moniliform profile (Fig. 13.6).
3. The probe is then applied in a transverse axis. A measurement in a longitudinal axis would expose to overestimation of the caliper, when the vein is not perfectly located in a frontal axis.
4. The left renal vein should be looked for (Fig. 13.7). This landmark has two advantages: it is a reliable place, and we are definitely far from the hepatic bulge.
5. Measurement should be from face to face, not from border to border.
6. An end-expiratory measurement is needed (see
»Normal and Pathological Patterns« below).
7. The increase in caliper with heart beats was not taken into account in our practice.
In addition, we did not index IVC caliper with body surface for two reasons. Risk is involved in determining these data in a critically ill, unstable patient, since it is necessary to weigh the patient.
Second, IVC dimensions are not correlated with the morphotype . Human eye diameter varies little in relation to weight and height as well.
Fig. 13.5. Correlation between expiratory caliper of the inferior vena cava at the left renal vein (VCI) and central venous pressure (PVC) in 59 ventilated patients
Fig. 13.6. Irregular pattern, mostly collapsed, of the in- ferior vena cava. Hypovolemic patient. Note the bulge (saber profile) at the left of the image
Normal and Pathological Patterns
In spontaneous ventilation, inspiratory caliper diminishes. This is seen in ambulatory abdominal examinations, as the patient is fasting (i.e., in mod- erate hypovolemia).
In mechanical ventilation, inspiratory caliper increases, for positive thoracic pressure creates an obstacle to venous return. Inspiratory collapses are found in nonsedated patients.
The expiratory caliper seems more constant. It does not vary after intubation of a patient, where- as inspiratory caliper is usually seriously disrupted.
Inspiratory collapse of a spontaneously breath- ing patient (Fig. 13.8) can be explained by a dysp- nea with use of accessory respiratory muscles, since the inspiratory collapse of thoracic pressure creates aspiration of the systemic blood, with the Venturi effect. This situation is striking in acute asthma (where fluid therapy is not at all con- traindicated). However, not all dyspneic patients, even with substantial use of accessory respiratory muscles, have inspiratory collapse.
An enlarged IVC (Fig. 7.1, p 41), with enlarged hepatic veins, is seen in right heart failure or hypervolemia, or can again be normal. Central venous pressure can be low but is rarely so.
A flattened IVC in a shocked patient (Fig. 13.7) is correlated with low central venous pressures, and indicates a hypovolemic part.
When the central venous pressure is rapidly altered, by fluid therapy, variations of PEEP or
disconnecting the ventilator, IVC caliper follows (Fig. 13.9).
Advantages of the Ultrasonic Method
One should first note that the possible errors of this noninvasive method should be compared with the numerous errors in the measurements and interpretations of central venous pressure or wedge pressure . Then the advantages can be delineated:
∑ These data are immediately available.
∑ The technique is simple (simple unit, without Doppler).
∑ There is no invasive procedure.
∑ The measurement does not affect the treatment (whereas measurement of the central venous Fig. 13.7. This transverse epigastric view shows the renal
veins’ point of arrival. The left renal vein is particularly visible, passing between the aorta and the superior mesenteric artery (v), the point where we chose to mea- sure the IVC caliper. Here, an expiratory caliper of 8 mm (arrows) indicates low central venous pressure
Fig. 13.8. Inspiratory collapse of the inferior vena cava.
Time-motion acquisition, showing a 12-mm diastolic caliper (V) that collapsed to 4 mm at inspiration in a patient with major bleeding and spontaneous breath- ing
Fig. 13.9. Caliper of the inferior vena cava (VCI) when the central venous pressure (PVC) is altered
pressure means clamping the catheter for a short time).
∑ The first measurement can be used. Conversely, the first information given by central venous pressure is not very useful: the intensivist mod- ifies therapeutic plans as its value evolves (a way to implicitly recognize the imprecision of this first value).
∑ A hydrostatic zero does not need to be defined, although this point can be debated.The supposed projection of the right auricle varies depending on habit. Error can be substantial in patients with widened anteroposterior thorax. Each measure- ment of the central venous pressure requires a number of verifications such as the height of the bed. These points cannot be checked a posteriori.
We could list more of these points.
∑ The intensivist tries to estimate a volume (the blood volume). Central venous pressure pro- vides a pressure (a rather indirect parameter).
IVC measurement provides a distance in mil- limeters, which is a less indirect parameter. In discordant patients (in the right-hand column of Fig. 13.5), one can then wonder which pa- rameter is misleading. It is in fact tempting to consider that the information given by a part of a volume is nearer the truth than the one given by a simple pressure.
In practice, this parameter will be integrated with others (heart or lung behavior; see Chaps. 17, 20, 28). We will conclude with this remark: even if a patient cannot benefit from cardiac or lung ultra- sound examination, any abdominal ultrasound test performed in a critically ill patient should include the degree of IVC filling.
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