Volumes

Volumes in general

One of the most obvious ways one can think to measure the circulating volume in the body would be to start by measuring volumes being pumped through the heart. If one could measure the volume of blood being ejected out of the heart, the amount of blood returning to the heart, or the volume of blood/plasma through, then certainly a measure circulating volume could be gleaned. Though they specifically measure an area, a diameter, or even total body conductivity, each of the techniques that follow measure some underlying volume to assess intravascular fluid status: an area merely requires a third dimensional extension to represent volume, a diameter a long cylinder stands as a proportional measure of volume, and total body conductivity is a function of the volume of conductors.imensional extension to represent volume, a diameter a long cylinder stands as a proportional measure of volume, and total body conductivity is a function of the volume of conductors.

 

Right ventricular end-diastolic volume

End-diastolic volume is the upper bounds of the stroke volume calculation. It is the most blood that is ever present in the ventricles and one of two parameters influencing stroke volume. Measuring end-diastolic volume should therefore be a good indicator of volume status and a key predictor of stroke volume and cardiac output. Pulmonary artery catheters outfitted with fast acting thermistors enable calculation of the right ventricular end-diastolic volume by measuring the temperature drop between two successive heart beats and correlating that to the right ventricular ejection fraction. Once the ejection fraction is known, the right ventricular end-diastolic and end-systolic volumes can be calculated from the stroke volume. Though some researchers have found end-diastolic volume to accurately predict preload, cardiac indices, and more accurately guide fluid resuscitation treatments [43, 44, 45], others have not found the measure reliable [4, 38, 46]. The reason for these conflicting results is not clear, but some believe it stems from the calculation from the thermodilution-derived right ventricular ejection fraction [46].

Figure 1.11: Right ventricular end-diastolic volume represent the amount of blood left in the right heart at the end of diastole. The volume of the right ventricle at the end of diastole is difficult to measure with bedside techniques and is thus usually back-calculated from thermodilution based methods for determining ejection fraction.

 

Left ventricular end-diastolic area

Here as elsewhere, pressure-volume relationships serve as the foundation for intravascular volume. Fluids driven by venous pressure fill the heart during diastole. For a given cardiac compliance, how much fluid is driven into the ventricle should thereby be a function of the filling pressures, and how large the ventricles are should be a function of the fluid within them. In this way it is thought that if one could measure the dimensions of the ventricles, one could determine preload, and with that, gain some insight into the fluid status of the patient. Indeed, evidence exists that area of the ventricle at the end of diastole measured through transesophageal echocardiography in mechanically ventilated patients can predict volume status and responsiveness [47, 48, 49]. However, other studies have failed to replicate finding [50, 51]. One possible reason for the inconclusiveness of this technique lies in the difficulty of interpreting absolute values of the left ventricular end- diastolic area, as baseline cardiac anatomy and physiology vary widely among patients [52]. As a single static measure, ventricular dimensions cannot accurately assess where a patient lies on the Frank-Starling curve and thus have difficulty in providing metric by which decisions can be made.

 

Inferior vena cava diameter

There are two vessels that return deoxygenated blood from the systemic system to the heart from the upper and lower halves of the body, the superior vena cava and the inferior vena cava, respectively. Both vessels empty into the right atrium and thus the pressure in both vessels is equivalent to, or nearly equivalent to, right atrial pressure. Because the vena cavae are large, highly compliant vessels, their size can is thought to be reflective of right atrial pressure, like central venous pressure, and therefore venous return, itself an indication of volume status.

To measure the size of the vena cavae, subcostal echocardiography is employed, which in the majority of cases only allows the inferior vena cava to be visible, as seen in Figure 1.12. Since the two vessels are roughly equivalent pressures, it does not matter which one is measured. A few researchers have demonstrated that the mean end-diastolic diameter of the inferior vena cava correlates well with mean right atrial pressure in spontaneously breath- ing and mechanically ventilated patients [53, 54]. Other researchers have demonstrated the ability to distinguish between hemodynamically stable and unstable patient populations based on the maximum (end-expiratory) inferior vena cava diameter [55] and could pre- dict the recurrence of shock after fluid resuscitation better than mean arterial pressure or heart rate [56]. Unfortunately, these absolute measures of inferior vena cava diameter are essentially indirect indicators of central venous pressure and suffer from many of the same limitations.

Figure 1.12: The inferior vena cava (IVC) as measured through subcostal echocardiography. The heart, right atrium (RA), liver, hepatic vein (HV), and common bile duct (CBD) all provide landmarks by which to make a consistent IVC diameter measurement.