Measurement of intravascular volume has only recently studied, covering the use of diuretics and filtration during dialysis.
Dr Matthew Kaptein of Loma Linda University reviews available evidence that may validate the IVC collapsibility index for measuring relative intravascular volume.
He has developed a calculator tool which will help doctors navigate this important step for optimizing the intravascular volume of their patients to offer more effective treatments.
Learn more about Kaptein’s Calculator Tool: Calculator Tool
Read the original research: https://doi.org/10.1053/j.ackd.2021.02.003
Image source: DC Studio / Shutterstock
Hello and welcome to Research Pod. Thank you for joining us today. In this episode, we will be looking at the work of Dr Matthew Kaptein, a nephrologist at Loma Linda University Medical Center in California, who is also affiliated with the University of Southern California in Los Angeles in the USA.
Kaptein’s group is researching the clinical relevance of the variability arising due to respiration in inferior vena cava diameter, or IVC diameter for short, as measured using ultrasound. The IVC diameter method provides a non-invasive dynamic estimate of the relative volume inside the blood vessels, which is known as intravascular volume.
Accurately assessing relative intravascular volume is essential for the appropriate management of volume in acutely ill hospitalised patients. Kaptein argues that estimating how much volume is in a patient’s blood vessels allows doctors to know whether that patient needs more intravascular volume, less intravascular volume, or possibly vasopressor medications that induce blood vessel contraction to increase blood pressure. This is not something doctors can easily assess through physical examination alone.
People in a steady state are likely to have an intravascular volume that relates to their blood pressure and extravascular volume, which is the amount of volume in the body’s tissues. However, hospitalised patients are less likely to be in a steady state and may have a mismatch between their intravascular volume and their blood pressure or their extravascular volume.
In a 2021 study, in collaboration with Professor Elaine M. Kaptein, Dr Matthew Kaptein reviews available evidence that may validate the IVC collapsibility index for measuring relative intravascular volume.
For context, the IVC sits to the right of the aorta, along the spine, under the diaphragm and connects the veins below the diaphragm to the heart. It is the largest vein in the human body.
Static measures to predict how a person will respond to the addition or removal of blood volume, such as mean central venous pressure, peak right atrial pressure (RAP), or pulmonary artery occlusion pressure, which do not vary with the breathing cycle, have low sensitivity and specificity. The IVC variability is a dynamic measure which reflects the relationship between the cycles of the lungs and heart.
Kaptein suggests using different ultrasound approaches to overcome the unique challenges of individual patients. The subcostal view, taken from the front of the torso, shows the IVC on the long axis. This window is commonly used by cardiologists and is helpful in assessing patients with morbid obesity, as the distance from the costal margin to the heart does not vary much. This view has limitations. For example, in patients who have had surgery approaching from the midline of the front of the chest or abdomen, the doctor may not be able to place the probe in the standard position.
As an alternative, the midaxillary view is taken at the imaginary line that divides the body into the back and front halves of the torso, usually on the patient’s right side. In patients who are morbidly obese, the probe may be too far away to easily visualise the IVC from the midaxillary position.
If the doctor struggles to visualise the IVC due to morbid obesity, abdominal pain or bloating, bowel gas, air under the skin, surgical dressings, or an open chest or abdomen, visualising more distal veins such as the subclavian vein – also called the proximal axillary vein – or the internal jugular vein with the patients torso elevated at 30 to 45 degrees as is required to assess jugular venous distention, are other options.
Suppose a doctor can gain a good view of these veins using ultrasound. In that case, they can usually rule out one of two extremes: either the patient does not have abnormally low blood volume in their vessels, a condition known as hypovolemia – or, alternatively, the doctor can rule out hypervolemia, which is when a patient has abnormally high blood volume in their vessels.
Kaptein highlights the difference between dealing with patients who can breathe naturally and those on mechanical ventilation. During natural inhalation, there is negative pressure in the chest cavity. This increases the blood flow from the veins to the heart and decreases the diameter of the IVC. When the person finishes exhaling, the pressure in the chest cavity increases to zero. This decreases the blood flow to the heart and increases the diameter of the IVC. The cycle is inverted if a person is on a mechanical ventilator so that the IVC diameter at the end of expiration is the minimum – rather than the maximum. Many authors divide the difference between the maximum and the minimum IVC diameters by end-expiratory diameter to describe the degree of diameter variability. This yields two different results: one called Collapsibility Index or CI for naturally breathing patients, and one called Distensibility Index or DI for ventilated patients.
Kaptein points out that using CI for some studies and DI for others makes it hard for clinicians and researchers to compare the data between people on total ventilator control and those able to breathe naturally. To overcome this obstacle, his team uses CI for all patients, whether ventilated or not. The CI can be readily calculated from the DI value and vice versa.
There is some disagreement in the scientific literature as to whether using a variation of IVC diameters in line with breathing – to determine volume responsiveness – is more reliable for mechanically ventilated people or for those who can breathe naturally.
Kaptein has reviewed the accuracy of IVC ultrasound to predict volume responsiveness for people who breathe naturally compared to those requiring mechanical ventilation. For natural breathing, the pooled sensitivity for correctly identifying those who will respond is 71%. The pooled specificity of IVC ultrasound for correctly identifying those who will not respond was found to be 81%. These accuracy rates are similar to what has been found for those on mechanical ventilation.
Cardiac and IVC ultrasound have a range of practical uses, including differentiating the causes of shock, which have different treatments. For example, IVC ultrasound may reveal when a patient has low intravascular volume, which signals a high risk of reduction in blood flow through the kidneys. Knowing this can help doctors prevent the person from developing damage to the kidney’s tubule cells, acute tubular necrosis, which is the most common type of kidney injury occurring in hospitalised patients.
IVC ultrasound can help doctors to more accurately determine the cause of acute kidney injury in a person hospitalised with cirrhosis – long-term liver damage that causes scarring, as these patients frequently have a mismatch between intravascular and extravascular volume when acutely ill.
Repeatedly measuring IVC CI while adding blood volume via IV fluids, or removing volume via diuretics or dialysis, can help doctors best manage a person’s blood vessel volume and tailor the patient’s therapy to their specific situation.
Kaptein recognises the limitations of the IVC collapsibility index. There are many health conditions that affect the interpretation of the patient’s intravascular volume status based on the respiratory variation of the IVC diameter. Luckily, each condition has a systematic direction of bias. With a good understanding of the clinical picture, the doctor can predict whether the volume status may be overestimated or underestimated by IVC ultrasound. For example, doctors might overestimate the relative intravascular volume in a patient with a condition such as a heart valve problem, pulmonary hypertension, heart failure, or breathing problems related to the diaphragm. Conversely, in a patient with intra-abdominal hypertension, a doctor may underestimate relative intravascular volume.
Some doctors caution against the blanket use of IVC ultrasound. Mean central venous filling pressure is a static measure that is poorly predictive of the response to volume administration in critically ill patients. IVC diameter is notably determined by central venous pressure, but in a dynamic way, which is why diameter variation over the respiratory cycle is measured. For this reason, arguments citing the limitations of static central venous pressure or right atrial pressure are not valid reasons to discount IVC CI. Central Venous Pressure or Pulmonary Artery Occlusion Pressure are not recommended to routinely assess intravascular volume in clinical practice since these measurements are not associated with improved outcomes, and central line placement is associated with complications.
The ability to quickly – and noninvasively – assess multiple ICU patients via IVC CI during a day’s rounds is an advantage Kaptein is keen to highlight. IVC CI is particularly useful to identify patients at the extreme ends of the hypovolemic or hypervolemic spectrum – and may suggest whether or not to administer volume therapy, remove volume by giving diuretics, or remove volume with ultrafiltration during dialysis therapy.
After using ultrasound to determine the patient’s volume status – or at least rule out one extreme or the other – a doctor needs to work out which fluids to administer or limit. Kaptein has developed a calculator tool based on ongoing body fluids losses and the type and amount of fluids being given to the patient to help doctors navigate this important step for optimizing the intravascular volume of their patients. Check out the tool using the link in the notes for this episode, where you will also find the links to the original research.
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