Venous Circulation Monitoring: New Technologies and Applications
Hangzhou Hospital of Traditional Chinese Medicine, translated by Qian Xiaoling, and Zhang Meiqi
Critical Walker Translation Group
The aim of the review
Venous pressure is an often overlooked cause. However, bedside assessment of venous pressure is challenging. This article reviews the clinical implications of venous pressure monitoring, prior art, and introduces the Venous Congestion Ultrasound Scoring System (VExUS) as a new method for assessing venous pressure using Doppler ultrasound.
Latest discoveries
Studies have shown a significant association between elevated venous pressure and poor prognosis in critically ill patients. Current techniques for measuring venous pressure include physical examination, right heart catheterization (RHC), two-dimensional ultrasound, and a variety of labor-intensive, research-focused physiological procedures. Each of these technologies has certain limitations that limit their clinical application. To reduce these errors, Beaubien-Souligny et al. introduced the VExUS score, a new method based on Doppler ultrasound that combines IVC diameter with Doppler measurements of the hepatic, portal, and renal veins to form an assessment of the degree of venous congestion. Studies have shown a strong correlation between VExUS scores and RHC measurements, and between VExUS scores and improvement in acute kidney injury, diuretic responses, and changes in volume status due to cardiorenal syndrome. However, studies in non-cardiac populations were small and heterogeneous, and conclusions were uncertain.
summary
Early studies using Doppler ultrasound to assess venous congestion have brought new hope, but further research is still needed in different patient populations and clinical settings.
gist
1. Excessive venous congestion is increasingly associated with end-organ injury and patient injury, but the assessment of venous congestion is challenging.
2. A wide variety of techniques have been used to assess venous congestion, each with its own limitations.
3. New research shows that the use of spectral Doppler to assess blood flow patterns in large veins is a non-invasive method for measuring venous congestion.
4. Several studies are currently underway to validate the role of the new spectrum Doppler technology in disease management in various patient populations.
introduction
One of the central challenges of modern critical care medicine is the bedside assessment of venous pressure. Historically, the medical field has used the arterial system as a major driver of circulatory dysfunction. However, in recent years, there has been a growing recognition of the dangers of volume overload and venous congestion in patients with heart failure and critical illness. The deleterious effects of venous congestion were first observed in 1931 and are increasingly documented in the modern patient population, reviving interest in quantifying venous pressure. Unlike arterial pressure, which can be measured at the level of the brachial or radial arteries, venous pressure is more challenging to measure. The pathophysiology of venous congestion and the different measures of venous pressure will be described, with a focus on Doppler ultrasound and the Venous Congestion Ultrasound Scoring System (VExUS).
Pathophysiology of venous congestion
Recent awareness of the hazards associated with venous congestion has led to a redefinition of the concept of associated pathophysiology, leading to increased attention to the study of perfusion pressure, and the pressure gradient of the distal capillary bed has been shown to be strongly associated with important clinical outcomes in the ICU. Perfusion pressure is defined as the difference between mean arterial pressure (MAP) and venous pressure, which must be overcome in order to achieve effective circulation and venous return. Arterial hypotension (e.g., hemorrhagic or septic shock) or increased venous congestion (e.g., volume overload or cardiogenic shock) can lead to perfusion pressure disturbances. The exact determinants of venous pressure are complex and controversial, and the Starling, Guyton, and Permutt theories have made significant contributions to venous hemodynamics, although their specifics and controversies are beyond the scope of this review. Simply put, venous pressure is derived from the elastic retraction of the walls of the vascular system, right atrial pressure (RAP), resistance to retrograde return, or the pressure present in the venous system in a state of zero blood flow. The elastic component of venous pressure can be likened to a bathtub, which is filled by a faucet and discharged by a pipe at half the height of the tub wall. In this metaphor, the faucet is the cardiac output and the drain is the venous return, increasing the pressure into the right atrium (Figure 1). Venous volume is expressed as the volume of the bucket below the drain line, and the volume below this water mark does not form venous return or increase vascular pressure. This part of the volume is also called the "tension-free volume". The volume above the drain line is called the "tension volume", and increasing venous pressure and increasing the flow of the drain is equivalent to increasing venous return. When volume is overloaded, the ability of the drain to decompress the tension volume is limited, and excess pressure is redistributed through the venous system. In the traditional bathtub model, increasing the tension volume only increases the forward pressure through the drain, increasing the venous return to the right atrium. However, in reality, the increase in venous elastic pressure leads to an increase in pressure throughout the venous system, leading to extravasation of fluid into the extravascular space, as well as disturbances in perfusion pressure. One of the most important concepts is "starling resistance", which highlights the tendency of the vasculature to collapse due to increased cross-wall pressure from surrounding tissues, also known as "critical closure pressure (Pcc)"; Regardless of the downstream pressure, Pcc is the resistance that must be overcome to resume the forward cycle (Figure 2). Failure to do so may lead to a vicious cycle of edema, leading to increased tissue pressure, further raising the Pcc, and increasing retrograde venous pressure. Dr. Koratala and his colleagues have recently compiled evidence of the negative effects of increased venous pressure and decreased perfusion pressure, including hypoxia due to pulmonary edema, altered mental status due to cerebral hypoperfusion, acute kidney injury (AKI) due to cardiorenal syndrome, cholestasis and liver damage, altered cardiac tone and arrhythmias, and poor wound healing due to tissue edema.
Starling Resistance Effect
Techniques for monitoring venous pressure
ICU physicians have a wide variety of tools to assess venous pressure, from characteristic physical examinations to invasive measurements of right heart catheterization (RHC). Rapid clinician assessment of venous congestion relies on a variety of physical findings, but its guiding value is limited. Therefore, the final accepted gold standard for venous pressure assessment is RHC. Central venous, right atrial pressure, and right ventricular end-diastolic pressure are less variable among subjects than physical examination and are often used as surrogate measurements of venous pressure to guide the management of volume overload. However, RHC is an expensive, invasive, and time-consuming procedure, and is not readily available at the bedside. Therefore, a better bedside venous pressure assessment tool has become a clinical need.
Academic/Computational-based
The growing interest in venous physiology has led to an increasing number of studies related to the indirect calculation of systemic venous pressure. These technical features are similar in their theoretical complexity, labor-intensiveness, and dependence on existing physiological models.
The first method is suitable for mechanically ventilated patients, with the help of reliable cardiac output (CO) measurements, and the relationship between intrathoracic pressure, VR, and CO. Under the assumption of steady-state VR=CO, a series of inspiratory holding maneuvers are performed at increasing pressures, followed by a measurement of CO. A curve is constructed and a function is computed to infer the VR value at a theoretical CO of 0 l/min, which is the zero flow state required to assess ofv [23,24]. While academically innovative, this technique is limited to patients who are intubated, sedated, have a central venous catheter, and require time-consuming bedside manipulation and calculations. Similar computer algorithms using MAP, CO, and RAP have been proposed, but invasive hemodynamic parameters still limit their widespread application.
Another technique uses a separate measurement of hydrostatic pressure in the forearm as a model for systemic circulation, assuming that instantaneously measured venous pressure is consistent across different vascular chambers. To achieve the zero-flow state required to assess venous pressure, the pneumatic tourniquet is rapidly inflated to 50 mmHg above the SBP. Once blood flow is interrupted, researchers measure local venous and arterial pressures 60 seconds after the blood flow stops. Unlike the inspiratory hold technique, this method is suitable for spontaneously breathing patients, and in a study of 24 patients undergoing cardiac surgery, the method was shown to predict volume responsiveness with an area under the curve of 0.79. However, the procedure also requires specialized equipment and training to achieve upper extremity blood flow blocking and blood pressure measurement, limiting its application in clinical practice.
Other validated techniques for measuring abnormal venous flow include occlusion plethysmography, pool scintigraphy, and intravascular ultrasound. Although these methods are useful in research settings, they have not been widely adopted in daily ICU practice due to their cost and technical complexity.
Traditional ultrasound techniques
In recent years, point-of-care two-dimensional ultrasound (POCUS) to assess venous pressure has become an increasingly interesting and innovative field. Traditionally, POCUS has been used to assess the presence of pulmonary edema, the size and collapse rate of IVC, and to measure elevated JPV. Lung ultrasound (LUS), which focuses primarily on the presence of pleural B lines that mark extraalveolar pulmonary fluid, has a sensitivity of 88% and a specificity of 90% for pulmonary edema secondary to left-sided heart failure. However, LUS cannot assess the direct effect of elevated venous pressure on surrounding organs and, importantly, it lacks utility assessment for right-sided heart failure. Ultrasonography for IVC was originally a valid parameter for assessing venous pressure, but in recent years, ultrasonography for IVC has been rigorously re-evaluated due to a large number of potentially confounding physiological variables that complicate their interpretation, including chest and abdominal pressure, cardiac function, anatomical variation, and vascular compliance. A 2023 review highlighted that despite the widespread use of IVC, the pitfalls of IVC ultrasound complicate its interpretation. Finally, assessment of the internal jugular vein by ultrasound has been evaluated as a surrogate indicator of elevated RAP (>10 mmHg) with moderate sensitivity and specificity. However, a recent meta-analysis of studies evaluating ultrasound measurements of JPV showed that small sample sizes and inconsistent exclusion criteria limited the generalizability of the findings. In addition, ultrasound measurement of JPV requires precise technique, which can be confounded by patient-specific factors, such as tricuspid regurgitation, and the inability to quantify congestion at the peripheral organ level, where elevated venous pressure can cause harm. Traditional two-dimensional ultrasound has proven to be a useful tool for assessing venous pressure, but with widespread use, limitations have emerged, including an over-reliance on a single view to assess systemic congestion, a lack of standardization, and variability in test features in a broader patient population.
Doppler Ultrasound: Venous Congestion Ultrasound Scoring System (VExUS)
To address these shortcomings, Beaubien-Souligny et al. recently developed the VExUS score, an ultrasound technique to assess venous circulatory congestion by assessing blood flow to abdominal organs. This technique combines the results of examination at four sites of the IVC diameter, hepatic vein, portal vein, and renal vein pulse Doppler spectrum to provide a comprehensive assessment of congestion throughout the venous circulatory system (Figure 3). This technique is a novel combination of previously documented ultrasound findings indicative of elevated RAP, including characteristic Doppler ultrasound forms associated with different degrees of bruising. In their original study, the authors performed a series of Doppler ultrasound evaluations of the liver, portal vein, and renal veins in 145 postoperative cardiac surgery patients requiring cardiopulmonary bypass. They evaluated the ability of five different "VExUS prototypes" consisting of different ultrasound parameters to predict the development of cardiorenal AKI. They reported a positive likelihood ratio of 6.37 for the development of cardiorenal AKI with the "VExUS C" scoring system. This finding has led to great interest in the critical care community in the use of Doppler ultrasound to assess venous congestion compared to traditional two-dimensional ultrasound. Their innovative preliminary study has generated further interest and research into the validation and clinical utility of VExUS.
Venous congestion ultrasound scoring system (VExUS) grading
Validation of VExUS technology
The original VExUS study was conducted in a small population and did not compare the VExUS grade with the gold standard for venous congestion, which limited the generalizability of the authors' findings. To establish the physiological plausibility of VExUS, Longino et al. evaluated the correlation between VExUS grade and invasively measured right atrial pressure in 51 hospitalized patients who received RHC for various indications and found that the AUC for RAP greater than 12 mmHg was 0.99. In a subsequent study of 81 patients, a similar correlation was found between VExUS grade and RAP, mean pulmonary artery pressure (mPAP), and pulmonary capillary wedge pressure (PCWP). These studies, although limited in sample size, demonstrated the physiological principles of VExUS by comparing it to the known gold standard, showed its reproducibility, and demonstrated that VExUS can be applied to the general hospitalized population.
VExUS has also been further validated in different patient populations, including a study of 145 adult ICU patients documenting a prevalence of moderate and severe hyperemia of 16 and 6 percent, respectively. In a study of 33 critically ill children, VExUS grade was significantly associated with CVP. There is only one case report in the surgical literature describing the successful use of VExUS to relieve bruising in patients with congenital cardiomyopathy. A recent case series reviewed the rationale and supporting evidence for the wider use of VExUS in a variety of clinical settings. These studies laid the foundation for the assessment of venous pressure by Doppler ultrasound in different patient populations.
Clinical application of VExUS
Although validation of VExUS is necessary, it is even more important to understand the clinical utility of the technology. The association between AKI and VExUS grades, which was originally reported, has been revalidated in some populations, but the results of some small pilot studies have not been completely consistent. In a recent longitudinal prospective trial of 30 ICU patients diagnosed with cardiorenal syndrome, Bhardwajet et al. demonstrated a strong association between a reduction in VExUS grade and improvement in cardiorenal AKI, changes in VExUS grade, and fluid balance. Similarly, Longino et al. observed an association between VExUS grade and AKI in hospitalized patients. However, in a multicenter study of 145 ICU patients, a low proportion of bruising was observed, and no association between AKI and VExUS grades was observed. However, it should be noted that this study excluded patients with MAP below 65 mmHg, limiting its generalizability in the ICU population. Interestingly, a pre-experimental study of 90 unclassified AKI intensive care unit patients who underwent serial VExUS examinations showed that patients with a decline in VExUS scores between 0 hours and 48 hours had more renal replacement therapy (RRT) days than those without a decline, suggesting that there is some renal protection in the intensive care unit population. Similarly, a recent prospective study of 230 cardiac surgery patients evaluated the VExUS score and its components, as well as CPV as a predictor of AKI, and found that there was a positive association between AKI and liver, portal vein, and renal abnormalities, but not VExUS itself. The authors attribute this finding to the IVC parameters of VExUS. Their findings provide the possibility to improve the assessment of venous pressure by Doppler ultrasound, and provide the possibility for future iterations of the VExUS method. Another potential use of Doppler ultrasound is to predict the diuretic response. A prospective observational study of diuresis in 81 ICU patients showed a significant relationship between elevated VExUS grade and appropriate diuretic response, with an AUC of 0.66 [95% confidence interval (95% CI) 0.53–0.79]. Interestingly, the same study noted an AUC of 0.8 (95% CI 0.7–0.9) for the portal pulse index (a component of VExUS). This finding also suggests that other methods of Doppler ultrasound measurement should be explored in addition to VExUS in clinical settings.
Conclusion: The application prospect of Doppler ultrasound in venous monitoring
Looking to the future, researchers are moving quickly to adapt Doppler ultrasound to new patient populations and clinical applications. Although the VExUS scoring system has been validated in the general hospitalized population, there are some important subgroups that may confound Doppler ultrasound assessments, including patients with cirrhosis, end-stage renal disease, pulmonary hypertension, and positive pressure ventilation. Excitingly, we're already seeing clinicians apply venous pressure measurements in a wider range of clinical scenarios than ever before. One study proposed the combination of LUS with VExUS to prevent volume overload in sepsis patients undergoing fluid resuscitation. Similarly, a multicenter clinical trial is currently quantifying the relationship between Doppler-identified venous congestion and death or RRT requirements in patients with septic shock. Once the possibility of using Doppler ultrasound to guide treatment has been identified, large-scale prospective trials using Doppler ultrasound as part of a protocol intervention must be conducted to determine the value and place of these new technologies in standardized treatments.