Right Atrial Pressure

Right Atrial Pressure
(VEXUS)

Dr Steve Lobaz

Consultant Anaesthetist and Intensivist
Barnsley Hospital

Right atrial pressure

High right atrial pressure (RAP) can cause systemic venous congestion and tissue oedema. 

There are multiple different causes of a raised RAP. These are due to the relationship between pressure, volume and compliance within the heart. 

Causes of raised RAP:

  • Hypervolaemia
  • Cardiac failure
  • Pulmonary embolism
  • Pulmonary hypertension
  • Left to right shunted ASD
  • Cardiac tamponade
  • Mechanical ventilation & PEEP
  • Tension pneumothorax
 
Some of the main issues causing a raised RAP in critical care patients are hypervolaemia, cardiac failure, mechanical ventilation and pulmonary embolism. 
 
In recent times there has been uptake in the use of POCUS for haemodynamic assessment. VEXUS assessment has been shown to correlate well with RAP and its use is becoming more common within critical care to assess venous congestion. It is important to distinguish fluid overload from venous congestion as patients can be clinically overloaded without signs of venous congestion and vice versa. 
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Venous congestion

 In the presence of a raised RAP ongoing intravenous (IV) fluid resuscitation may lead to organ dysfunction, by inhibiting venous drainage and lead to complications for the patient. 

Venous congestion can affect multiple body systems including:

  • Brain: Cerebral congestion (encephalopathy)
  • Heart: Myocardial oedema (diastolic dysfunction, arrhythmias), worsen cardiac failure
  • Lungs: Pulmonary congestion (pulmonary oedema, prolonged need for ventilation), Respiratory failure
  • Liver: Hepatic congestion (cholestasis/liver dysfunction)
  • Kidneys: Renal congestion (Acute Kidney Injury (AKI), delayed renal recovery → persistent severe AKI, Acute Kidney Disease, Chronic Kidney Failure (CRF), reduced diuretic efficacy, increased need for renal replacement therapy (RRT).
  • Gut: Bowel oedema (malabsorption, prolonged ileus, intra-abdominal compartment syndrome, gut hypoperfusion, anastomotic leak, toxin-bacterial translocation.
  • Systemic Tissue Oedema (poor wound healing, pressure sores, increased infection, poor mobility, falls)
  • Electrolyte derangement
  • Patient Outcomes: Prolonged hospital stay, increased morbidity and mortality.

Fluid balance

Determining fluid balance status in critical care patients can be very difficult.  Excessive fluids will result in organ dysfunction related to venous congestion, but too little fluid can also be harmful, affecting every organ system. 

 Hypoperfusion related to dehydration / fluid losses can cause/exacerbate AKI, increase venous thromboembolism risk (VTE), cause confusion, falls, poor wound healing; adversely impacting ALL patient outcomes (Sylvester et al., 2022).

Any IV fluids given must follow the ‘Goldilocks principle’ of being ‘JUST RIGHT’ in terms of fluid type, volume, and rate for the patient fluid balance situation.  IV fluid therapy influences every aspect of patient outcomes.  Although, Central venous pressure (CVP) monitoring measurements >8mmHg is associated with fluid overload/harm, the absolute CVP value does not necessarily predict fluid responsiveness or whether venous congestion is present and is unreliable in patients who are ventilated due to the effect of dynamic mean airway pressures (Salerno et al. 2021).  Aggressive fluid removal/diuresis can also be harmful.

Putting it together with POCUS

POCUS can help determine if elevated RAP and venous congestion is present and that IV fluid therapy may be becoming harmful.   

The venous excess ultrasound (VExUS) score assesses systemic venous congestion by analysing the inferior vena cava size (IVC) and pulsed wave doppler (PWD) flow pattern abnormalities in the hepatic, portal and renal veins. (Miller et al., 2021 & Beaubien-Souligny et al., 2020)

POCUS can identify potential causes of venous congestion including causes of acute cardiorespiratory deterioration (e.g. LV/RV failure, MR, TR, Pulmonary disease, Pericardial effusion) and fluid balance overload, including excess IV fluid administration.   VExUS assessment is indicated in critically ill patients, to determine a patient’s fluid volume status, in the presence of new AKI or heart failure, delirium, newly deranged liver function tests, and for guiding fluid de-resuscitation/diuresis, and in the presence of a raised CVP >8mmHg (Miller et al. 2021). 

VEXUS: Early evidence

VExUS has been found to be useful in post-cardiac surgery patients, as they are more likely to develop systemic venous congestion and AKI, due to a higher prevalence of cardiac and chronic renal disease (Andrei, et al.; 2023)

In the general Intensive Care population, VExUS may not be indicated for everyone, as venous congestion relevance in non-cardiac surgery patients and what optimal interventions if present, have yet to be determined (Andrei, et al.; 2023).

VEXUS has been shown to correlate well with right atrial pressure and have a greater positive predictive value than IVC assessment. A VExUS grade of 3 had a sensitivity of 1 for a RAP ≥ 12. (Longino et al. 2023)

VExUS imaging

Typically, the phase array probe in the subcostal echo view (SC) or just below the rib cage between midclavicular line and anterior axillary line, is used to visualise the Inferior Vena Cava (IVC), Hepatic and Portal veins.  Alternatively, the abdominal curvilinear probe may also be used to visualise the Hepatic and Portal veins in the mid-axillary line.  It is useful to use ECG monitoring with either the phased array or curvilinear probe, as this helps with wave pattern interpretation.

1. IVC

To visualise the IVC, from the subcostal view fan the phased array probe to the patient’s right with the probe marker towards the patient’s head.  Small adjustments to the probe are often required to gain the optimal IVC image.    The IVC should be assessed 0.5-3cm from the caval-atrial junction in the SC view.

If the IVC is <2cm diameter on measurement, there is no significant venous congestion and no requirement to undertake a detailed VExUS scan.

If the IVC ≥2cm diameter, venous congestion may be present and warrants undertaking a VExUS assessment.

IVC diameter 2.57cm - indicating further VEXUS assessment

2. Hepatic Vein

In the midaxillary line or subcostal view, identify the thin-walled hypoechoic hepatic veins draining into the IVC.  Apply colour flow doppler (CFD) to identify the best area of measurable blood flow (normally away from the probe, blue flow). Nyquist limit can be turned down as velocity is low in these vessels. Select Pulsed Wave Doppler (PWD) and position the gate where there is best hepatic vein blood flow.  Activate and save the Hepatic vein PWD waveform.  

Original
Labelled

Hepatic vein from subcostal view - seen draining into the IVC.
PWD placed in the hepatic vein.
S>D wave below the baseline (labelled). Identified by the ECG at the bottom - S wave after the QRS, then D wave following after T wave. This is normal flow in the hepatic vein. A wave is seen above the baseline.

  • The hepatic veins transport deoxygenated blood from the liver to the inferior vena cava (IVC) and into the right atrium (RA).
  • Normally, blood flows towards the IVC and the RA.
  • The Hepatic venous pressure waveform is a tetra-phasic waveform and represents the venous pressure transmitted from the right ventricle (RV). The Hepatic vein is normally <10mm diameter at the proximal site of the IVC.
  • Antegrade flow towards the heart (away from the transducer and liver) is seen as a blue colour on doppler (CFD) and is represented graphically below the baseline.
  • Retrograde flow away from the heart (towards the transducer and liver) is seen as a red colour on CFD and is represented graphically above the baseline.

Hepatic vein waveform

The Hepatic vein waveform is composed of 4 parts (Field, 2017):

  • A-wave (atrial contraction/systole) – generated by RAP and atrial contraction. Blood flow is towards the RV (antegrade), but also towards the IVC/Liver/probe (retrograde).  The net flow creates a small positive retrograde wave.
  • S-wave (ventricular systole) – in ventricular systole a large amount of blood flows from the hepatic veins into the RA – atrial relaxation (antegrade). The RV contracts pushing blood out of the RV into the RVOT.  The tricuspid annulus moves upward into the RV, creating a negative RA pressure and decrease in RAP. Blood flow is towards the RV away from the liver / probe.
  •  V-wave (transitional wave) – here there is a decrease in RA filling and blood flow towards the heart. The atria over fills and the tricuspid annulus returns to its resting position. This creates a small pressure wave away from the heart towards the liver and probe (retrograde).
  • D-wave (ventricular diastole) – here the TV opens, the RA and RV fill passively with blood moving from the liver towards the heart, away from the probe (antegrade). There is a decrease in RAP due to early rapid diastolic RV filling.

Hepatic vein waveform assessment

  • Normal RAP:  (S >D) here the Hepatic Vein PWD shows both the Systolic (S)-wave and Diastolic (D)-wave below the baseline.  S-wave is seen just after the QRS.  D-wave is after the S-wave and falls after the T-wave.  There are no A-waves in patients with atrial fibrillation.
  • Elevated RAP: (S<D: mildly abnormal) here the S-wave is less than D-wave due to elevated RAP.
  • High RAP: (S-reversal: severely abnormal) here there is retrograde flow back into the hepatic vein. The S-wave flips above the baseline and often fuses with A.

Hepatic vein pitfalls

  • S-wave reversal may occur without significant venous congestion, particularly in patients with severe tricuspid regurgitation.
  • Monophasic or blunted waveforms may occur in patients with liver cirrhosis, fatty liver disease and lymphoma. A Valsalva manoeuvre can also blunt the waveform.

3. Portal vein

In the mid-axillary line or subcostal view, identify the smaller, thick, hyperechoic (bright) walled right portal vein located posterior to the hepatic vein.  Portal veins do not drain into the RA and are normally non-pulsatile.  Apply CFD to locate the best location of measurable portal vein blood flow (typically red colour).  Select the PWD and place the gate in this location of best blood flow (normally towards the probe, red flow).  Activate the PWD tracing and record the waveform.  .

Note:  sometimes hepatic arterial flow can superimpose over the portal vein and be mistaken for portal vein pulsatility.  If this occurs, the image should be optimised to get the portal vein only.  See below for pattern explanation.

Renal vein assessment pitfalls

  • Measurement of renal venous PWD is challenging due to small calibre vessels and may not be possible. In the general ICU population where there is a high prevalence of AKI, venous PWD may not be useful.
  • Patients with severe AKI or requiring renal replacement therapy may not have measurable PWD flow.
  • Hepatic PWD congestion predicts adverse kidney events in the general ICU population (Spiegal et al.). Portal and renal venous flow may be predictive of AKI and renal events in the cardiac surgical population (Beaubien-Souligny).
  • Fluid limitation and diuresis should be considered in patients with hepatic/portal venous congestion, even when hypotension present.

Portal vein waveform assessment

Normal RAP:   A non-pulsatile monophasic continous waveform is seen in systole and diastole.

Pulsatility Index (PI) = (Vmax – Vmin) / Vmax

Pulsatility normal <30%

Elevated RAP:  Mildly abnormal pulsatile flow.  Pulsatility 30-50%

High RAP:  Severely abnormal.  Retrograde flow in systole.  Pulsatility >50%

4. Renal vein

Using the phased array or curvilinear (abdominal) probe, the kidneys should be located from the liver or spleen basally either side (typically 1-2cm below and laterally) between the anterior axillary and midclavicular lines or in the post-axillary line.  The probe marker is directed towards the patient’s head, so the liver or spleen is seen on top of the right or left kidney, respectively.  Adjust the depth to centre on the kidney and apply CFD over the kidney.  Optimise the CFD scale to 10-25cm/sec (most patients between 15-20cm/sec).  Unlikely to see colour flow <10cm/sec.  Select PWD and place the gate where there is high renal blood flow, usually over interlobar vessels.  Activate and record the waveform. 

Adjust the PWD baseline so that both renal arterial and venous flow can be seen – artery above the baseline, venous below the baseline.  Sometimes only one flow pattern can be optimised.  See below for pattern explanation.

Renal vein waveform assessment

Renal venous flow changes with elevations in RAP.  The Renal Artery on PWD is above the baseline, whereas the Renal Vein is seen below the baseline.

Normal RAP:  the Renal Vein flow is continuous throughout systole and diastole.

Elevated RAP: Mildly abnormal.  As RAP rises, the Renal Venous flow PWD pattern becomes pulsatile.  Initially, flow is Biphasic with S > D.  As RAP rises further, S < D. 

High RAP:  Severely abnormal.  Only Mono-phasic diastolic flow seen.  Systolic venous flow is absent.

Renal Arterial Assessment

Renal Resistive Index (RRI)

The RRI doesn’t make up part of the VEXUS assessment but it may be worth knowing about.

The RRI is calculated by measuring the Peak Systolic Velocity (PSV) and End-Diastolic Velocity (EDV) on the Renal Arterial waveform.

Renal Resistive Index (RRI) = (PSV – EDV) / PSV

With normal renal artery flow, there is inflow throughout the cardiac cycle.  In renal injury, there is greater resistance to blood flow with lower diastolic velocities, so the RRI increases.  A RRI >0.7 is associated with a higher risk of developing AKI in some critical care patients e.g. sepsis, trauma.  There is currently no clear use of RRI in the management of shock, with more studies required to investigate potential uses.

In renal artery stenosis, the arterial waveform appears significantly compressed (peak wave form is flattened).

VEXUS Scoring

  • If the IVC < 20mm, this is Grade 0 and no further assessment required.

  • Grade 1: IVC >20mm and combination of no (normal) or mild PWD abnormalities.

  • Grade 2: IVC >20mm and severe PWD in 1 vessel

  • Grade 3: IVC >20mm and severe PWD abnormalities ≥2 vessels

VEXUS pitfalls

  • VExUS likely to be high in patients with pulmonary hypertension. Diuresis in patients with pulmonary hypertension needs to be undertaken carefully as stroke volume may become compromised.
  • In patients with Right Heart Failure, VExUS is a surrogate for heart function and not volume status and therefore inaccurate.
  • In patient with VExUS grade 3, IV fluids should be avoided as these patients may be fluid intolerant. They may also be diuretic resistant, requiring a combination of diuretic medication to achieve diuresis.  The aim in these patients is to aim for fluid tolerance and not overload.

Summary

VExUS along with other fluid balance parameters can be used to assess systemic venous congestion in critically ill patients. 

VExUS has been shown to outperform isolated CVP measurements in predicting AKI in post-cardiac surgery patients (Beaubien-Souligny, 2020).  

VExUS waveforms may be used to monitor the efficacy of fluid de-resuscitation or diuresis in heart failure patients and in management of elevated RAP and systemic venous congestion.  However, the optimal use of VExUS in critically ill patients has yet to be fully determined, with caution advised (Koratala, 2022). 

Treatment to normalise waveforms, may not being appropriate for everyone.  If AKI worsens despite improvements in VExUS waveforms, cardiac impairment (forward flow) should be suspected and managed accordingly.  

In patients with chronic pulmonary hypertension, normalisation of VExUS waveforms may lead to decreased preload and heart failure.  Determining a VExUS grade baseline in such patients may be of value (Koratala, 2022).  

Further studies on the optimal use of VExUS in the management of critically ill patients and systemic venous congestion are warranted.

References

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