Hemodynamic Monitors are not all the same: Why this Matters

Grisly scenes from COVID-19 critical care will be etched into our memories forever. And through the fog of the front line, through the despair and devastating reality, much will have been learned. Especially when it comes to hemodynamic monitors.

Hemodynamic Monitors in Critical Care: Home Turf for Deltex

At Deltex Medical we’re heavily involved in the critical care picture. After all, this was the original inspiration and proving ground for what is acknowledged to be the ‘reference standard’ among hemodynamic monitors, esophageal Doppler monitoring (EDM+).

Haemodynamic monitoring in critical care
Deltex Medical TrueVue with esophageal Doppler at its heart

We continue to be closely involved with developments, offering support and technical expertise when and where demanded. In editorials such as this one we’ve also been sharing what we hope are useful insights into a cornerstone in COVID-19 critical care… hemodynamic management.

What follows is a little more detail on the benefits of using the most precise of hemodynamic monitors for the vital organs of the COVID-19 patient.

Protecting the Kidneys

The emerging picture sees EDM+ coming into its own more each day. In recent weeks, something that insiders have known for a while, COVID-related kidney damage reached the national papers. Renal failure is providing just one more stressor on care providers, some units reporting up to 30% of patients with acute kidney injury. Others quote 21% of patients receiving dialysis. A proportion of this kidney damage is caused directly by the virus itself. However it’s likely that in many cases it’s an unintended consequence of the therapeutic approach of keeping patients “dry” as
we’ve discussed elsewhere. Kidneys don’t respond well to either hypovolemia (or, for that matter, hypervolemia). Clinicians increasingly recognize the importance of finding the hemodynamic “goldilocks zone.” They know that there is a very fine line between being fluid- restrictive and delivering “too much.”

The common therapeutic denominator ought to be tight hemodynamic control. This is now accepted as a desirable standard of care. COVID-19 is a disease that is as much about organ perfusion as it is about respiration. This means getting the fluid balance right, because keeping the patient in that
very small sweet spot is vital.

When the target is small, accuracy matters

The benefits of EDM+ are well understood by users. This is the only system that measures cardiac output directly from the aorta. When timing and accuracy are everything, it makes sense to rely on the monitoring solution that delivers both. While other systems derive fluid status information by applying complex algorithms to indirectly acquired data, EDM+ delivers real information, in real time, straight from the heart. Study after study demonstrate improved outcomes with esophageal Doppler. Improved outcomes that include reduction in acute kidney injury.

Protecting the Lungs

So to respiration. We’ve spoken of the two patient types, again now becoming understood by the general press. The presentation of many COVID-19 patients demands a “lung-protective” strategy of low tidal volumes (6 ml/Kg body weight) and low PEEP. This can be problematic for hemodynamic monitoring systems that derive fluid-responsiveness information from Stroke Volume Variation (SVV) and Pulse Pressure Variation (PPV).

“Several studies have shown that PPV does not reliably predict fluid responsiveness during low tidal volume ventilation”. Myatra et al. Critical Care (2017) 21:60

Sadly, 6ml/Kg falls outwith the (8ml/Kg) tidal volume required for these systems. This has seen the emergence of the so-called Tidal Volume Challenge (TVC). Performing a TVC means temporarily raising this parameter to fit the monitoring protocol. It’s moving the goalposts in an attempt to work around the fundamental unreliability of these systems in a dynamic, real world setting. Which is all well and good except that returning the patient to the optimal tidal volume after the “challenge” means continuous monitoring is lost and a repeat TVC (performed by the clinician) is just another demand on them.

These systems require patients to be stable, with normal cardiac function and functioning vasculature. Without stability, the predictive link between PPV and SVV, implicit in their algorithms is lost.

There’s another factor too. Prone positioning:

“In ventilated patients with low tidal volume, a prone position may have a direct effect on the heart that alters the performance of SVV in predicting fluid responsiveness”. Berger et al, Minerva Anestesiol 2015;81:628-35

When the target is moving, continuous monitoring matters

COVID-19 patients are often anything but stable. The recently published Perioperative Quality Initiative (POQI) reads across to the critical care setting quite well in this regard. It tells us that SVV-based systems require:

  • patients to be in regular cardiac rhythm (COVID-19 patients are often not),
  • a lack of patient inspiratory efforts (ie patient must be on a ventilator with no spontaneous breaths),
  • consistent changes in intra-thoracic pressure produced by mechanical ventilation with tidal volumes of at least 8 ml/kg predicted body weight.

Our question of the clinician is the obvious one: Why rely on unstable, indirectly acquired data from your hemodynamic monitors when the simple act of deriving this vital information from an indwelling esophageal Doppler probe takes all that variation out of the equation?

In Conclusion… Good news?

It may be anecdotal, but we’re close to the front line so happy to hear and share reports from EDM+ users. Feedback suggest they are not experiencing difficulties in achieving reliable hemodynamic information in COVID-19 patients. Little wonder that (as discussed on TopMedTalk recently) the general consensus is that COVID-19 patients need individualized hemodynamic optimisation. The mantra is to adopt a standard stroke volume optimization protocol then pause to think: Resuscitate, replace, maintain. Don’t ‘run dry’ but do avoid salt and water excess after resuscitation and replacement. 

COVID-19 patients can be on the edge, which means therapeutic windows are often very small. This demands decisions that must be based on the best technological guidance. Faced with a choice of hemodynamic monitors, if ever there was a case for adopting the ‘reference’ standard, this is surely it.

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