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The concept of fluid tolerance: where POCUS brings Guyton’s physiology to the bedside

This post is a commentary to the article The emerging concept of fluid tolerance: A position paper(1) following my previous post where I stated that “The development of handheld devices, with applications that go beyond the 2D mode, has created a new era of hemodynamic POCUS within everyone’s reach. But it takes a lot of study and training, without forgetting the laws of physics, for the integration of these laws into biological systems (in Guyton’s spirit), and, more importantly, evaluating patients as a whole (as we already do well)”.

Nonetheless, we need to refine the whole physiology-based matrix regarding the treatment of patients in shock. The recently published article in the Journal of Critical Care by Kattan et al (1), is very relevant, bringing about a revolution in the traditional approach to shock, based on the concept of “fluid responsiveness” and proposing an alternative and operational definition of “fluid tolerance”. The article is last authored by Philippe Rola, one of the gurus of the VEXUS protocol (2).

There are aspects that I want to highlight where POCUS enters this definition of fluid tolerance in order to make it operational.

The authors define the organ systems that will suffer the most from the consequences of volume overload in the resuscitation of the patient in shock, anticipating the important role that POCUS can play in the clinical evaluation of the patient at his/her bedside (spoiler alert: they announce an upcoming article where POCUS will be covered in greater depth on the definition of fluid tolerance).

There is a great emphasis on the systematization of where we should clinically evaluate the patient in shock in the resuscitation phase, in the least invasive way possible.

In contrast to fluid tolerance, the authors point to data in which 30% of patients showed a lack of fluid responsiveness before intensive care resuscitation, in a subanalysis of the ANDROMEDA-SHOCK trial (3).

Hence, the clinic at the patient’s bedside is one of the touchstones in the treatment of patients in shock, especially in their initial presentation in the Emergency Department (ED), within the window of opportunity to which the authors refer (Figure 1)

Figure 1: Window of opportunity in the treatment of shock, based on the concepts of fluid tolerance, venous stress, and organ damage (from Kattan et al (1)).

The window of opportunity is a window of clinical evaluation at the patient’s bedside, which encompasses the right branch of the hemodynamic system, the venous, historically neglected in resuscitation. The anatomical division by systems is fundamental, and hence the division between non-encapsulated versus encapsulated organs, the brain and kidney, which suffer the most from the effects of venous congestion. Thus, the definition of fluid tolerance arises, as well as the level that the patient can tolerate of fluid administration without causing organ dysfunction. I will not undertake an exhaustive analysis of the article, as it must be read in its entirety. I will rather frame it with some examples of bedside cases from my own experience (with an emphasis on POCUS), and on the effect of the anatomical region on venous stress in patients who presented with syndromic shock in the ED, referring to some sections of the article:

Non-encapsulated organs: Lungs and Heart

Pulmonary: oxygen requirements, lung ultrasound (B-lines), pleural effusions, elevated pulmonary capillary wedge pressure or echographic signs of elevated left atrial pressure.

Youth is accompanied by a decrease in risk in most clinical scores, however, being young does not give the ability to neglect fluid tolerance, even reasoning from a Bayesian perspective of pre-test probabilities. A 41-year-old young male (younger than the young man who writes this post!), was admitted to the ED, without any known antecedents, where he had already gone twice for the same condition, due to progressive dyspnea on exertion, with no evident clinical signs of hypervolemia, stating a decrease in urinary output and presenting hypotensiveness on the current admission to the ED. The decision to perform POCUS overlaid that of fluid therapy, and the following images were obtained:

The presentation of acute heart failure was later clarified in relation to the abusive intake of alcohol, not evident on admission, and a diuresis strategy was established in the ED.

Cardiovascular – lack of fluid responsiveness, D-shaped septum, right to left ventricular (RV:LV) ratio > 1, markers of either systolic or diastolic dysfunction, etc .

The condition of obstructive shock often superimposes to that of other causes of shock; however, the immediate reflex of hypotension>>>fluids in the presence of obstruction of the right cavities can have fatal consequences. Consider the case of an 18-year-old female, who for the second time goes to the ED for chest pain with pleuritic characteristics, a recent history suggestive of an upper respiratory infection, and who, upon admission, presented with complaints of difficulty in remaining upright and evidence of hypotension on physical examination. The decision to administer fluids was fortunately accompanied by the decision to perform POCUS, being obtained an image in the short-axis parasternal window:

The diagnosis of massive PTE was made in a timely manner, and a decision was made to administer a thrombolytic (instead of a fluid strategy). POCUS in post-thrombolysis correlated with both symptom and patient’s blood pressure profile improvement:

Encapsulated Organs: Brain and Kidney

Cerebral – acute injury

Congestive encephalopathy is taking its first steps in the jargon of clinical medicine, where, due to the “encapsulated” nature of the brain, the direct assessment of venous stress in the cerebral circulation is a challenge to clinical practice and research. On the other hand, the clinical gestalt of the assessment of acute cognitive dysfunction and delirium, without the use of scales and systematic methods, has a very low sensitivity, especially in the ED. A lot could be said about this subject (I’m both a “Guyton geek” and a “delirium geek”), so a development of this theme with a “sleep-integrated” focus will be left for future posts. It is common to make the objective assessment of the critically ill patient with the qSOFA, which includes the Glasgow Coma Scale (ECG). But data are lacking that allow to calibrate ECG sensitivities and specificities across the spectrum of cognitive dysfunction associated with shock states. A sensible approach is to understand the problem (not blindly trust in our gestalt!) and apply scales aimed at assessing delirium, alongside scores such as the qSOFA. If we want to apply the concept of fluid tolerance in the sense of being “early”, specific delirium assessment scales in the ED must be adopted. One of my favorite scales is the bCAM (Figure 2), but other ED-validated scales can be reviewed in (4).

Figure 2: Brief Confusion Assessment Method decision algorithm. Copyright © 2012. Vanderbilt University.

Renal – stress biomarkers rise, oliguria

The kidney is an encapsulated retroperitoneal organ, whose acute injury has profound implications for the outcome of the patient in shock, largely related to its greater sensitivity to venous congestion. Usual markers based on urine output and serum creatinine values ​​show a considerable late delay in relation to other earlier signs of dysfunction, which can be measured with ultrasound. POCUS can guide the diuretic treatment, showing signs of decongestion that precede the usual biochemical markers (and, hence, helping in the most assertive and objective treatment strategy), evidenced in the following images of a patient in a “de-resuscitation” plan for septic shock in the interlobar renal vessel windows:

Before diuretics:

After diuretics:

The message is relevant:

As important as opening a good imaging window in POCUS is opening the window of opportunity in the resuscitation of the patient in shock. The logical path (plausible from a biological point of view) is the integration of clinical data stratifying the risk, the use of bedside tests that assess the stress in the venous system, having as a matrix of action the tolerance to fluids, and from there explore the relationship between hemodynamics, organ damage, and patient outcomes. There is no doubt that POCUS is a crucial tool for this understanding. Soon, I will talk more about these subjects (always as a “Guyton geek”!).


1- Kattan E, Castro R, Miralles-Aguiar F, Hernández G, Rola P. The emerging concept of fluid tolerance: A position paper. J Crit Care. 2022 Jun 2;71:154070. doi: 10.1016/j.jcrc.2022.154070. Epub ahead of print. PMID: 35660844. https://pubmed.ncbi.nlm.nih.gov/35660844/

2- Beaubien-Souligny W, Rola P, Haycock K, Bouchard J, Lamarche Y, Spiegel R, Denault AY. Quantifying systemic congestion with Point-Of-Care ultrasound: development of the venous excess ultrasound grading system. Ultrasound J. 2020 Apr 9;12(1):16. doi: 10.1186/s13089-020-00163-w. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7142196/

3- Kattan E, Ospina-Tascón GA, Teboul JL, Castro R, Cecconi M, Ferri G, Bakker J, Hernández G; ANDROMEDA-SHOCK Investigators. Systematic assessment of fluid responsiveness during early septic shock resuscitation: secondary analysis of the ANDROMEDA-SHOCK trial. Crit Care. 2020 Jan 23;24(1):23. doi: 10.1186/s13054-0202732-y. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6979284/

4- José Mariz, Teresa Costa Castanho ,Jorge Teixeira ,Nuno Sousa and Nadine Correia Santos, Delirium Diagnostic and Screening Instruments in the Emergency Department: An Up-to-Date Systematic Review. Geriatrics 2016, 1(3), 22; https://www.mdpi.com/2308-3417/1/3/2

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