A difficult task is to estimate the dry weight of children on HD. Many tools are available, but not for every HD centers, i.e., evaluation of the diameter of inferior vena cava with US (pre and post-HD), bioimpedance... Does anyone know of a mathematical model that includes clinical parameters, like body surface, weight gain etc?
We obtained "ideal" extracellular volume (indexed to height) using body surface area and body mass index as:
ECW/height = -0.00682 + 0.03532 BSA + 0.00123 BMI + 0.00823 Gender (*: 0 females, 1 males). This was validated in mexican adults. You have a measured ECW/H (by means of impedance meter) GREATER than the ideal ECW obtained by the formula. You have to ultrafiltrate the patient to minimize the difference between this measures (Measured - Calculated ECW near to ZERO).
I would be extremely careful to rely on mathematical modelling alone. For my knowledge, all available methods ECV (Critline, bioimpedance, ultrasonic ...) have a greater signal /noise ration that expected autonomous regulation of patients. That means, these tools are helpful for deciding on, but not for automated guiding ultrafiltration. Second, the true clinical status of the patient is expressed by means of sympathetic activity: blood pressure, heart rate variability, skin resistance and so on. It would be extremely helpful to have these data online to combine them with ECV calculation and have a feedback on fluid removal.
Interestingly I found a specific calculation for pediatric PD patients at J Am Soc Nephrol 17: 285–293, 2006. doi: 10.1681/ASN.2005050568. I am still looking for ped patients on HD, despite the fact that I believe that (as Joachim stated)sympathetic activity (as measureable parameters) do contribute in praxis to our understanding and conduct regarding dry weight. Best regards
Concerning the equation described by Mauricio (please send the reference), we should validate it in ped patients.
The issue of noise is really a concern. I previously used Fresenius BVM (relative volume) module and BCM (body composition monitor) device. My personal opinion is that relative blood volume performs worse than any other gizmo. There is also evidence showing limitations with this technique (the rejected issue of "crash crit", intergroup and intraindividual variability) that makes it unsuitable for clinical application. Spectroscopic bioimpedance derived parameters where successfully related to outcomes (specifically the term named "Overhydration" (OH) Nephrol Dial Transplant (2009) 24: 1574–1579), but performs better with groups, not individuals...
We use interdialytic wt gain and clinical assessment of pulse ,BP and signs of pulmonary edema for UF and not formula
Dear Mauricio
before giving any answer I prefer to pose a question .
please explain how much is specificity or sensitivity of your formula ?
it seems that it should be measured by bioimpedence and clinically simultaneously .
I read earlier about measuring ECW by ultrasound (measure skin layer in scalp) and after dialysis and some other techniques but I couldnot study on this here in my country .I cannot say my suggestion is the best but I can say that it is a safe non invasive method that we can try it at least in a project .
Why we should use a formula? Measurements and calculations are difficult specially in children ,and we can,t trust to them because of challangeable accuracy of these formulas.I think if we follow our patients closely ,we will not have any problem with clinical evaluation of dry weight in children.
According to my experience the best way to estimate the dry weight is by clinical means, that is the blood pressure initially, biological investigations at the bigining and the end of dialyis (sodium, total serum protein levels) so that we can check for signs of hemoconcentration. Moreover, if a patient experience hypotension during the dialysis session (after excluding secondary causes of hypotension)or cramps this might be a sign that we are removing much fluids and a need to adapt his dry weight.
I prefer to stay away from formulas - children are growing, and growing fast, and thus we have to adjust and to allow some "extra" over the mathematically "ideal" calculations. I agree with those, who rely on clinical approach - blood pressure, lab prior and post dialysis, UF, HB, HTC, Ca, Mg, etc. We've tried bioimpedance and other gizmos - contrary to the adults, neer worked properly in kids
I realy decide for UF on clinical assessment not used any formula in my experience of 15 years.Growing age may be responsible for weight gain and we may take as hypervolumia but we should check pulse,BP with appropriate sized cuff in quiet environment and examine chest for crepitations or rhonchi,liver size to decide whether child is hypervolumic or euvolumic.Also intradialyltic hypotension is an important determinant of dry weight.
I agree with clinical judgment, but I think to avoid intradialytic hypotension and it is too late to find pulmonary edema clinical signs. Rather you can check regularly pulse oximetry, indeed spO2 equal to or greater than 97% tells you that there isn't dangerous water even if your patient is slightly over hydrated
Clinical judgement, certainly no formulas. However, there are some helpful tools, i.e. growth charts. If you know the anthropometry when starting dialysis, with adequate dialysis the growth should follow the same curve. The rest is more art than science, to my opinion
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