By Peter Laird, MD
In Dr. Kjellstrand's Annual Dialysis Conference lecture in 2009, he focussed on two of the most important basic physiology lessons for those prescribing dialysis is the fluctuation and "unphysiology" of potassium levels and volume status. Prior to dialysis, most hemodialysis patients have potassium levels higher than 5.0 and after dialysis, especially if a 1K bath is used, the serum potassium levels may fall below 3.0 leaving the patient always above or below the normal range for potassium levels. Daily dialysis eliminates the high and low fluctuation of potassium levels keeping levels between 3.5 - 4.8 at all times.
A recent article in the Journal of the American Medical Association on potassium fluctutions in heart attack patients treated with infusions of intravenous potassium to keep the levels above 4.0 experience more episodes of cardiac arrhythmias and death when the levels of potassium were higher than 4.5 and lower than 3.5 in a U-shaped outcomes profile:
RESULTS: There was a U-shaped relationship between mean postadmission serum potassium level and in-hospital mortality that persisted after multivariable adjustment. Compared with the reference group of 3.5 to less than 4.0 mEq/L (mortality rate, 4.8%; 95% CI, 4.4%-5.2%), mortality was comparable for mean postadmission potassium of 4.0 to less than 4.5 mEq/L (5.0%; 95% CI, 4.7%-5.3%), multivariable-adjusted odds ratio (OR), 1.19 (95% CI, 1.04-1.36). Mortality was twice as great for potassium of 4.5 to less than 5.0 mEq/L (10.0%; 95% CI, 9.1%-10.9%; multivariable-adjusted OR, 1.99; 95% CI, 1.68-2.36), and even greater for higher potassium strata. Similarly, mortality rates were higher for potassium levels of less than 3.5 mEq/L. In contrast, rates of ventricular fibrillation or cardiac arrest were higher only among patients with potassium levels of less than 3.0 mEq/L and at levels of 5.0 mEq/L or greater.
CONCLUSION: Among inpatients with AMI, the lowest mortality was observed in those with postadmission serum potassium levels between 3.5 and <4.5 mEq/L compared with those who had higher or lower potassium levels.
If we extrapolate these results to the hemodialysis setting, it shows just how dangerous potassium fluctuations are in patients with myocardial ischemia (insuficient blood flow and oxygen delievery to the heart tissues). Lest some would criticize this as a different population than the dialysis population, I would remind the readers that dialysis patients have a very high prevalence of myocardial ischemia during each dialysis session. (here, here and here).
Likewise, patients experience constant difficulties managing fluid balance with thrice weekly dialysis protocols, especially following the "killer" dialysis weekend. Nephrologists have adopted a variety of methods to avoid intradialytic hypotension (low blood pressure while undergoing dialysis) but most reviews do not acknowledge the simple physiology of depleting the cardiovascular compartment (heart and vessels) and the slower replenishment of fluid back into the blood vessels from edematous "3rd space" tissues that most patients recognize as swelling in their legs but instead focus on various treatment options except for the most important, increasing time or frequency of dialysis to prevent hypotension in the first place. With only 5 liters of blood and plasma in the the blood vessels, patients undergoing dialysis with 4 liters taken off by ultrafiltration will always experience various symptoms of cardiovascular collapse if taken off too rapidly. The rate limiting step is how fast, or more aptly, how slowly these fluids in the swollen tissues return to the blood system. Dr. John Agar has an excellent overview of fluid and solute shifts during dialysis on Home Dialysis Central. (here and here)
With daily dialysis, patients do not experience the wide variations in fluid balance and because they keep the extreme elevations seen in thrice weekly conventional in-center dialysis to a minimum, these patients do not experience many of the "common" or "expected" symptoms associated with dialysis including low blood pressure, severe cramping, nausea and vomiting and passing out. Dr Scribner believed in sodium restriction as the primary method of controlling blood pressure by controlling the volume status. However, daily dialysis lessons the burdens of severe fluid restrictions for patients enhancing the quality of life.
All of these factors are described in an extensive body of literature describing potassium and fluid balances with daily vs thrice weekly schedules. Paradoxically, the medical specialty most established on the principles of basic physiology is also the specialty that dismisses this knowledge during the practice of dialysis. Daily dialysis is the only modality that aligns with the physiology of the normal state. Standard dialysis does not restore the "interior milieu" to normal at any time. Daily dialysis on the other hand does restore this balance to more physiologic fluctuations and eliminates the extremes of change that cause much of the morbidity and mortality associated with dialysis. By avoiding these highs and lows, dialysis patients are at less risk of many of the symptoms and complications associated with dialysis therapy. The FHN studies did not contribute new evidence to what has been known for over 40 years already. Knowing physiology is the first step in understanding optimal dialysis.
Slides by permission from Dr. Carl Kjellstrand.