Haemodynamic consequences of changing potassium concentrations in haemodialysis fluids

Kidneys are responsible for long-term potassium homeostasis; this exposes patients with end-stage renal disease to a high risk of hyperkalaemia [1–5]. Recovering potassium homeostasis is one of the important objective of dialysis. Considering that its location is mainly intracellular (98% of the pool [1]), its potential removability during a haemodialysis session is quantitatively modest (between 40 and 80 mmol corresponding to 1-2% of total body potassium) [6]. As a consequence, even if, in order to be suitable, potassium removal during dialysis should be equal to the amount accumulated during the inter-dialytic phase, in clinical practice the potassium concentration in the dialysate is usually adjusted with the suboptimal goal of avoiding pre-dialysis hyperkalaemia [7].

The importance of the body content and serum concentration of potassium to control blood pressure remains controversial. Epidemiological data suggest a role for potassium depletion as a co-factor in the development and severity of hypertension, while dietary potassium inversely correlates with blood pressure [8–10]. In animal models, an acute decrease in serum potassium concentration produces vasocostriction mediated by the vascular endothelium and an increase in myocardial contractility; the opposite effect is observed if it increases [11, 13].

In haemodialysis nephrologists are faced with sudden changes in blood pressure and haemodynamic fragility phases that have a multi-factorial origin; ultrafiltration, decrease in osmolarity with imbalance and correction of metabolic acidosis play a predominant role [7, 14–19]. Despite this, and thanks to some strategies based on current practice, with particular reference to calcium and magnesium concentration in the dialysate [16, 20], dialysate temperature [21] and ultrafiltration and sodium concentration profiles [7, 22–25], pressure stability is guaranteed as a general rule. Some electrolytes, particularly sodium and bicarbonate, can be modulated in profiles with the purpose of better respecting the gap in osmolarity or concentration that is established during the haemodialysis session, but their haemodynamic effect still remains controversial [21, 23, 25].

Serum potassium is an electrolyte whose concentration - in order to guarantee a negative balance - varies rapidly and significantly during dialysis, frequently resulting in going from pre-dialysis hyperpotassaemia to intra-dialysis hypopotassaemia. In a study performed by Dolson, designed to analyze the consequences of acute potassium changes on haemodynamics, differences in intra-dialytic blood pressure were not found between the groups treated with dialysates containing 1, 2 or 3 mmol/l of potassium [6]. However, at the end of the dialysis session those patients treated with the lower potassium concentrations showed what was called a "rebound hypertension" [6].

With the purpose of better characterising this phenomenon, we redesigned the study dividing the dialysis session into 3 phases (in fact, clinical practice suggests that the haemodynamic pattern at the beginning, intermediate and final phases of the dialysis are not the same) and programming for each a more or less sharp drop in serum potassium concentration, respecting in the meantime the need to remove the amount of potassium that usually keeps the patient in steady-state. Using a crossover protocol, we divided the dialysis session into 3 tertiles where the potassium concentration in the dialysate was modulated between the usual concentration for the study subject and two cut-off points at +1 e -1 mmol/l respectively. To complete the information provided by blood pressure, haemodynamics were measured in a non-invasive manner using a finger beat-to-beat monitor.

The primary end point was the difference in haemodynamic parameters between the extremes in potassium concentration of the dialysate, while the incidence of hypotension during dialysis was considered a secondary end point.

The haemodialyses were performed using a 4008 H machine, equipped with a cartridge of bicarbonate Bibag^©, and a high flux single use polysulfone membrane, all from Fresenius Medical Care (Bad Homburg, Germany). The prescribed dialyser effective surface area, dialysis fluid conductibility, dialysate temperature and composition (with the exception of potassium concentration), effective blood flow, and dry weight were recorded at the enrolment in the study and were then left unchanged. The medications of the patients were also left unchanged. Serum potassium and patient weight were measured at the beginning and at the end of each dialysis session. Blood samples were taken from the arterial limb of the shunt.

Kt/V was used to quantify haemodialysis adequacy and was calculated using a second generation single-pool Daugirdas formula (Kt/V = -ln(R-0.03) + [(4-3.5 × R) × (UF/W)], where R = post-dialysis BUN/pre-dialysis BUN, UF = net ultrafiltration, W = weight, K = dialyzer clearance of urea, t = dialysis time, and V = patient's total body water.

The incidence of hypotension episodes (defined as a systolic blood pressure <90 mmHg) was recorded.

Systolic and diastolic blood pressures, heart rate, stroke volumes (integrated mean of the flow waveform between the current upstroke and the dicrotic notch) and total peripheral resistances (ratio of mean arterial pressure to stroke volume multiplied by heart rate) were evaluated at the beginning of the session and then every 30 minutes using a Finometer^© finger beat-to-beat monitor (Finapres Medical Systems BV, Arnhem, The Netherlands). Finometer^© measures finger blood pressure noninvasively on a beat-to-beat basis and gives waveform measurements similar to intra-arterial recordings.

Mean blood pressure (BP_mean) was calculated using the following formula: BP_mean = (BP_syst+2BP_dias)/3, where BP_syst and BP_dias are systolic and diastolic blood pressure, respectively.

The fluid loss as a function of the time was considered to be constant during the dialysis session and was recorded as total ultrafiltration. The dry weight was established on the basis of clinical assessment and bioimpedance (Body Composition Monitor, Fresenius Medical Care; Bad Homburg, Germany).

Statistical analyses were performed using the SAS System (Statistical Analysis System). Comparisons between body weight, potassium concentration and haemodynamic parameters were done first with an ANOVA followed, if significant by a paired t-test performed between the mean values obtained in each patient with each modality. To improve the probability of showing significant differences, the haemodynamic parameters within the tertiles were compared against the dialysate potassium concentration cut-off points (-1 vs. +1 mmol/l). Percentages were compared using a Fisher Exact test. In all cases, a P ≤ 0.05 was considered statistically significant; P was expressed as ns (not significant) or as significant (P ≤ 0.05).

The protocol of the study was approved by the local Ethical Committee (Comitato Etico Cantonale del Cantone Ticino). All the patients gave written informed consent prior to enrolling in the study.

The study showed that a low potassium concentration in the dialysate, inducing a rapid decrease in serum potassium, causes a decrease in systolic and mean blood pressure correlated with a decrease in peripheral resistance. The decrease in pressure recorded when a dialysate with a potassium concentration 1 mmol/l lower than the one usually employed for the patient was used, translated into a higher incidence of hypotension, defined as episodes with systolic blood pressure <90 mmHg (72% of the episodes happened during the tertile using the lowest dialysate potassium concentration). The dialysate potassium concentration in the initial tertile affects mean blood pressure for the whole dialysis session in that a larger gap between serum and dialysate potassium concentrations results in a lower pressure. The effect of removing potassium is progressively reduced during dialysis, with haemodynamic consequences that are no longer significant during the final tertile. The lesser impact during the intermediate and final stages of dialysis could be due to the gap between serum and dialysate potassium, which progressively narrows during the dialysis session.

The data obtained agree with what was observed by Dolson [6] who, while not highlighting pressure discrepancies during the dialysis using dialysates with potassium concentrations of 1, 2 or 3 mmol/l, had observed hypertension, defined as "reflex" (or "rebound"), at the end of the session that used the two lower potassium concentrations. The lack of differences during dialysis in the aforementioned study could have been a consequence of (i) the lower statistical power (11 versus 24 subjects investigated) and (ii) the different method used (based on the steady state prescription, we randomly reduced and increased the dialysate potassium concentration in the whole group). The cited blood pressure rebound after dialysis suggests a counterregulatory phenomenon compatible with an undetected intradialytic hypotension/hypoperfusion phase.

However, if we take into account the experimental data obtained from animals besides the typical metabolic circumstances of kidney failure that requires dialysis, it is surprising to observe the haemodynamic pattern that is traced and which, contrary to expectations, shows a hypertensive effect for acute decrease in serum potassium. In fact, the experimental hypokalaemia model demonstrates a vasoconstriction and an increase in myocardial contractility [11–13]. The difference between the theoretical and the observed pattern could be due to the method employed to evaluate peripheral resistance (indirect, non-invasive measurements using beat-to-beat in our case) and the metabolic circumstances of dialysis with sharp variations in other factors that together can modify haemodynamics (calcaemia, osmolarity, acid-base balance, temperature), as well as concomitant counterregulatory phenomena, particularly the sympathetic and renin-angiotensin systems. The fact that hypokalaemia sensitizes myocardium to hypoxic related dysfunction [26] together with the selection for the study of an elderly population (mean age 70.3 years) with a high incidence of ischemic cardiomyopathy (10 out of 24 subjects) could have influenced the incidence of intra-dialytic hypotensions and the results. Moreover two other reasons could potentially explain a blood pressure reduction related to acute potassium decrease in the dialysis population: hypokalaemia may exacerbate autonomic dysfunction while intra-dialytic potassium loss accounts for a decrease in total osmoles [7].

Regardless of the pathophysiological explanations of the haemodynamic consequences, the results are potentially relevant in that (i) dialysate potassium concentration could theoretically be modulable in a profile, as proposed for other electrolytes like sodium and bicarbonate and (ii) more attention could be paid to controlling the potassium balance with alternative measures (diet, chelating agents, avoidance of medications which inhibits the renin-angiotensin system if unnecessary [27] and possibly prescription of mineralocorticoids [28]).

In conclusion, a rapid decrease in the concentration of serum potassium during the initial stage of the dialysis - obtained by reducing the concentration of potassium in the dialysate - translates into a decrease of systolic and mean blood pressure mediated by a decrease in peripheral resistance. The risk of intra-dialysis hypotension inversely correlates to the potassium concentration in the dialysate. Based on the results obtained by this study, modulating potassium concentration in the dialysate during haemodyalisis sessions could have favourable haemodynamic consequences.