In this pilot study, 30 prevalent hemodialysis patients will undergo three regular hemodialysis sessions during which various potassium assessments will be performed. Pre- (t1) and post-dialysis (t2) plasma potassium levels (K+Pl) will be measured using standard ion-selective electrodes. These values will be correlated to K+ determine in saliva probes (K+Sa) using genetically encoded potassium ion indicators (GEPIIs). Additionally, continuous ECG will be recorded during each hemodialysis treatment and potassium assessment (K+ECG) will be performed upon ECG-based parameters and correlated to K+Pl.

Cardiovascular diseases are among the leading causes of death in industrialized countries. Medical therapy for these diseases has seen significant progress, much of which is based on agents interfering with the renin-angiotensin-aldosterone system (angiotensin-converting enzyme inhibitors, angiotensin receptor antagonists, mineralocorticoid receptor antagonists, neprilysin inhibitors). However, these agents bear the side effect of reducing renal potassium (K+) excretion and may thus lead to elevation of blood K+ Levels (hyperkalemia). Hyperkalemia is a potentially life-threatening condition, which in its most severe forms requires immediate medical attention, since there is imminent danger of dangerous arrhythmias and sudden cardiac death. Therefore, hyperkalemia is a leading reason to withdraw potentially lifesaving therapy in a significant number of patients, which is considered to have a negative impact on patient outcomes. Recently, novel intestinal potassium binders have been shown to be efficient in reducing incidence and severity of hyperkalemia These compounds have very recently been shown to allow extending the benefits of antihypertensive therapy with spironolactone to patients with chronic kidney disease with refractory hypertension . Yet due to preanalytical problems as well as time and cost-restraints, out-patient monitoring of serum or plasma potassium levels has proven problematic.

The plasma potassium level and kinetics at two timepoints before and after a hemodialysis session as determined by either standard of care (i.e. ion selective electrode) will be compared to the potassium level measured in (i) saliva (salivary potassium [K+Sa]) or (ii) determined based on electrocardiogram (K+ECG). Patients suffering from end-stage renal disease undergoing hemodialysis (HD) frequently present with severe hyperkalemia prior to a HD session. During HD treatment, K+ levels undergo unphysiologically rapid changes due to K+ removal via HD as well as due to changes in acid-base status. Thus, HD patients represent a unique population in whom significant K+ derangements and rapid K+ Level changes predictably occur and where these phenomena can be investigated in a safe environment.

Inclusion Criteria:

• 18 to 90 years of age
• End stage-renal disease or acute kidney injury patient undergoing hemodialysis
• Ability to provide oral and written informed consent
• Ability and willingness to comply with study procedures
• Willingness to not consume foods or drinks other than water during dialysis session

Exclusion Criteria:

• Intraventricular conduction abnormalities (left- or right bundle branch block, trifascicular block) which interfere with K+ECG determination
• Active inflammation or infection of the oral mucous membranes or dentition