Hyperkalemia: Causes, Effects on the Heart, Pathophysiology, Treatment, Animation.

Hyperkalemia refers to abnormally high levels
of potassium in the blood. In normal circumstances, more than 90% of
the total body potassium is INTRAcellular; the remaining is in the EXTRAcellular fluid
and blood plasma. The ratio of INTRAcellular to EXTRAcellular
potassium is important for generation of action potentials and is essential for normal functions
of neurons, skeletal muscles and cardiac muscles. This is why potassium levels in the blood
are strictly regulated within a narrow range between 3.5 and 5mmol/L. As the normal daily
dietary intake of potassium varies widely and can be as much as 100mmol a day, the body
must quickly and precisely react to keep blood potassium levels within the normal limits. This is achieved by 2 mechanisms:
– Excretion of potassium through the kidneys and intestines; with the kidneys playing a
predominant role. – Shifting of potassium from the extracellular
fluid into the cells by the sodium/potassium pump. The pump is mainly regulated by hormones such
as insulin and catecholamines. Hyperkalemia is defined as a serum potassium
concentration HIGHER than 5mmol/L. Hyperkalemia may result from decreased excretion, excessive
intake, or shift of potassium from INSIDE the cells to EXTRA-cellular space. Usually, a combination of factors is responsible. The most common scenario is a RENAL INsufficiency
combined with excessive potassium supplements OR administration of certain drugs. Impaired kidney function is most prominent;
excessive intake or extracellular shift is rarely the only cause. Mild hyperkalemia is often without symptoms,
although some patients may develop muscle weakness. Slow or chronic increase in potassium levels
is less dangerous, as the kidneys eventually adapt by excreting more potassium. Sudden onset and rapid progression of hyperkalemia,
on the other hand, can be fatal. Primary cause of mortality is the effect of
potassium on cardiac functions. As potassium levels INcrease in the EXTRAcellular
space, the MAGNITUDE of potassium gradient across the cell membrane is REDUCED, and so
is the ABSOLUTE value of the resting membrane potential. Membrane voltage becomes less negative, moving
closer to the threshold potential, making it EASIER to initiate an action potential. The effect this has on excitability of myocytes,
however, is complex. While initial changes seem to increase myocyte
excitability; further rise of potassium has the OPPOSITE effect. This is because the value of membrane potential
at the onset of an action potential DETERMINES the number of voltage-gated sodium channels
activated during depolarization. As this value becomes less negative in hyperkalemia,
the number of available sodium channels DEcreases, resulting in a SLOWER influx of sodium and
subsequently SLOWER impulse conduction. In experimental models, ECG changes produced
by hyperkalemia follow a typical pattern that correlates with serum potassium levels: peaked
T-wave, P wave widens and flattens, PR interval lengthens, QRS complex widens and eventually
blends with T-wave. In practice, however, this pattern is present
only in a fraction of hyperkalemia patients and does NOT always correlate with potassium
levels. This makes diagnosis on the basis of ECG alone
very difficult. Given the dangerous nature of acute hyperkalemia,
it must be suspected in any patient having new bradycardia or conduction block, especially
in those with renal problems. Severe hyperkalemia is treated in 3 steps:
– Calcium infusion is given to rapidly REVERSE conduction abnormalities. Calcium antagonizes the effect of potassium
at the cellular level, stabilizing membrane potential. However, it does not remove potassium, and
should not be used in the case of digoxin toxicity. – Insulin is administered to stimulate the
sodium/potassium pump, promoting INTRA-cellular shift of potassium. – Hemodialysis is performed to remove potassium
from the body. Longer term treatment for hyperkalemia without
conduction problems consists of reducing intake and increasing excretion.

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