Hemodialysis: A model for extreme physiology in a vulnerable patient population.

Hemodialysis (HD) is a lifesaving treatment for patients with end-stage renal disease, which is very efficient in the correction of abnormalities of the internal environment. However, this efficiency also induces significant hemodynamic, thermal, and respiratory stressors. These have parallels with the extreme physiologic demands which are normally mainly experienced by healthy subjects under adverse environmental conditions, with the difference that they must be endured by a vulnerable patient population. Hemodynamic stress induced by ultrafiltration leads to a decline in circulating blood volume, which may result in intradialytic hypotension (IDH) and changes in tissue perfusion, which may have long-term consequences for the function of vital organs such as the brain and the heart. Pronounced declines in central venous oxygen saturation have been observed during routine HD, which are related to the circulatory stress imposed upon the patient. Apart from patient-related factors, thermal stress induced by HD may lead to skin vasodilation, counteracting the normal hemodynamic response to hypovolemia, which has important pathophysiologic correlates in heat syncope. Lastly, respiratory stress is reflected by prolonged arterial hypoxemia during HD, which is both related to patient-related factors, but may also be partly because of the treatment itself, especially during the first 30-60 minutes. Whereas hypoxemia during HD is related to increased mortality, its role in the reduced tissue oxygen delivery during HD should be further defined. Treatment modifications, such as cool or temperature-controlled HD, may reduce circulatory and thermal stress, which also may translate into a reduced risk of long-term cardiac or cerebral damage. However, as circulatory stress is mainly time-dependent, prolonged, or more dialysis treatment may reduce the homeostatic burden on the patient.