Continuous monitoring of interstitial fluid potassium during hemorrhagic shock in dogs

Interstitial fluid potassium ion activity (IF K+) may be an accurate indicator of adequacy of cellular perfusion because normal transmembrane K+ gradient appears to be related to oxygen-dependent energy supply, and with a large gradient, even a small absolute increase in IF K+ may represent an easily detected percentage increase in that compartment. Ion sensitive field effect transistor (ISFET) devices combine integrated circuit and ion selective electrode technology to produce electrochemical sensors which are sufficiently small, rugged, and biocompatible to allow placement in the interstitium for continuous in vivo monitoring in unparalyzed subjects. K+ ISFETs were mounted at the tips of 19 gauge vascular catheters. In each of 10 dogs, 1 such probe was inserted in a central vein and another was surgically placed between medial thigh muscle fascicles. Shock (MAP = 40 mm Hg) was produced and maintained for 1 h by exsanguination into citrated blood bags. Shed blood was then reinfused. Continuous monitoring of arterial pressure, serum potassium activity (S K+) and IF K+v and intermittent determinations of cardiac output (thermodilution) and S K+ (off-line clinical analyzer) were continued for another 5 h. Mean IF K+ rapidly and monotonically increased from a control value of 3.6 to 5.1 mEq/L after 1 h of shock (p < 0.005), and then temporarily returned to a value indistinguishable from control within 2 h after resuscitation. During most of these same intervals and events, variations in S K+ were statistically indistinguishable (p > 0.05) from control values. It is concluded that ISFET probes can reliably and continuously monitor IF K+ in vivo for extended intervals. The consistently observed increase in IF K+ in response to hemorrhage, a phenomenon invisible systemically (S K+), suggests that such probes may provide clinically valuable information regarding perfusion related events at the cellular level during onset of and resuscitation from hypoperfusion states.