Continuous urinary electrolyte measurement in a swine model of mechanical ventilation

Introduction

The kidney is the organ of the human body designed to sense and regulate intravascular volume. The KING (Kidney Instant Monitoring; Orvim, Paderno Dugnano, Italy) is a new device that allows continuous measurement of urinary electrolytes and real-time body response to changes in intrathoracic and intra-abdominal pressures and to lung injury.

Methods

Sixteen pigs (weight 20 ± 3 kg) were anaesthetized, tracheotomized, catheterized and mechanically ventilated. Pigs were ventilated for approximately 10 hours with a TV of 10 ml/kg, RR 15 breaths/minute, FIO₂ 0.5 and no PEEP. After TV was increased, pigs were divided into two groups: eight pigs with a lower TV and eight pigs with higher TV. A variable dead space was added to maintain normocapnia. Pigs were mechanically ventilated with the new TV up to 48 hours. The KING measured urinary output and urinary concentration of sodium and potassium every 10 minutes. NaCl 0.9% was exclusively infused. The average urinary concentration of each electrolyte was expressed as the total concentration for each electrolyte divided by the length of mechanical ventilation. Data were compared using the Wilcoxon test.

Results

The higher TV group was ventilated with a TV of 890 ± 251 ml and a plateau pressure of 31 ± 8 cmH₂O while the lower TV group had a TV of 472 ± 262 ml (P = 0.006) and a plateau pressure of 17 ± 9 cmH₂O (P = 0.02). The length of mechanical ventilation for the high TV group was 1,850 ± 633 minutes and for the low TV group was 3,341 ± 749 minutes (P = 0.0006). The urinary output for the high TV was 37 ± 16 ml/hour and for the low TV was 52 ± 19 ml/hour (P = 0.10). The high TV group showed lower urinary sodium (40 ± 35 mEq/l vs 93 ± 26 mEq/l, P = 0.007), higher urinary potassium (52 ± 12 mEq/l vs 35 ± 23 mEq/l, P = 0.04) and a lower urinary sodium/potassium ratio (0.77 ± 0.7 vs 3.86 ± 3.3, P = 0.001). Before TV was increased, urinary output, urinary concentrations of sodium, potassium and sodium/potassium ratio were not different.

Conclusions

Data show that injurious ventilation is sensed by the kidney as a relative hypovolemia with reduction in sodium urinary output, increased excretion of potassium and inverted sodium/potassium ratio.