Материал: Oxford American Handbook of Urology ( PDFDrive )

JJ STENTS 597

much more effective for relieving obstruction by an intrinsic problem (e.g., a stone).

Placement of two stents may provide more effective drainage (figure- of-eight configuration may produce more space around the stents for drainage).

For acute, ureteric stone obstruction with a fever, should I place a JJ stent or a nephrostomy?

In theory, one might imagine that a nephrostomy is better than a JJ stent—it can be done under local anesthetic (JJ stent insertion may require a general anesthetic) and it lowers the pressure in the renal pelvis to 0 or a negative value. A JJ stent, by contrast, results in a persistently positive pressure, is less likely to be blocked by thick pus, and allows easier subsequent imaging (contrast can be injected down the ureter—a nephrostogram—to determine if the stone has passed).

In practice, both seem to be effective for relief of acute stone obstruction and associated infection.3,4

3 Pearle MS, et al. (1998). Optimal method of urgent decompression of the collecting system for obstruction and infection due to ureteral calculi. J Urol 160:1260–1264.

4 Ryan PC, et al. (1994). The effects of acute and chronic JJ stent placement on upper urinary tract motility and calculus transit. Br J Urol 74:434–439.

600 CHAPTER 16 Urological surgery and equipment

Diathermy

Diathermy is the coagulation or cutting of tissues through heat.

Monopolar diathermy

When an electric current passes between two contacts on the body, there is an increase in temperature in the tissues through which the current flows. This increase in temperature depends on the volume of tissue through which the current passes, the resistance of the tissues, and the strength of the current. The stronger the current, the greater the rise in temperature.

If one contact is made large, the heat is dissipated over a wide area and the rise of temperature is insignificant. This is the earth or neutral electrode, and under this the rise in temperature is only 1 or 2°C. The working electrode or diathermy loop is thin, so that the current density is maximal and, therefore, so is the heating effect.

When a direct current is switched on or off, nerves are stimulated and muscles will twitch. If the switching on and off is rapid enough, there is the sustained contraction familiar to the physiology class as the tetanic contraction. If a high-frequency alternating current is used (300 kHz to 5 MHz), there is no time for the cell membranes of nerve or muscle to become depolarized, and nerves and muscles are not stimulated (they are stimulated at lower frequencies).

The effect of the diathermy current on the tissues depends on the heat generated under the diathermy loop. At relatively low temperatures, coagulation and distortion of small blood vessels occurs. If the current is increased to raise the temperature further, water within cells vaporizes and the cells explode. This explosive vaporization literally cuts the tissues apart.

Bipolar diathermy

Bipolar diathermy involves the passage of electrical current between two electrodes on the same handpiece. It is inherently safer than monopolar diathermy, since the current does not pass through the patient, and diathermy burns cannot therefore occur.

Potential problems with diathermy

The diathermy isn’t working

•Do not increase the current.

•Check that the irrigating fluid is glycine (sodium chloride conducts electricity, causing the diathermy to short-circuit).

•Check that the diathermy plate is making good contact with the skin of the patient.

•Check that the lead is undamaged.

•Check that the resectoscope loop is securely fixed to the contact.

Modern diathermy machines have warning circuits that sound an alarm when there is imperfect contact between the earth plate and the patient.