Extra Topic 4.8 -- Lithotripsy

Question 1

How does shock wave lithotripsy disintegrate renal calculi without causing an unacceptable amount of tissue damage?

(A 64-year-old man with an automatic internal cardiac defibrillator (AICD) and pacemaker is scheduled for extracorporeal shock wave lithotripsy (ESWL) of right renal calculi. Your hospital utilizes a first-generation lithotripter.)

Answer 1

During ESWL, the sudden vaporization of water by an energy source generates a pressure wave that is focused (F2 focal zone) on the urinary stone.

When this generated shock wave encounters a sudden change in impedance, such as occurs at the tissue-stone interface, compressive energy is released causing shear forces on the stone.

Since the acoustic impedance of water and body tissues is similar, the shock wave travels through body tissues without a significant dissipation of energy, causing minimal tissue damage.

However, some signs of tissue injury, such as skin bruising, flank ecchymosis, and hematuria (secondary to endothelial injury to the kidney and ureter), do commonly occur (especially with the more powerful first generation lithotripters).

Moreover, when the shock waves are focused on an air-tissue interface, such as found in the lungs or intestine, the difference in acoustic impedance can lead to a dissipation of energy and significant tissue damage.

Question 2

How would you provide anesthesia for this case?

(A 64-year-old man with an automatic internal cardiac defibrillator (AICD) and pacemaker is scheduled for extracorporeal shock wave lithotripsy (ESWL) of right renal calculi. Your hospital utilizes a first-generation lithotripter.)

Answer 2

While intravenous analgesia and sedation are usually adequate for procedures performed with second and third generation lithotripters, the increased discomfort associated with the more powerful first-generation lithotripters often requires general anesthesia, neuraxial anesthesia, or flank infiltration with/without intercostal blocks.

Therefore, considering the advantages of general anesthesia (when compared to neuraxial anesthesia), such as – rapid onset, reduced diaphragmatic excursion (less stone movement with respiration), and quicker recovery, my preference would be to provide general anesthesia for this procedure.

In choosing this technique, I would recognize that his lack of consciousness increases the risk of – positional injury (due to patient positioning in the bath while unconscious), and complicates patient transport to other locations for adjunctive procedures, such as cystoscopy or stent placement (which are sometimes necessary).

Question 3

Would your anesthetic plan change if the patient had a Mallampati III airway?

(A 64-year-old man with an automatic internal cardiac defibrillator (AICD) and pacemaker is scheduled for extracorporeal shock wave lithotripsy (ESWL) of right renal calculi. Your hospital utilizes a first-generation lithotripter.)

Answer 3

If I were concerned about difficult airway management, I would consider utilizing an epidural or spinal anesthetic for the procedure.
Either one of these choices eliminates the need for airway manipulation and reduces the risk of peripheral nerve injury associated with positioning and transporting an unconscious patient.

The principal disadvantage of epidural anesthesia is a slow onset and recovery,

while the principal disadvantage of spinal anesthesia is the increased incidence of hypotension.

While these techniques are associated with increased diaphragmatic excursion (due to a spontaneously ventilating patient), the subsequent stone movement, in most cases, does not significantly interfere with the procedure (stone movement is usually limited to the F2 focal zone).

Question 4

Could you proceed with flank infiltration and intercostal nerve block placement?

(A 64-year-old man with an automatic internal cardiac defibrillator (AICD) and pacemaker is scheduled for extracorporeal shock wave lithotripsy (ESWL) of right renal calculi. Your hospital utilizes a first-generation lithotripter.)

Answer 4

While this technique is a viable option for the procedure,

I believe that neuraxial anesthesia would more reliably provide adequate analgesia (as compared to flank infiltration combined with intercostal nerve blocks),

thus reducing the potential necessity for additional sedation.

Since one of my principal goals in managing this patient with a potentially difficult airway is to maintain spontaneous ventilation and avoid the necessity for airway manipulation,

I would prefer to proceed with the technique that most reliably avoids the need for supplemental intravenous sedation.

Question 5

What are the risks associated with performing ESWL on a patient with an AICD?

(A 64-year-old man with an automatic internal cardiac defibrillator (AICD) and pacemaker is scheduled for extracorporeal shock wave lithotripsy (ESWL) of right renal calculi. Your hospital utilizes a first-generation lithotripter.)

Answer 5

While the overall risk is low, there is some risk that patients with a cardiac rhythm management device (CRMD),

such as a pacemaker or automatic implantable cardioverter-defibrillator (AICD),

will experience shock wave-induced intraoperative arrhythmias during ESWL (as are those with a history of arrhythmias).

Moreover, lithotripter-induced shock waves can lead to CRMD malfunction, such as – switching to magnet mode, pacing suppression, oversensing of asynchronous shocks, and damage to rate-sensing piezoelectric crystals (affects rate-responsive CRMDs).

However, as long as the CRMD generator is not located in the abdomen (usually located in the pectoral region), ESWL is not contraindicated in patients with these devices.

To minimize the risk of CRMD associated complications, I would:

1. ascertain the indication for placement, the patient’s underlying rhythm and rate, and the degree of pacemaker dependency;
2. determine the type, manufacturer, programmability, and functionality of the device (i.e battery life, lead integrity, the presence of any alert status, and sensing/pacing thresholds);
3. verify the behavior of the device when exposed to a magnet (usually disables tachydysrhythmia detection and therapy);
4. ensure the availability of a programming device, trained pacemaker programmer, and alternative pacing modality in the operating room;
5. make sure that the patient’s CRMD is not in the shock wave path (the focal point of the lithotripter should be kept at least six inches away from the pacemaker);
6. employ continuous telemetry;
7. begin lithotripsy with low energy shock waves followed by gradually increasing energy levels, while closely monitoring pacemaker function;
8. terminate lithotripsy if the patient developed an arrhythmia; and
9. use a magnet only if there were inhibition of the device’s pacemaker function.

I would NOT require preoperative interrogation as long as his device had been checked within the last 6 months

(Recommendation: implantable cardioverter-defibrillators should ideally be checked within last 6 months, and pacemakers within the last 12 months).

Moreover, given the low risk that the acoustic pulse from the lithotripter will interfere with his CRMD, I would NOT require preoperative reprogramming to disable tachyarrhythmia sensing and treatment, or that his device was set to asynchronous mode.

In this case, interrogation would only be necessary if the patient experienced device-related complications.

Question 6

Postoperatively his hemoglobin has dropped from 14 mg/dL to 10.4 mg/dL.

Are you concerned?

What do you think is the cause?

(A 64-year-old man with an automatic internal cardiac defibrillator (AICD) and pacemaker is scheduled for extracorporeal shock wave lithotripsy (ESWL) of right renal calculi. Your hospital utilizes a first-generation lithotripter.)

Answer 6

A drop of 3.6 mg/dL in the hemoglobin concentration is significant, and would elevate my suspicion of intra-abdominal or retroperitoneal hemorrhage.

In evaluating his progressive anemia, I would –

examine the patient’s abdomen,

stabilize his hemodynamics,

look for other sources of bleeding, and

consider radiographs or CT to identify any hematoma formation.

During this evaluation, I would also –

consider other potential causes of his postoperative anemia, such as hemodilution from excessive fluid administration and/or shock wave-induced damage to the gastrointestinal, pulmonary, or urinary systems.