Re: Increase in Intracranial Pressure During Carbon Dioxide Pneumoperitoneum with Steep Trendelenburg Positioning Proven by Ultrasonographic Measurement of Optic Nerve Sheath Diameter

Dear Editor:

We read with interest the article by Kim and associates, published electronically.

Robot-assisted laparoscopic radical prostatectomy has become an often used surgical procedure for patients with prostate cancer because of its superiority in controlling intraoperative bleeding, postoperative pain, and shortening the patient's hospital stay more than for those patients who undergo conventional radical retropubic prostatectomy. The unfavorable effects of pneumoperitoneum and the steep Trendelenburg position, however, both of which are necessary for creating an appropriate surgical field, raise concerns regarding the disturbed function of major organs. Of the several major organs of concern, the authors evaluated the effects of pneumoperitoneum and the steep Trendelenburg position on the patient's cerebral physiology by sonographically measuring the optic nerve sheath diameter, which has been known to be a surrogate for intracranial pressure.

Kalmer and associates ¹ previously investigated the influence of the steep Trendelenburg position on cerebrovascular homeostasis during robot-assisted laparoscopic radical prostatectomy. They found that the difference between the end-tidal CO₂ partial pressure (ETCO₂) and the arterial CO₂ partial pressure (PaCO₂) increased significantly from 1.06 kPa (7.95 mm Hg) in the normal supine position up to 1.41 kPa (10.58 mmHg) after 120 minutes in the pneumoperitoneum combined with the Trendelenburg position, and they also demonstrated that this difference between the ETCO₂ and the PaCO₂ increased as the ETCO₂ increased. Similarly, Choi and colleagues ² found that the difference between the ETCO₂ and the PaCO₂ increased gradually in the pneumoperitoneum combined with the steep Trendelenburg position over time (approximately 10 mm Hg of difference was reported during the pneumoperitoneum combined with the steep Trendelenburg position).

Based on these previous studies, we are attempting to control the ETCO₂ by using an optimal ventilator strategy, because the ETCO₂ reflects the PaCO₂, which is a well-known factor affecting the cerebral blood flow and subsequently the intracranial pressure. We continuously monitor the ETCO₂ rather than the PaCO₂ in daily clinical practice, because there is otherwise no continuous measurement of the PaCO₂. Therefore, it is clinically relevant whether the difference between the ETCO₂ and the PaCO₂ during pneumoperitoneum and the steep Trendelenburg position differ from that in the supine position and, furthermore, what is the extent of the difference between the ETCO₂ and the PaCO₂.

Moreover, this is becoming an important issue for patients undergoing robot-assisted laparoscopic prostatectomy because they are usually old; Choi and coworkers ² found a greater difference between the ETCO₂ and the PaCO₂ in patients age 65 years or older. In contrast to previous studies, Kim and colleagues found an unexpectedly small difference between the ETCO₂ and the PaCO₂ during pneumoperitoneum combined with the steep Trendelenburg position, and this very small difference was surprisingly left unchanged even 30 minutes after patients remained in those positions.

Based on the results of Kim and colleagues, we remain basically uncertain regarding the management of the ETCO₂ during robot-assisted laparoscopic prostatectomy. We are unsure why the difference between the ETCO₂ and the PaCO₂ during pneumoperitoneum combined with the steep Trendelenburg position was very small and remained nearly constant throughout the entire surgery, unlike that seen in previously published studies.