In Reply:-We appreciate the opportunity to respond to the letters by Steinbrook et al. and Shankar et al.
Answering the letter by Steinbrook et al., case reports [1–7] of laparoscopic procedures during pregnancy with good outcomes were published previous to our study. Also, papers warning against the indiscriminate use of CO2pneumoperitoneum appeared in the literature. [8] It was the lack of controlled prospective studies that prompted our prospective experimental study. [9]
In our animal model, we demonstrated a large and variable PaCO sub 2 -ETCO2gradient using direct measurement methods, whereas Steinbrook et al. did not attempt to directly evaluate the adequacy of ventilation during their procedures. It has been shown that the use of ETCO2to monitor ventilation in this group of patients might be inaccurate leading potentially to the development of respiratory acidosis. [10–11] Based on their study design and number of patients, we believe that no conclusions can be made from it to recommend against the necessity of using arterial blood gas sampling to monitor the adequacy of ventilation during laparoscopic procedures in third-trimester pregnant patients.
In response to the interesting letter by Shankar et al., we agree that it would be an ideal situation to assume a constant 10 mmHg difference in PaCO2to ETCO2gradient values between insufflated and noninsufflated gravid patients. Unfortunately, we obtained PaCO2to ETCO2gradients in the range of 10–16 mmHg with ETCO2values in the range of 23–30 mmHg. This large range of values would prevent us from assuming a constant 10 mmHg gradient difference, which may lead to hypo- or hyperventilating the patient. This fact supports our recommendation of using arterial blood gas sampling to accurately monitor adequate ventilation in this group of patients.
The work by Shankar et al. [12–13] merits recognition as they provide an interesting piece of information regarding cardiovascular physiology of late pregnancy and in the early postpartum period in anesthetized patients. However, there are physiologic differences when CO sub 2 pneumoperitoneum is induced, and therefore Shankar et al.'s results are difficult to apply to our study.
With reference to the second point, in our study, ventilation was controlled as necessary to maintain maternal normocarbia by increasing tidal volume. Changes in respiratory rate were not significantly different between study groups, and we did not observe zero or negative PaCO2to ETCO2differences, which could indicate a large mismatch of ventilation-perfusion. In addition, we do not know the extent of CO2absorbed from the peritoneal cavity and its contribution to ETCO2measurement. Despite that, the PaCO2to ETCO2difference remained significantly larger in the insufflated group. It is our impression that the magnitude of arterial CO2changes are great enough to raise a concern regarding the use of noninvasive methods to monitor ventilation.
We cannot answer precisely what happens in the alveoli during ventilation of patients undergoing intraabdominal CO2insufflation, but the results of our study strongly suggest a large and unpredictable difference between PaCO2and end-tidal CO2values; thereto our recommendation of monitoring PaCO2by direct methods.
Addressing the third point, despite the disadvantages of using animal models with regards to their applicability to humans, they help us to understand and to obtain vistas and ideas of the mechanisms of biological operation. Naturally, results from animal models must always be interpreted with caution in lieu of physiologic differences between species. Nonetheless, the sheep model has been used in obstetric research for decades and has been accepted by the scientific community. However, a well-conducted clinical investigation in humans is still necessary to support our results.
Antonio M. Cruz, D.V.M.
Lesley-Ann Crone, M.D.
Lucy C. Southerland, M.D.
Tanya Duke, D.V.M.
James G. Ferguson, D.V.M.
Department of Veterinary Anesthesiology, Radiology, and Surgery; Western College of Veterinary Medicine; University of Saskatchewan; 52 Campus Drive; Saskatoon SK S7N 5B4; Canada
(Accepted for publication August 24, 1997.)
