The retroperitoneal laparoscopic surgery significantly improves postoperative outcomes due to less invasiveness compared to traditional open abdominal surgeries. However, due to the large area of CO2 contact and abundant blood supply in the loose connective tissue (6), CO2 gas was rapidly absorbed, more severe hypercapnia was prone to happen (7).
Nowadays, there are both noninvasive and invasive techniques of measuring blood CO2 content. PaCO2 and PETCO2 are commonly used methods, while TcPCO2 has been initiated recently. Arterial blood gas analysis is the gold standard for measuring PaCO2, however, it is invasive, cumbersome and time-consuming when multiple measurements are required. In normal circumstances, the difference between PETCO2 and PaCO2 is a constant, thus PETCO2 can provide indirect estimates of PaCO2. However, in patients with complicated coexisted diseases, PETCO2 is reliable in predicting the PaCO2 decreases (8). In particular, during the retroperitoneal laparoscopic surgery, the excessive ventilation may increase intrathoracic pressure, reduce pulmonary blood flow, and increase proportion of dead space ventilation. Thus, CO2 partial pressure difference (PaCO2-PETCO2) at the end of expiration will significantly increase (3, 9). TcPCO2 causes the temperature of skin surface increase to increase the blood flow velocity in capillary, resulting in arterialization. Higher solubility and diffusivity of CO2 are used to measure the concentration of CO2 dispersing from arterialized capillaries in skin, and furthermore to predict PaCO2. In our study, the skin at the flexor side of brachioradialis was monitored to obtain stable and accurate data. It has been indicated that TcPCO2 can accurately (11-15) predict PaCO2 in a real-time manner (16), and has been widely used (8). The most important thing is that TcPCO2 is a valuable supplement to PETCO2 monitoring in patients with a large gap between PaCO2 and PETCO2 and in those concurrently requiring continuous accurate non-invasive control of CO2 levels (4, 8, 9). But it has also been shown that TcPCO2 -PaCO2 difference will increase (8) in the following settings: PaCO2 above 60 mm Hg (16), hypoperfusion in measurement site, shock, edema, thick skin and vasocontractive drugs.
In this study, 3 time points were chosen to analyze the correlation between PaCO2 and PETCO2: before pneumoperitoneum (baseline), 30 minutes [when CO2 absorption was the fastest (7)] and 60 minutes [when PaCO2 was relatively stable (7)] after establishment of pneumoperitoneum. Prior to pneumoperitoneum, there was a significant correlation between PaCO2 and PETCO2. After establishment of pneumoperitoneum, the hyperventilation setting (small tidal volume and high frequency) was used (10) to increase pulmonary ventilation volume. Due to the increased dead space ventilation and decreased pulmonary blood flow, PETCO2 increased as well and correlation between PaCO2 and PETCO2 was lost. According to our results, in elderly patients who underwent retroperitoneal laparoscopic surgery, it is not reliable to use PETCO2 to predict the PaCO2 level.
Our results indicated that significant correlation between TcPCO2 and PaCO2 still existed until 60 minutes of insufflation. Therefore, TcPCO2 may effectively predict the value and change trend of PaCO2. TcPCO2 value was not significantly affected by intrathoracic pressure, age-related changes in skin and subcutaneous circulation, pneumoperitoneum and hyperventilation modes in laparoscopic surgeries. Further evaluation using Bland - Altman method on consistency of TcPCO2 and PETCO2 with PaCO2 was performed and the maximum measurement error allowed clinically was assumed as plus or minus 5 mm Hg (17). Our results showed a poor correlation existed between PETCO2 and PaCO2 in retroperitoneal laparoscopic surgery, while TcPCO2 was highly correlated with blood gas PaCO2, hence it could be a good alternative measurement of blood CO2 level (17).
Studies by Mohsen et al. (18) showed that mild hypercapnia (PaCO2 45-53 mm Hg) provided a slight effect on cardiopulmonary function, while moderate to severe hypercapnia (PaCO2 54-70 mm Hg) may result in a significant change of cardiopulmonary function. In elderly patients with impaired organ function and compensatory mechanism, the tolerance to refractory hypercapnia drop dramatically. Intractable hypercapnia without effective treatment may further lead to acidosis, cause systemic inflammatory response syndrome and even multiple organ function failure (19-22). On the other hand, refractory hypercapnia may affect the cerebral oxygen saturation and increase the risk of delayed postoperative awakening in elderly patients.
It has been recognized that moderate excessive ventilation can correct CO2 accumulation and treat intractable hypercapnia. Moderate excessive ventilation, which means an increase of 10-15% in minute ventilation, is usually achieved by increasing tidal volume or breathing rate (23).
In elderly patients, certain lesions in small airways may exist. Insufflation easily induces increase in physiological dead space due to limited lung volume. High volume plus low frequency ventilation was considered capable of treating CO2 accumulation in elderly patients, but the improvement was not obvious (24); Further comparison of VA/Q ratio before and after insufflation was performed, the results indicated that this VA/Q ratio still significantly fell after insufflations. This suggested that high volume ventilation was not sufficient to improve lung function; on the other hand, blindly increasing the ventilation volume would increase the possibility of airway injury, affect the venous reflux and damage hemodynamic stability (24). Therefore, the small tidal volume plus high frequency ventilation strategy may be a better choice (25).
PEEP means that the ventilator produces a positive pressure higher than the atmospheric pressure at the end of expiration. PEEP could prevent alveolar atrophy or collapse, increase functional residual capacity (FRC) by opening up closed alveolus, decrease arteriovenous shunt, and restore ventilation/blood flow ratio and diffusion function. In that case, respiratory function could be improved in elderly patients with lung diseases (26). However, an even higher level of PEEP will induce an obvious elevation in airway pressure or intrathoracic pressure, which leads to ventilation injury and declined cardiac output.
Recently, a series of researches have recommended that small tidal volume (5-7 ml/kg) with a certain level of PEEP could be a safer and effective ventilation strategy for elderly patients (7). Unfortunately, It is still controversial which level (high [4, 27, 28] or low [25, 29]) of PEEP is proper.
In our study, comparison of PaCO2 before and after PEEP was performed. Our data suggested that small tidal volume (7 ml/kg) with PEEP could, to a certain extent, effectively reverse pneumoperitoneum induced hypercapnia, and PEEP of 4-10 cm H2O was more effective compared to higher PEEP levels (Figure 5).
Until now, the therapeutic role of PEEP in hypercapnia has been confirmed. We continued to investigate the adverse effects of PEEP (4-10 cm H2O), and its impact on pulmonary and cardiovascular system was used as indicators.
In this study, adequate depth of anesthesia, proper analgesia and muscle relaxtion were maintained during operation to eliminate their effect on pulmonary and cardiovascular system. Our results showed that MAP and HR stopped to elevate in group II and III after application of PEEP, this illustrated that the effect of PEEP (4-6 cm H2O) was enough to compensate the impact of hypercapnia. Furthermore, MAP and HR decreased more significantly in Group IV and V, indicating that higher levels of PEEP (> 8 mm Hg) had reversed the effect of hypercapnia and relieved the overactivation of sympathetic nerves. Another possible explanation is that high level of PEEP could increase intrathoracic pressure and damage venous reflux, resulting in drop in cardiac output. In spite of this, the decrease in MAP or HR was still within the clinical acceptable range (30) and did not harm the perfusion of vital organs in elderly patients. Our data demonstrated that PPLAT and PPEAK increased gradually with application of PEEP. The average value of PPEAK was over 30 mm Hg in group IV and V, simultaneously, mean PPLAT was above 25 mm Hg in those two groups. It has been recognized that PPLAT>25 mm Hg is the main risk factor for barotrauma, indicating that PEEP>8 cm H2O may increase risk of ventilator-induced lung injury. By contrast, several meta-analysis focused on reducing postoperative pulmonary complications by different ventilation strategies. Their results recommended that a higher level of PEEP (3 to 12 cm H2O) is more advantageous to prevent postoperative lung injury, infections and atelectasis (31). Our data also indicated that 4 to 10 cm H2O of PEEP could effectively improve intraoperative oxygenation without any significant adverse hemodynamic effects in elderly patients. So in general, a PEEP value of 10 cm H2O could be accepted, which is a preferred strategy for its better effect of reversing hypercapnia.
To conclude, the strategy to ventilate patients using small tidal volume (7 ml/kg) plus 10 cm H2O PEEP can effectively alleviate the hypercapnia without obvious adverse effects in retroperitoneal laparoscopic surgery. A point worth emphasizing is that only relatively healthy elderly patients (ASA I -II ) were enrolled into our study and the ventilation strategy could just induce a certain extent decrease in PaCO2 after pneumoperitoneum. In elderly patients with moderate to severe lung disease or perioperative severe hypercapnia, further studies are still needed to test the efficacy and safety of the forementioned strategy. Several impacting factors, such as preoperative status, operation time and insufflation pressure, should be carefully considered.
After 60 minutes of pneumoperitoneum, PETCO2 and PaCO2 became unmatched. However, at 20 minutes after the application of PEEP, their consistence restored and PETCO2 - PaCO2 correlation coefficient increased with the level of PEEP. The increase in PETCO2 - PaCO2 difference was more marked due to the impact of pneumoperitoneum, lateral clasp-knife position, and shunt in the lung. At this point, the further increase in ventilation frequency would shorten exhalation time and worsen the CO2 expiration. The use of PEEP was a better choice to solve this problem. This strategy could effectively narrow the gap between PETCO2 and PaCO2, and restore their consistence. This effect was proportional to the level of PEEP in the range of 4-10 cm H2O.
In conclusion, in elderly patients subject to retroperitoneal laparoscopic surgery for more than 1 hour, correlation and consistency between PaCO2 and TcPCO2 are higher compared to PETCO2. Hence, TcPCO2 can effectively predict PaCO2 level. In addition, during this kind of surgery, the ventilation strategy using small tidal volume (7 ml/kg) plus 10 cm H2O PEEP can be a safe and effective choice to treat pneumoperitoneum-induced hypercapnia, and restore the consistency between PETCO2 and PaCO2 .