Hemodynamics and tissue oxygenation effects after increased in positive end-expiratory pressure in coronary artery bypass surgery
In the present study we investigated the influence of PEEP on hemodynamics and tissue oxygenation in patients after CAB. It should be noted that increases in PEEP caused a significant reduction in tissue oxygenation (ScvO2), even with higher levels of oxygen delivery (PaO2 and PaO2/FiO2 ratio).
Collapsed lung and hypoxia are common complications after cardiac surgery, mainly due to anesthesia (decreased muscle tone and predisposition to atelectasis) [4] and these changes may be accentuated by surfactant reduction and inflammatory response. In addition, the inflammatory process contributes to pulmonary interstitial edema, which cause a significant decrease in pulmonary gas exchange [5]. Our results indicate increases in PaO2, oxygen hemoglobin saturation, and PaO2/FiO2 ratio, suggesting a reversal of alveolar collapse. These results are consistent with those found in the scientific literature, mainly in patients with acute respiratory distress syndrome [5]. Studies have shown that higher values of PEEP can prevent hypoxemia, with a significant increase in PaO2 in post-cardiac surgery patients [5–7].
Despite the benefits of high PEEP after cardiac surgery in improving oxygenation, some precautions should be taken with PEEP increases in patients at the intensive care unit, mainly because PEEP might interfere with intra-thoracic pressure with hemodynamic effects. Some studies have demonstrated that reductions in cardiac output and tissue oxygenation could occur after increased PEEP [4, 8]. Moreover, different PEEP levels reduced cardiac output without changes in PaO2/FiO2 ratio, ScvO2, or hemodynamic values (heart rate or mean arterial pressure). The authors suggested that the reduced cardiac output was due to high levels of PEEP, as cardiac output was restored after PEEP decreased to 5 cmH2O [8].
Studies of normovolemic patients have reported maintenance of cardiac output after increased PEEP [9]. However, it should be noted that our study exhibited positive water balance and higher values of blood lactate after study protocol, suggesting that increased PEEP values were responsible for reducing cardiac output and, consequently, for reductions in tissue oxygenation (lower ScvO2 values). Recently, Gutierrez (2016) [10] described a higher respiratory muscle workload with a decreased ScvO2 and this finding was not associated with sepsis. Furthermore, Rivers et. al. (2001) [11] demonstrated that increases in ScvO2 should have been related to respiratory muscles unloading by mechanical ventilation. Our study was not designed to access the respiratory muscle function (overloading or unloading) by PEEP increases, but this fact might have influenced the reduction in ScvO2 values. This issue should be performed in further studies with PEEP hemodynamics after CAB surgery. Moreover, some studies have demonstrated that higher levels of PEEP required more sedatives and increase the mechanical ventilation time and compromise the mechanical ventilation withdrawal [6, 12, 13]. Our data support the notion that high PEEP levels directly affect tissue oxygenation in patients after cardiac surgery, even with preserved cardiac function and without surgery complications. However, it should be noted that our protocol did not measure cardiac output directly and we suggest a reduction in cardiac output using ScvO2.
Moreover, some studies have reported that short periods of increased PEEP may be beneficial to post-cardiac surgery patients without compromising hemodynamic or tissue oxygenation [14]. In this scenario, even in short periods of high PEEP levels, certain precautions should be taken, such as ScvO2 measurements. Our results suggest a reduction in tissue oxygenation and higher levels of blood lactate during only 30 min of rising PEEP; and it has been demonstrated that reductions in ScvO2 are related to poor outcomes after cardiac surgery. The maintaining ScvO2 above 70% resulted in a reduction in mortality [15] and our findings support our suggestions that ScvO2 should be monitored in patients without cardiac failure or without surgery complications.
Study limitations
Some limitations of this study should be addressed. The inclusion criteria of PaO2/FiO2 ratio 200 may have limited the number of patients in our study. Thus the extrapolation of our findings to other studies of CAB surgery should be viewed with caution. On the other hand, there is evidence that the PaO2/FiO2 ratio 200 is the most common hypoxemia finding after CAB surgery [16] and PaO2 alterations in patients after CAB surgery do not depend on PEEP only but the level of O2 delivery/utilization ratio [11]. Moreover, we did not measure cardiac output before or after PEEP increase, which could have better demonstrated tissue oxygen delivery and utilization. Rather, ScvO2 values were used to reflect tissue oxygen delivery and utilization indirectly during the PEEP maneuver. The venous blood sample has been taken by a catheter positioned in right atrium, we recognize that this could rise some questions, but all patients had normal right ventriculum and with adequate volemic status and central venous pressure. Further studies with Swan-Ganz catheter should be performed to access the directly effect of hemodynamics repercussions with PEEP in hypoxemic CAB patients. We believe that ScvO2 is a good alternative to measure whole-body oxygen extraction, it is minimally invasive and have low cost. In addition, our results demonstrated positive water balance during the intensive care unit, but other measurements could also have been addressed, such as Swan-ganz catheter. Moreover, we did not measure the inflammation status after cardiac surgery or after PEEP maneuver.
Our study aimed to evaluate the hemodynamics effects after PEEP increases, and we recognize that a control group, without PEEP increases, was not performed; but the PEEP titration was not main objective of the study; we aimed to investigate hemodynamics and tissue oxygenation after a simple PEEP increase in hypoxemic patients after CAB. The choice of 12 cmH2O for the PEEP value was made because it is commonly used in intensive care units to reverse arterial hypoxemia [17]. Further studies for hemodynamics measurements and tissue perfusion with control group and with different groups of PEEP are necessary, both in CAB and other cardiac procedures.