Gaseous microemboli (GME) remain a persistent and clinically relevant concern in cardiopulmonary bypass (CPB), as they are associated with neurological injury, postoperative cognitive dysfunction, and systemic inflammatory responses. The study titled “The Effect of Surgical Field Suction Flow Rate and Venous Reservoir Levels on Gaseous Microemboli Transmission” provides an in-depth experimental analysis of how two modifiable perfusion variables—suction flow rate and venous reservoir level—affect GME generation and transmission within the CPB circuit.
The researchers designed a controlled in vitro experimental model using a mock CPB circuit filled with bovine blood to simulate real clinical conditions. This setup allowed precise manipulation of variables while minimizing confounding factors commonly encountered in clinical environments. A key strength of the study is the use of the Gampt BCC300 ultrasonic bubble counter, which measured bubble count, size, and volume at multiple circuit locations, including post-reservoir (venous), post-oxygenator (arterial), and recirculation points.
To simulate real surgical conditions, room air was introduced into the suction line at a controlled rate (200 mL/min), mimicking air entrainment during cardiotomy suction. The investigators then systematically varied two primary factors: suction flow rate and venous reservoir level. Suction flow rates ranged from 25 RPM (0.32 L/min) to 100 RPM (1.32 L/min), while reservoir levels were set at 200 mL, 500 mL, and 1000 mL. A total of 284 measurements were collected across multiple trials and different commercially available reservoir-oxygenator systems.
The findings demonstrated a strong and statistically significant relationship between suction flow rate and GME production. As suction speed increased, the number of microemboli detected at the venous sensor rose dramatically. This is attributed to increased turbulence and air-blood mixing within the cardiotomy reservoir, leading to foam formation and bubble fragmentation.
Equally important was the role of venous reservoir level. Lower reservoir levels were associated with increased GME transmission. This occurs because reduced blood volume shortens the distance bubbles must travel to exit the reservoir, decreasing the opportunity for air separation and removal. The study found a significant interaction between suction flow rate and reservoir level, meaning that the combined effect of high suction and low reservoir level produced the highest GME counts.
The box plot on page 6 (Figure 4) visually illustrates this relationship, showing a clear upward trend in GME counts as suction speed increases, particularly at lower reservoir levels. At 200 mL, the effect of increasing suction flow was most pronounced, whereas at 1000 mL, the system demonstrated greater resilience to changes in suction speed.
At the arterial level (post-oxygenator/filter), the results were more reassuring but still clinically relevant. The oxygenator and arterial filter significantly reduced GME counts, as shown in the data on page 8 (Figure 5). However, higher suction speeds still resulted in increased arterial GME, indicating that filtration systems are not completely effective at eliminating microemboli—especially smaller bubbles.
The study also analyzed bubble size distribution, revealing that most venous microemboli ranged between 30–80 microns, with smaller bubbles (<40 microns) frequently detected in the arterial line. This is particularly important because smaller microemboli can pass through filtration systems and enter the systemic circulation, potentially contributing to microvascular obstruction and inflammatory responses.
Mechanistically, the study suggests that bubble fractionation plays a key role in GME generation. When air mixes with blood at high suction speeds, larger bubbles break into numerous smaller “daughter bubbles,” increasing total embolic load. Additionally, blood viscosity and composition influence bubble stability and transmission, with protein-rich blood promoting microbubble persistence compared to crystalloid solutions.
Clinically, these findings have important implications for perfusion practice. The study reinforces that GME exposure is multifactorial, but modifiable intraoperative variables—such as suction flow rate and reservoir level—can significantly impact embolic load. Excessive suction not only introduces more air into the system but also enhances bubble fragmentation, while low reservoir levels reduce the circuit’s ability to filter and trap air.
The authors recommend maintaining the lowest effective suction flow rate and the highest practical reservoir level to minimize GME transmission. Specifically, suction speeds below 50 RPM (0.65 L/min) were associated with significantly lower embolic counts, particularly when reservoir levels were maintained above 500 mL.
The study also highlights broader clinical concerns. GME can contribute to cerebral ischemia, endothelial irritation, and systemic inflammation, even when individual bubbles are small. While the exact threshold at which GME causes clinical harm remains unclear, the cumulative effect of microemboli exposure during prolonged CPB procedures may be significant.
Despite its strengths, the study has limitations. It is an in vitro model, meaning it does not fully replicate the complexity of clinical scenarios, such as surgical manipulation, multiple suction devices, and patient-specific variables. Additionally, the use of bovine blood instead of human blood may affect viscosity and emboli dynamics. Nevertheless, the controlled design allows for clear isolation of the variables under investigation.
In conclusion, this study provides compelling evidence that both suction flow rate and venous reservoir level are critical determinants of GME transmission during CPB. By optimizing these parameters, perfusionists can potentially reduce embolic exposure and improve neurological outcomes. The findings support a shift toward more conservative suction practices and vigilant reservoir management during cardiac surgery.
