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The Influence of Positive End-Expiratory Pressure on Surgical Field Conditions During Functional Endoscopic Sinus Surgery

DeMaria, Samuel MD; Govindaraj, Satish MD; Huang, Alice BS; Hyman, Jaime MD; McCormick, Patrick MD, MEng; Lin, Hung Mo PhD; Levine, Adam MD

doi: 10.1213/ANE.0000000000000550
Ambulatory Anesthesiology and Perioperative Management: Research Report
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BACKGROUND: Functional endoscopic sinus surgery (FESS) is the mainstay of surgical treatment for sinonasal disease. This surgery carries certain risks. Most of these risks relate to the quality of the surgical field. Thus, mechanisms by which the surgical field can be improved are important to study. We sought to determine whether positive end-expiratory pressure (PEEP) had a deleterious effect on the quality of the surgical field in patients undergoing primary FESS.

METHODS: Forty-seven patients were randomized to a ventilation strategy using either 5 cm H2O of PEEP or zero added PEEP. The quality of the surgical field was measured every 15 minutes using a validated surgical scoring method.

RESULTS: The addition of PEEP did not have any measurable effect on the surgical field scores after onset of surgery (odds ratio [OR] (95% confidence interval [CI]) = 1.06 (0.44–2.58), P = 0.895 for side 1; OR (95% CI) = 0.56 (0.16–1.93), P = 0.356 for side 2). The peak inspiratory pressure did have an effect on surgical grades. Every cm H2O of added pressure over 15 cm H2O total pressure contributing to increased odds of higher surgical field score. For each cm H2O increase in inspiratory pressure above 15cm H2O increased the surgical field score (OR [95% CI] 1.13 [1.04–1.22], P = 0.002).

CONCLUSIONS: During FESS surgery if PEEP is added, it is important to keep the mean inspiratory pressure below 15cm H2O to avoid worsening surgical field conditions.

Published ahead of print November 25, 2014.

From the Department of Anesthesiology, Icahn School of Medicine at Mount Sinai, New York, New York.

Accepted for publication October 6, 2014.

Published ahead of print November 25, 2014.

Funding: This study was supported solely with departmental funds.

Conflict of Interest: See Disclosures at the end of the article.

Reprints will not be available from the authors.

Address correspondence to Samuel DeMaria, MD, Department of Anesthesiology, Icahn School of Medicine at Mount Sinai, One Gustave Levy Place, Box 1010, New York, NY 10029. Address e-mail to: samuel.demaria@mountsinai.org.

Functional endoscopic sinus surgery (FESS) is the mainstay of surgical treatment for sinonasal disease. Because of the limited surgical field and the proximity of the sinuses to critical structures (e.g., eye, carotid artery, brain), even small amounts of surgical site bleeding can obscure the surgeon’s view and lead to rare but devastating and even life-threatening complications.1–3 The benefit of a clear, “bloodless” surgical field on outcomes in these patients is widely accepted by surgeons and anesthesiologists.4,5 Therefore, identifying perioperative measures that decrease excessive intraoperative bleeding to optimize surgical visualization is important to anesthesiologists, surgeons, and patients alike.

The use of positive end-expiratory pressure (PEEP) has become commonplace because of its beneficial respiratory effects in the postoperative phase,6 especially in obese patients7 and those patients undergoing laparoscopy.8 However, PEEP may increase intrathoracic pressure, thereby impairing venous return. Some authors have suggested the resultant head and neck venous congestion may thereby be associated with poor surgical visualization due to increased bleeding.9,10 In 1 study, decreasing intrathoracic pressure through the use of high-frequency jet ventilation during FESS was associated with a reduction in intraoperative bleeding and an improved surgical field.11 While this is compelling work, high-frequency jet ventilation can be impractical in most locales.

In this study, we sought to determine whether patients with sinonasal disease scheduled for FESS would have different surgical grades intraoperatively when randomized to a PEEP ventilation mode as compared to zero added PEEP (ZEEP). Given previous studies showing the positive effects of decreased intrathoracic pressures on surgical grade, we hypothesized that the addition of a low level of PEEP, as might be seen in typical anesthetic practice (i.e., 5 cm H2O) would worsen bleeding scores, potentially via effects on overall intrathoracic pressures as measured by inspiratory pressures.

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METHODS

Study Design

After obtaining IRB approval from the Mount Sinai Hospital Program for the Protection of Human Subjects, we offered study enrollment to adult patients (ages 18–80 years) ASA physical statuses I through III, scheduled to undergo primary (nonrevision) bilateral endoscopic sinus surgery without septoplasty from May 2012 to August 2013. Patients were recruited from 1 surgeon’s practice and all underwent 1 of 2 attending anesthesiologists’ care during this time period. Written informed consent was obtained for all subjects who agreed to participate. Participants with relative contraindications to a controlled hypotensive anesthetic technique (e.g., coronary artery or cerebrovascular disease, hypertension, use of cardiovascular system-altering drugs) and those with potentially increased anesthetic or analgesic requirements (e.g., chronic alcohol or opioid use) were excluded. Those with known allergies to amide-based local anesthetics were also excluded because all patients were to receive regional anesthetic placement after induction of general anesthesia. Patients with bleeding diatheses or those taking medications that could have affected surgical hemostasis were excluded. Also, patients with severe lung disease (e.g., chronic obstructive pulmonary disease and severe asthma) were excluded. Patients with a body mass index > 40 kg/m2 were also excluded from participation because their body habitus would be more likely to affect respiratory variables. If the anesthesiologist believed that enrollment in the PEEP or ZEEP group was causing untoward effects intraoperatively, enrollment in the study could be terminated immediately.

Patients were randomized to either a PEEP or ZEEP group. ZEEP group patients had their lungs ventilated with volume control ventilation at 5 to 8 mL/kg ideal body weight to maintain an end-tidal CO2 (EtCO2) between 30 and 35 mm Hg with ZEEP added. The PEEP group had the same ventilator variables but with a PEEP of 5 cm H2O added. PEEP was set to 5cm H2O pressure on the ventilator of the Datex Ohmeda Aisys Machine (GE Healthcare, Wauwatosa, WI). Women were tracheally intubated with a 6.5 anode endotracheal tube (ETT) and men with a 7.0 anode ETT. The surgeon was blinded as to group assignment throughout the study. All patients underwent a standardized anesthetic protocol as described previously by our group.12 Briefly, this comprised a nitrous (50%–70%) narcotic technique with infusions of remifentanil (0.05–0.5 mcg/kg/min) and propofol (25–50 mcg/kg/min). Postinduction bilateral sphenopalatine ganglion and infraorbital nerve blocks were performed. No neuromuscular blockade was provided. The sphenopalatine ganglion block was performed via the palatal approach with a slight modification using a Macintosh 3 blade to illuminate and expose the hard and soft palates and a curved 25 gauge 1.5 inch needle used to inject 1 mL 1% lidocaine with 1:100,000 epinephrine into the greater palatine foramina. The infraorbital nerve block was placed transnasally using a 25 gauge 1.5 inch needle directed toward the infraorbital notch. Two mL 0.5% bupivicane was injected midway between the nasal fold and the notch after confirming a negative aspiration.

An automated record was initiated upon the patient’s entry into the operating room and terminated just before departure. Vital signs, anesthetic technique (e.g., bolus and infusion drug totals, end-tidal gas concentrations, narrative comments), and inputs and outputs for the case were recorded directly into the computerized record (Compurecord©, Philips Electronics North America Corp, Andover, MA) from the anesthesia machine. Surgical field grading was performed in real time by the otolaryngologist every 15 minutes, with time zero occurring when the endoscope was first inserted into the nasal cavity, time 1 occurring 15 minutes later, time 2 occurring 15 minutes after that, and so on. The Boezaart grading system was used to assess the quality of the surgical field (Table 1).13 When and if the surgical side was switched, the timing began again at the introduction of the endoscope into the operative nares and scoring occurred every 15 minutes.

Table 1

Table 1

Vital signs, including arterial blood pressures, SpO2, end-tidal gas concentrations, ventilator pressures, and PEEP were recorded every 15 seconds as available per a routine recording scheme used in our department. Noninvasive arterial blood pressures were measured every 3 minutes throughout the case for all patients and were only available at that interval. The electronic medical record was queried electronically at the time at which the “Boezaart score” was obtained. For each study patient, the values for each blood pressure within the time period 3 minutes before and after the score were logged. These values were used to calculate median values for each group. For all other continuously recorded vital signs (e.g., SpO2, EtCO2, ventilator pressures), data recorded within the 15 seconds of the Boezart score were used to calculate median values for each group. Blood loss calculations were performed by a study group member who was not actively managing the study participant’s anesthetic. The blood suction canisters were examined as were surgical pads and irrigation totals to calculate the final blood loss.

As part of the protocol, if any difficulties with oxygenation were encountered intraoperatively, the ventilation strategy was to be changed and optimized per the anesthesiologist and if variables were outside of the predefined study variables, the patient was to be automatically withdrawn from the study so as not to interfere with proper care.

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Surgical Considerations

After the initiation of general and regional anesthesia, stereotactic frameless computer-assisted surgical navigation was set up. Topical epinephrine 1:1000-impregnated pledgets were administered into the nasal cavity and sinuses were injected with 2 mL (1 mL each side) of 1% lidocaine with 1:100,000 epinephrine. Standard maxillary antrostomies, total ethmoidectomies, sphenoid and frontal sinusotomies were performed as indicated with a combination of powered and manual instrumentation. Conservative suction cautery was administered for any hemostasis that did not respond to topical vasoconstriction. Finger cot merocels were placed within the middle meatus, and silastic stents were fashioned for use in the frontal sinuses as needed.

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Statistical Analysis

Our initial power calculation during design stage was based on the following: Group sample sizes of 17 and 17 achieve 80% power to reject the null hypothesis of equal means when the population mean difference on Boezaart grading scale is −1 (ZEEP versus PEEP) with a standard deviation for both groups of 1.0 and with a significance level (α) of 0.050 using a 2-sided 2-sample equal-variance t test. However, in the analysis stage, we realized that the grading data were not normally distributed and thus opted to analyze data using logistic regression, both the traditional approach defining an event as grading scale ≥ 3 and the proportional odds model assuming PEEP effect is associated with higher score in general.

Because our initial assumptions were for a less robust analytic method, we performed the following post hoc power determination. In our sample, 40% of the sample from the ZEEP group had grading ≥ 3 in the first surgical side. Thus, we assumed that the proportions of observations that had grading ≥ 3 after time 0 are 0.6 and 0.4 for the PEEP and ZEEP group, respectively. With group sample sizes of 23 and 23, the study would achieve 90%, 81%, or 66% power in a design with 8, 6, or 4 grading readings that has an AR(1) covariance structure, and the correlation between observations on the same subject is 0.2 (based on for a 2-sided and α = 0.05) test. If the proportion of grading ≥ 3 in the PEEP group is 55%, the corresponding power would be 68%, 57%, or 43% (PASS 12.0.2. 1983-2013 NCCC, LLC).

Patient and disease characteristics are described as percent, median (interquartile range [IQR]), or means (standard deviation). For comparisons between PEEP and ZEEP, χ2 tests were used for gender, hypertension, and American Society of Anesthesiologists status variables. Fisher exact tests were used for asthma, coronary artery disease, and obstructive sleep apnea variables. Student t tests were used for age, body mass index, mean Lund-Mackay scale. For intraoperative variables, the averages were first taken for each patient and then the Wilcoxon rank sum tests were used for comparison between ZEEP and PEEP, as appropriate.

The generalized estimating equation (GEE) method for proportional odds model was used to test the association between PEEP treatment and the surgical score, controlling for intraoperative factors, which included heart rate, mean arterial blood pressure (MAP), tidal volume, mean peak inspiratory pressures, and EtCO2 levels. For time effect, the model only considered time zero because surgical grades appeared to be similar thereafter. Three-way interaction among PEEP, nostril side, and time zero and their pairwise 2-way interactions were also tested for statistical significance at the 0.1 level. Since the P value for the 3-way interaction term was 0.07, all relevant pairwise interaction terms and the main effects were kept in the model regardless of their P values. As a confirmatory analysis, the traditional logistic regression model using the GEE method, where the outcome was defined as having surgical score ≥3, was repeated. A working independent correlation matrix was applied in GEE for the within-patient observations. However, empirical standard error estimates were used to compute P values for hypothesis testing and confidence intervals. Analysis was performed using SAS 9.3. (SAS Inc., Cary, NC).

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RESULTS

Sixty-five patients were approached for enrollment and 47 patients fit the exclusion/inclusion criteria (72%), ranging in ages from 19 to 80 years. No patients were omitted after enrollment due to intraoperative complications or other clinical circumstances. The groups did not differ significantly in most pre- or intraoperative characteristics, estimated blood loss, or Lund-Mackay scores (Tables 2 and 3). There was a difference between median EtCO2 recordings between groups. To confirm that group assignment was maintained, the median measured PEEP was analyzed; in the ZEEP group, the measured PEEP was 2.19 cm H2O (IQR = 1.98–2.29) and in the PEEP group, it was 6.13 cm H2O (IQR = 6.01–6.48) (P < 0.001).

Table 2

Table 2

Table 3

Table 3

Patients were compared for the primary outcome of Boezaart surgical scores by group assignment. For the entire cohort, 395 separate observations were analyzed. The box-plots of the Boezaart surgical scores by side and by time are displayed in Figure 1. The difference in median of the average Boezaart scoring over repeated readings was 0.17 (95% CI = −0.25 to 0.58; P = 0.36). The Wilcoxon rank sum tests comparing the medians between the 2 groups stratified by time and side all yielded nonsignificant P values (all >0.2, except time 1 side 1 P = 0.100 and time 2 side 2 P = 0.139).

Figure 1

Figure 1

In the proportional odds model that adjusted for SpO2, EtCO2, heart rate, or tidal volume, no significant difference in Boezaart surgical scores was observed after time zero between the 2 groups (For PEEP versus ZEEP, OR (95% CI) = 1.06 (0.44–2.58), P = 0.895 for side 1; OR (95% CI) = 0.56 (0.16–1.93), P = 0.356 for side 2). It was noted that time zero for side 1 had a significantly lower Boezaart surgical score than time zero for side 2 for both the ZEEP (odds ratio (OR) = 18.45, 95% CI: 5.03–67.66, P < 0.001) and PEEP groups (OR = 5.34, 95% CI: 1.41–20.21, P = 0.014). (Table 4) In addition, inspiratory pressure was found to be associated with increased odds of having a higher Boezaart score irrespective of group assignment (the OR (95% CI) for every 1 cm H2O increase in median peak inspiratory pressure above 15 cm H2O was 1.13 (1.04–1.22, P = 0.002). MAP showed a trend toward a relationship with a Boezaart score, but this did not reach significance (OR (95% CI) for 10 mm Hg MAP increase was 1.14 (0.96–1.35, P = 0.132).

Table 4

Table 4

The regular logistic regression analysis that used Boezaart score ≥ 3 as the outcome confirmed the following findings: Side 2 was more likely to have an elevated surgical grade than side 1 at time zero for either group and side 2 was less likely to have an elevated surgical grade after time zero for the PEEP group but not the ZEEP group. However, the 3-way interaction term and the time zero by group interaction term were not significant (P = 0.992 and 0.760, respectively). Higher inspiratory pressure and MAP were associated with increased likelihood of bleeding. The OR (95% CI) for every 1 cm H2O increase in inspiratory pressure above 15cm H2O was 1.11 (1.02–1.20), P = 0.011) and the OR (95% CI) for 10 mm Hg MAP increase was 1.19 (1.00–1.40, P = 0.051).

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DISCUSSION

The bloodless surgical field is a major focus for perioperative teams performing FESS since compromise of the surgical field can lead to many untoward events. We sought to determine whether the addition of PEEP to the ventilator strategy of patients undergoing primary FESS would have a deleterious effect on surgical visualization as measured by a validated grading system (the Boezaart surgical score). We found that PEEP may not impact the surgical field at the levels we studied. However, if related to increased inspiratory pressures throughout the case, then it may impact bleeding scores.

The importance of the anesthetic plan (i.e., total IV anesthesia versus inhaled anesthesia),14,15 use of deliberate hypotension,16 surgical preparation,17 and the use of reverse Trendelenburg positioning18 have been demonstrated to influence surgical conditions during FESS. Our results shed light on the potential importance of ventilator strategy in FESS and they strengthen the concept that increased intrathoracic pressures may worsen surgical conditions. We found a significant association with worse surgical grades, irrespective of PEEP status, with increasing inspiratory pressure. This would be in line with previous work in similar patient populations.11 Atef and Fawaz19 examined the use of the laryngeal mask airway (LMA) during FESS and speculated that the improved surgical conditions with a LMA were a result of lower levels of circulating catecholamines when compared to cases managed with an ETT. It is possible that negative intrathoracic pressure from spontaneous respirations imparted the improvements seen with the LMA in this study, and not a catecholamine-driven mechanism.

Although low tidal volume strategies have been demonstrated to improve pulmonary outcomes in patients with acute respiratory distress syndrome or at high risk for pulmonary compromise,6 and though lung-protective strategies with lower tidal volumes and added PEEP have become more prevalent,20 the use of large tidal volumes (>10 mL/kg) by anesthesiologists may still be as high as 20% of all cases.21 Larger tidal volumes may be deliberately used in sinus endoscopy surgery to avoid atelectasis, especially if the anesthesiologist is attempting to avoid PEEP or to deliberately hyperventilate the patient to promote hypocarbia. Carbon dioxide levels have not been shown to impact surgical conditions,22 and our findings suggest this finding as well given the difference between groups’ EtCO2 measurements did not have an association with surgical conditions. Overall, it would seem that PEEP and normocapnea could be maintained safely so long as inspiratory pressure is minimized. To this end, it is possible that higher PEEP levels could be used, as long as inspiratory pressure was not elevated (well above 15 cm H2O).

The results presented herein must be tempered by the acknowledgement of their limitations. First, though these results approached statistical significance at many levels, the overall N was modest, and some of the significant predictors of surgical grade were not primary objects of this study. The effect of the inspiratory pressure on surgical field was not the primary investigated variable (at least not directly), and it is possible that multiple other factors have biased the result. Also, any grading of surgical field is subjective and since we did not use a second blinded rater, we were reliant upon the ratings of 1, albeit trained, observer. Calculations of blood loss cannot be considered completely rigorous since we used practitioners’ recorded estimated blood losses based on suction canister contents less the volume of irrigation used. The same may be true of arterial blood pressure measurements, as we did not use an arterial catheter for any of the participants. These factors may further diminish the reliability of our findings. Finally, we chose to control for various factors by studying a small team of 1 surgeon and 2 anesthesiologists at 1 major academic center. This allows for potentially decreased confounding variables but hampers generalizability to other centers and/or perioperative teams.

Optimization of surgical conditions for FESS is multifactorial and is within the purview of both otolaryngologists and anesthesiologists. Unfortunately, achieving the bloodless field is not without risk and is generally accomplished via interventions with significant impact on hemodynamic stability. It is clear that there is no 1 variable that can be used, avoided, or manipulated that will assure good surgical conditions, but determining which variables play a role is exceedingly important to minimize intraoperative risks from both poor conditions and overzealous use of controlled hypotension or local vasoconstrictive drugs.

In this study, we determined that the inclusion of PEEP had no independent effect on intraoperative conditions, but we were able to determine that higher inspiratory pressures had a direct effect on intraoperative conditions. Since inspiratory pressure is influenced by many variables controlled by the anesthesiologist (e.g., ventilator settings, ETT size, use of neuromuscular blocking drugs), we recommend a ventilator strategy to reduce inspiratory pressure until such time that independent variables can be studied further. Also, the well-established safe practice of ventilating patients’ lungs with lower tidal volumes and added PEEP should likely not be denied to these patients for fear of impaired surgical visualization in FESS unless proven otherwise. E

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DISCLOSURES

Name: Samuel DeMaria, MD.

Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.

Attestation: Samuel DeMaria has seen the original study data, reviewed the analysis of the data, approved the final manuscript, and is the author responsible for archiving the study files.

Conflicts of Interest: This author has no conflicts of interest to declare.

Name: Satish Govindaraj, MD.

Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.

Attestation: Satish Govindaraj has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Conflicts of Interest: This author has no conflicts of interest to declare.

Name: Alice Huang, BS.

Contribution: This author helped design the study, conduct the study, and write the manuscript.

Attestation: Alice Huang has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Conflicts of Interest: This author has no conflicts of interest to declare.

Name: Jaime Hyman, MD.

Contribution: This author helped design the study, conduct the study, and write the manuscript.

Attestation: Jaime Hyman has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Conflicts of Interest: This author has no conflicts of interest to declare.

Name: Patrick McCormick, MD, MEng.

Contribution: This author helped collect the data and analyze the data.

Attestation: Patrick McCormick approved the final manuscript.

Conflicts of Interest: This author has no conflicts of interest to declare.

Name: Hung Mo Lin, PhD.

Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.

Attestation: Hung Mo Lin has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Conflicts of Interest: This author has no conflicts of interest to declare.

Name: Adam Levine, MD.

Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.

Attestation: Adam Levine has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Conflicts of Interest: Adam Levine received honoraria from Mylan Pharmaceuticals and consulted for Mylan Pharmaceuticals.

This manuscript was handled by: Peter S. A. Glass, MB ChB, FFA.

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