Factors Influencing Pain Scores and Opioid Demand after Robotically Assisted Cardiac Surgery

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Objective.Introduction of minimally invasive cardiac surgery anticipated the reduction in postoperative pain but little quantitative data are available on this efect.Tis study investigated factors infuencing pain scores and opioid demand after robotically assisted cardiac procedures.Methods.Using data derived from prospective robotic cardiac surgery and anesthesia databases, we analyzed 75 patients undergoing robotic cardiac surgery between July 2021 and December 2022.Study endpoints were mean cumulative pain scores measured on a 0-10 scale and opioid use on postoperative days (PODs) 1 to 4. Pain scores and oral morphine equivalent (OME) were correlated with perioperative variables.Results.Postoperatively, 39/75 (52%) of patients were extubated in the operating room (OR) and 34/75 (45.3%) were extubated within 24 hours of surgery.Mean pain scores declined from 5.8 (SD 1.5) on POD 1 to 3.8 (SD 1.6) on POD 4 and OME fell from 34.9 (SD 29) mg to 8.2 (SD 11.8) mg.OME use correlated signifcantly with pain scores (p < 0.01).Higher pain scores on postoperative day (POD) 1 were associated with diabetes (p � 0.006), tobacco use (p � 0.006), and extubation in the OR (p � 0.017).Opioid utilization was higher in younger patients (p < 0.001), heavier patients (p � 0.033), active tobacco users (p � 0.01), longer procedure times (p � 0.002), and those extubated in the OR (p < 0.001).Conclusion.Pain and opioid consumption after robotically assisted cardiac surgery are moderate but decline steadily within the frst four postoperative days.Tobacco use and extubation in the OR were associated with increased pain and opioid consumption.

Background
Compared to its counterparts in abdominal and thoracic surgery disciplines, adoption of robotic surgery for cardiac procedures has been relatively slow to gain popularity.A minimally invasive surgical approach is often associated with less pain postoperatively, improved respiratory function, shorter hospitalization, and faster recovery.However, few studies have demonstrated clear and consistent beneft of robotic cardiac surgery compared to traditional sternotomy concerning pain [1].
Te course of pain scores has been described for classic heart surgery but not for robotic cardiac surgery [2].Known factors that are associated with increased pain after traditional cardiac surgery are female gender, young age, and use of cardiopulmonary bypass [3,4].Data on factors that infuence pain after robotic cardiac surgery are lacking.Tis study therefore describes trends in postoperative pain scores and opioid utilization across various robotic cardiac surgery procedures and analyzes perioperative factors contributing to pain.

Methods
Tis study was approved by the Institutional Review Board (IRB #21080141).

Patients and Data Collection.
Data for patients undergoing robotic-assisted cardiac surgery were derived from prospectively maintained surgery and anesthesia databases.Between July 2021 and December 2022, 76 patients underwent robotic-assisted cardiac surgery.One patient underwent robotic-assisted epicardial lead placement and was excluded in this analysis.Te remaining procedures included robotic mitral valve repair (MVr, n � 32), minimally invasive direct coronary artery bypass (MIDCAB, n � 9), and totally endoscopic coronary bypass grafting (TECAB, n � 18).Of the 75 included patients, no patients had chronic preoperative pain.Baseline patient characteristics and intraoperative and postoperative data are reported in Table 1.

Anesthetic Considerations.
A standard analgesic protocol was utilized for patients undergoing robotic cardiac surgery consisting of (1) preoperative thoracic paravertebral block at T4 with 20 mL ropivacaine 0.2%; (2) acetaminophen scheduled every 6 hours (intravenous, then oral when able); (3) intraoperative analgesia with low-dose fentanyl; and (4) postoperative analgesia with intravenous hydromorphone and oral oxycodone.Ketorolac may be used starting on postoperative day #1 in case of normal kidney function.Patient-controlled analgesia (PCA) was not used.
Endpoints of this retrospective study were mean cumulative pain scores measured on a 0-10 scale and opioid use on postoperative days (PODs) 1 to 4. Opioid utilization during this time was converted to oral morphine equivalents (OMEs) [5].All patient-reported pain scores entered into the electronic medical record system were collected.

Statistical Analysis.
Statistical analysis was conducted using SPSS Statistics, version 28.0.1.1.Patient-reported pain scores and OMEs were correlated with preoperative and intraoperative variables.Categorical variables are shown as absolute values and percentages and continuous variables are shown as means and standard deviations (SDs).Correlations were calculated using Pearson's correlation coefcient.Diferences between groups were calculated using Student's t-test and one-way ANOVA.Time-related measures of pain scores and OME were assessed using ANOVA for repeated measures.p value < 0.05 was considered statistically signifcant.
Mean pain scores declined from 5.8 (SD 1.5) on POD 1 to 3.8 (SD 1.6) on POD 4 (p < 0.001), Figure 1(a).OME fell from 34.9 (SD 29) mg on the frst postoperative day to 8.2 (SD 11.8) mg on the fourth postoperative day, Figure 1(b).As shown in Figure 2, the highest pain scores on POD 1 were seen in robotically assisted MIDCAB followed by robotic secundum ASD and myxoma resection.Te lowest pain score was noted for robotic mitral valve repair.Opioid requirements were not signifcantly diferent among procedure types, Figure 2.
Mean pain scores on POD 1 were higher in patients with diabetes (p � 0.005), in those with tobacco use (p � 0.006), and those extubated in the OR (p � 0.017), Table 3. Pain score on the frst postoperative day was associated with higher pre-op hemoglobin A1c levels (p � 0.031) and was negatively correlated with shorter myocardial ischemia time (p � 0.044), Figure 3.
Opioid utilization showed a strong negative correlation with age (p < 0.001), while OME demand was higher in heavier patients (p � 0.033) and current smokers (p � 0.01), Figure 4 and Table 3.A longer procedure time and extubation in the operating room were intraoperative factors associated with increased opioid demand (p � 0.002 and p < 0.001, respectively), Table 4.
Te strong positive correlation of pain score on POD 1 with opioid consumption (p < 0.001) is depicted in Figure 5. Factors including gender, New York Heart Failure class, lung disease, preoperative creatinine, diabetes mellitus, hypertension, mini-thoracotomy approach, conversion to sternotomy, and total incision length did not infuence pain scores or OME use.Additionally, preoperative lab values of white blood cell count, albumin level, and platelet count did not impact pain scores or OME utilization.
No signifciant correlation was found between mean pain scores and OMEs with postoperative hospital length of stay (p value 0.294 and 0.096, respectively).

. Discussion
Te present study demonstrates that mean postoperative pain scores after robotically assisted cardiac surgery reach a moderate level and decrease by 35% on the fourth postoperative day.Opioid demand on postoperative day one reaches an average of 35 mg and decreases by 77% on postoperative day four.Several perioperative factors such as tobacco use and extubation in the operating room strongly infuence both the level of pain and opioid demand in this study.
Adequate pain control is emphasized in our current practice.Pain studies often report pain peaking at POD 2 and associated with a variety of baseline characteristics such as gender, body mass index, and smoking [6,7].Previous data suggest that improvements in postoperative outcomes can be further enhanced by preoperative optimization of physical rehabilitation, smoking cessation, and comorbidity management.

Pain Scores.
Generally, the course of pain reduction within the frst four postoperative days can be regarded as relatively slow.Andrade and coworkers described a decrease 2 Journal of Cardiac Surgery of postoperative pain in cardiac surgery performed through sternotomy from 2.6 to 1.3 on a one to fve level scale during the frst four postoperative days, a 50% reduction [2].Pain in this analysis was in most instances described as moderate in the initial few postoperative days.We found that mean pain scores declined from 5.8 on POD 0 to 3.8 on POD 4 (p < 0.001), a 35% reduction.Tis diference between Andrade et al.'s results and ours could be due to the higher pain associated with thoracotomy incisions.Specifcally in this study, the patients undergoing robotic-assisted MIDCAB reported statistically signifcant higher pain scores.Te thoracic mini-incision in the MIDCAB procedure was described as specifcally painful since the introduction of the procedure in the mid-1990s [7].Te damage or trauma to the intercostal nerve from an incision involving rib spreading can be particularly painful [3].Walther et al. describe that the pain from a lateral thoracotomy is signifcant in the postoperative period; it typically peaks around day 3 without signifcant diferences in pain outcomes compared to traditional sternotomy by POD 7 [7].Although robotic-assisted mitral valve surgery also involves a mini-thoracotomy, the combination of a mini-thoracotomy with internal mammary artery harvesting may contribute to increased infammation and pain Journal of Cardiac Surgery noted in the MIDCAB procedure [8].Notably, in our practice, intercostal sutures are placed for all minithoracotomies to help alleviate early postoperative pain and intercostal nerve blocks are performed [9].Patients with elevated hemoglobin A1c levels more often underwent robotic-assisted MIDCAB rather than TECAB in this study.Tis may explain the higher level of pain in found in this diabetic patient population.Additionally, the association of shorter myocardial ischemic times with higher pain scores may be due to the predominance of MIDCABs, in which the procedure is carried out on the beating heart.
Te enhanced recovery pathway aims to achieve extubation within the frst six hours.Studies comparing extubation in the operating room after cardiac surgery compared to the intensive care unit have shown higher reintubation rates and bleeding but lower resource utilization [10,11].When comparing pain intensity and analgesia use, Lin et al. found no signifcant diferences between early extubation in sternotomy and thoracotomy incisions in cardiac surgery [12].However, in this study, patients who were able to be extubated in the operating room at the completion of the procedure reported higher pain scores on POD 1 than those that were extubated in the intensive care unit.Extubation in the OR being associated with higher opioid requirements is most likely a result of earlier withdrawal of sedation and anesthesia.Additionally, after early extubation, patients may have also been able to express their pain to nursing staf and physicians, leading to higher opioid administration.
Multiple studies have demonstrated signifcant pain associated with coughing, movements, turning around, getting up from bed, and deep breathing [3].Our results support higher pain and opioid use in patients with recent or active smoking history, which causes increased coughing, atelectasis, and mucus production.
Interestingly, conversion to sternotomy was not associated with increased pain or narcotic use.Tis may be explained by the known fact that sternotomy is less painful than lateral thoracotomy and the added trauma may not have played a major role.As there was not even a trend towards increased pain with conversion, it will have to be seen if larger patient numbers can demonstrate an infuence.

Opioid Demand. OME requirements in various robotic
procedures did not difer signifcantly but followed a similar pattern as pain scores.Tere was a very strong negative correlation between narcotic pain medication demand and 4 Journal of Cardiac Surgery age.Younger patients required morphine equivalents in the 40 to 60 mg range, whereas older patients asked for less than 20 mg on postoperative day one.Data in the literature show that age is a signifcant factor infuencing the perception of pain after heart surgery [3,13,14].
It has been previously reported that women report a higher pain intensity than men after heart surgery [13,15,16].In our analysis, interestingly, men had a higher opioid demand than women by approximately 13 mg, but the diference did not reach statistical signifcance.Although previous studies have found higher pain with increasing body mass index [17], our study did not demonstrate a statistical signifcance between these factors.However, we did fnd OME requirements signifcantly increased with body weight.Tis can probably be explained by the simple fact that pain medication dosage is calculated according to body weight.
We found a very strong correlation between procedure time and opioid demand.Tis is in line with data presented by Mazzef and Khelemsky which demonstrate an association of postoperative pain and duration of surgery [18].

Factors Infuencing Both Pain Scores and Opioid
Demand.Te two factors that strongly infuenced both the level of pain on the frst postoperative day and the corresponding demand for narcotic pain medication were extubation in the operating room and a history of smoking.Patients extubated in the OR had an average pain level of 6 as compared to a level of 5 in patients extubated in the ICU; additionally, the early extubation group required more than twice the dose of morphine equivalents.Current smokers also showed POD 1 pain scores of 6 as compared to a score of  Extubation in the operating room may not allow for enough time to fully recruit the lungs after single lung ventilation.Tese patients, especially if they are smokers, may exhibit more atelectasis and frequent coughing, leading to higher pain levels postoperatively.A prospective study in 705 patients undergoing traditional cardiac surgery clearly demonstrated that the most severe postoperative pain was associated with coughing and movement [19].
Te present study is one of the few in the literature demonstrating this pain diference in early extubation and smokers.Despite our eforts with enhanced recovery protocols and decreasing length of stay with early extubation, this fnding is important in managing postoperative pain and pulmonary rehabilitation.Perhaps, patients with a smoking history would not beneft from early extubation, but further investigation is needed.Although there is increased pain in the patients who undergo extubation in the operating room, it is worth considering that this avoids patients remembering waking up with an endotracheal tube, as this can be considered an uncomfortable experience.

Importance of Appropriate Pain Management after
Cardiac Surgery.It has also been demonstrated that initial postoperative pain severity has been associated with persistent pain [8,15], which stresses the importance in management of acute pain in the frst few days after surgery.A meta-analysis of 23 studies found that up to 37% reported persistent postoperative pain at 6 months after cardiac surgery and up to 17% at 2 years [4].Furthermore, new persistent pain and opioid use after cardiac surgery were found to be dependent on opioid prescription size at the time of discharge [20].
Data from the Mayo Clinic did not demonstrate a signifcant diference in postoperative pain between traditional sternotomy and robotic approach for mitral valve repair but found earlier return to work and improved quality of life [1].Less chest pain frequency was reported within the frst year using surveys, but the diference was indistinguishable after 1 year.
Te national attention to the opioid epidemic has highlighted the importance of pain management in surgical practice.Holst et al. demonstrated signifcant variation in opioid prescribing guidelines, thus urging the importance of standardization of pain regimen [21].When compared to thoracotomy, patients after robotic-assisted cardiac surgery were less likely to require opioid reflls [21].However, pain diferences were not signifcantly evident between the robotic-assisted method and partial sternotomy and sternotomy.Te pain management in our institution encourages less use of narcotic pain medication and increased prescription of alternatives such as acetaminophen, ketorolac, ibuprofen, and gabapentin.Te 77% decrease in opioid consumption on the fourth postoperative day is an encouraging fnding in this study.By comparison, Balkhy and coworkers found 80% free of opioids within one week after robotic cardiac surgery [22].

Study Limitations.
Tis is a small retrospective study in a relatively inhomogeneous group of patients undergoing the spectrum of robotic cardiac surgery.Although procedure type did not infuence pain scores and opioid utilization, comparing pain across multiple procedures is a limitation of this study.Due to the size of the study population, we carried out univariate analysis and multivariate analysis was not performed but will be carried out in future studies.Propensity-matched comparative studies and randomized comparisons with cardiac surgery through sternotomy are warranted.We collected pain score data points from electronic medical records, which were recorded at the discretion of nursing staf.Nursing staf are instructed to record pain scores at every shift but there is potential bias where patients with more pain were more frequently asked to rate their pain score.Study limitations also include the inability to assess psychosocial aspects that can contribute to pain.Preoperative expectation of pain is notably an important determinant of postoperative pain perception but was not a focus of this study.

Conclusion
As robotic-assisted techniques become more common in cardiac surgery, it is important to understand that this minimally invasive alternative does not necessarily translate into minimal pain as patients may expect.In this study, pain was found to be moderate after robotic-assisted cardiac surgery but signifcantly declined within the frst 4 postoperative days.Pain scores and opioid consumption correlated.Higher pain scores were found in the MIDCAB group compared to other procedures.Factors infuencing both higher pain scores and opioid consumption were smoking and extubation in the operating room.Additionally, young patients and longer procedure times required more opioids.As we delineate factors infuencing pain, patient perception and counseling can be optimized in anticipation of robotic cardiac procedures.

Figure 2 :Figure 1 :
Figure 2: Mean pain score and opioid demand by procedure type.

Figure 3 :
Figure 3: Mean pain score on postoperative day (POD) 1 based on preoperative hemoglobin A1c level and myocardial ischemic time.

Figure 4 :
Figure 4: Opioid demand on postoperative day (POD) 1 based on factors of age, weight, and procedure time.

Figure 5 :
Figure 5: Correlation of opioid utilization and pain score on postoperative day (POD) 1.

Table 3 :
Univariate correlation of preoperative patient characteristics with pain scores and opioid demand on postoperative day 1.

Table 4 :
Univariate correlation of intraoperative variables with pain scores and opioid demand on postoperative day 1.