High oxygen concentration exacerbates cardiopulmonary bypass-induced lung injury

Three Logistic Predictive Models for the Prediction of Mortality and Major Pulmonary Complications after Cardiac Surgery

Background and Objectives: Pulmonary complications are a leading cause of morbidity after cardiac surgery. The aim of this study was to develop models to predict postoperative lung dysfunction and mortality. Materials and Methods: This was a single-center, observational, retrospective study. We retrospectively analyzed the data of 11,285 adult patients who underwent all types of cardiac surgery from 2003 to 2015. We developed logistic predictive models for in-hospital mortality, postoperative pulmonary complications occurring in the intensive care unit, and postoperative non-invasive mechanical ventilation when clinically indicated. Results: In the "preoperative model" predictors for mortality were advanced age (p < 0.001), New York Heart Association (NYHA) class (p < 0.001) and emergent surgery (p = 0.036); predictors for non-invasive mechanical ventilation were advanced age (p < 0.001), low ejection fraction (p = 0.023), higher body mass index (p < 0.001) and preoperative renal failure (p = 0.043); predictors for postoperative pulmonary complications were preoperative chronic obstructive pulmonary disease (p = 0.007), preoperative kidney injury (p < 0.001) and NYHA class (p = 0.033). In the "surgery model" predictors for mortality were intraoperative inotropes (p = 0.003) and intraoperative intra-aortic balloon pump (p < 0.001), which also predicted the incidence of postoperative pulmonary complications. There were no specific variables in the surgery model predicting the use of non-invasive mechanical ventilation. In the "intensive care unit model", predictors for mortality were postoperative kidney injury (p < 0.001), tracheostomy (p < 0.001), inotropes (p = 0.029) and PaO2/FiO2 ratio at discharge (p = 0.028); predictors for non-invasive mechanical ventilation were kidney injury (p < 0.001), inotropes (p < 0.001), blood transfusions (p < 0.001) and PaO2/FiO2 ratio at the discharge (p < 0.001). Conclusions: In this retrospective study, we identified the preoperative, intraoperative and postoperative characteristics associated with mortality and complications following cardiac surgery.

Hyperoxemia During Cardiac Surgery Is Associated With Postoperative Pulmonary Complications

The use of hyperoxemia during cardiac surgery remains controversial. We hypothesized that intraoperative hyperoxemia during cardiac surgery is associated with an increased risk of postoperative pulmonary complications.

DESIGN

Retrospective cohort study.

SETTING

We analyzed intraoperative data from five hospitals within the Multicenter Perioperative Outcomes Group between January 1, 2014, and December 31, 2019. We assessed intraoperative oxygenation of adult patients undergoing cardiac surgery with cardiopulmonary bypass (CPB). Hyperoxemia pre and post CPB was quantified as the area under the curve (AUC) of Fio₂ above 0.21 in minutes when the corresponding peripheral oxygen saturation was greater than 92% measured by pulse oximetry. We quantified hyperoxemia during CPB as the AUC of Pao₂ greater than 200 mm Hg measured by arterial blood gas. We analyzed the association of hyperoxemia during all phases of cardiac surgery with the frequency of postoperative pulmonary complications within 30 days, including acute respiratory insufficiency or failure, acute respiratory distress syndrome, need for reintubation, and pneumonia.

PATIENTS

Twenty-one thousand six hundred thirty-two cardiac surgical patients.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

During 21,632 distinct cardiac surgery cases, 96.4% of patients spent at least 1 minute in hyperoxemia (99.1% pre-CPB, 98.5% intra-CPB, and 96.4% post-CPB). Increasing exposure to hyperoxemia was associated with an increased risk of postoperative pulmonary complications throughout three distinct surgical periods. During CPB, increasing exposure to hyperoxemia was associated with an increased odds of developing postoperative pulmonary complications (p < 0.001) in a linear manner. Hyperoxemia before CPB (p < 0.001) and after CPB (p = 0.02) were associated with increased odds of developing postoperative pulmonary complications in a U-shaped relationship.

CONCLUSIONS

Hyperoxemia occurs almost universally during cardiac surgery. Exposure to hyperoxemia assessed continuously as an AUC during the intraoperative period, but particularly during CPB, was associated with an increased incidence of postoperative pulmonary complications.

Year in Review 2021: Noteworthy Literature in Cardiothoracic Anesthesia

In 2021, progress in clinical science related to Cardiac Anesthesiology continued, but at a slower rate due to the ongoing pandemic and disruptions to clinical research. Most progress was incremental and addressed persistent questions related to our field. To identify articles for this review, we completed a structured review using our previously reported methods (1). Specifically, we used the search terms: "cardiac anesthesiology and outcomes" (n = 177), "cardiothoracic anesthesiology" (n = 34), "cardiac anesthesia," and "clinical outcomes" (n = 42) filtered on clinical trials and the year 2021 in PubMed. We also reviewed clinical trials from the most prominent clinical journals to identify additional studies for a narrative review. We then selected the most noteworthy publications for inclusion in this review and identified key themes.

Impact of partial pressure of oxygen trajectories on the incidence of acute kidney injury in patients undergoing cardiopulmonary bypass

BACKGROUND

To investigate the impact of the dynamic oxygenation status on the incidence of acute kidney injury (AKI) in patients undergoing cardiopulmonary bypass.

METHODS

This retrospective study was performed using data extracted from the Medical Information Mart for Intensive Care III database. A group-based trajectory approach was used to identify partial pressure of oxygen (PaO₂) trajectories using dynamic change in PaO₂ within 48 hours after intensive care unit admission.

RESULTS

In total, 5,824 patients were included. Four PaO₂ trajectories were identified: Trajectory 1 (Traj-1), hyperoxia and rapid decrease; Trajectory 2 (Traj-2), hyperoxia and rapid decrease similar to that of Traj-1; Trajectory 3 (Traj-3), normoxemia and rapid increase in PaO₂; and Trajectory 4 (Traj-4), hyperoxia and gradual decrease. Compared with the Traj-1 group, the Traj-3 group had a significantly lower initial Sequential Organ Failure Assessment score, similar vasopressor use rate, and a higher fraction of inspired oxygen. However, the risk of developing AKI was significantly higher in the Traj-3 [adjusted odds ratio (OR): 1.7, 95% confidence interval (CI): 1.1-2.7] and Traj-4 groups (OR: 1.9, 95% CI: 1.4-2.5) than in the Traj-1 group.

CONCLUSIONS

Patients with persistent hyperoxia had a higher incidence of AKI than those with transient hyperoxia. Further studies are required to determine potential underlying mechanisms.

Different strategies for mechanical VENTilation during CardioPulmonary Bypass (CPBVENT 2014): study protocol for a randomized controlled trial

Background

There is no consensus on which lung-protective strategies should be used in cardiac surgery patients. Sparse and small randomized clinical and animal trials suggest that maintaining mechanical ventilation during cardiopulmonary bypass is protective on the lungs. Unfortunately, such evidence is weak as it comes from surrogate and minor clinical endpoints mainly limited to elective coronary surgery. According to the available data in the academic literature, an unquestionable standardized strategy of lung protection during cardiopulmonary bypass cannot be recommended. The purpose of the CPBVENT study is to investigate the effectiveness of different strategies of mechanical ventilation during cardiopulmonary bypass on postoperative pulmonary function and complications.

Methods/design

The CPBVENT study is a single-blind, multicenter, randomized controlled trial. We are going to enroll 870 patients undergoing elective cardiac surgery with planned use of cardiopulmonary bypass. Patients will be randomized into three groups: (1) no mechanical ventilation during cardiopulmonary bypass, (2) continuous positive airway pressure of 5 cmH₂O during cardiopulmonary bypass, (3) respiratory rate of 5 acts/min with a tidal volume of 2–3 ml/Kg of ideal body weight and positive end-expiratory pressure of 3–5 cmH₂O during cardiopulmonary bypass. The primary endpoint will be the incidence of a PaO₂/FiO₂ ratio <200 until the time of discharge from the intensive care unit. The secondary endpoints will be the incidence of postoperative pulmonary complications and 30-day mortality. Patients will be followed-up for 12 months after the date of randomization.

Discussion

The CPBVENT trial will establish whether, and how, different ventilator strategies during cardiopulmonary bypass will have an impact on postoperative pulmonary complications and outcomes of patients undergoing cardiac surgery.

Trial registration

ClinicalTrials.gov, ID: NCT02090205. Registered on 8 March 2014.

Electronic supplementary material

The online version of this article (doi:10.1186/s13063-017-2008-2) contains supplementary material, which is available to authorized users.

Pulmonary Complications After Cardiac Surgery

Over the past two decades there has been a steady evolution in the practice of adult cardiac surgery with the introduction of “off-pump” surgery. However, respiratory complications remain a leading cause of postcardiac surgical morbidity and can prolong hospital stays and increase costs. The high incidence of pulmonary complications is in part due to the disruption of normal ventilatory function that is inherent to surgery in the thoracic region. Furthermore, patients undergoing such surgery often have underlying illnesses such as intrinsic lung disease (e.g., chronic obstructive pulmonary disease) and pulmonary dysfunction secondary to cardiac disease (e.g., congestive heart failure) that increase their susceptibility to postoperative respiratory problems. Given that many patients undergoing cardiac surgery are thus susceptiple to pulmonary complications, it is remarkable that more patients do not suffer from them during and after cardiac surgery. This is to a large degree because of advances in anesthetic, surgical and critical care that, for example, have reduced the physiological insults of surgery (e.g., better myocardial preservation techniques) and streamlined care in the immediate postoperative period (e.g., early extubation). Moreover, the development of minimally invasive surgery and nonbypass techniques are further evidence of the attempts at reducing the homeostatic disruptions of cardiac surgery. This review examines the available information on the incidences, consequences, and treatments of postcardiac surgery respiratory complications.

Adjusting Oxygen Fraction to Avoid Hyperoxemia during Cardiopulmonary Bypass

Although an adverse influence of hyperoxemia during cardiopulmonary bypass is well documented, there is a wide range of oxygen settings during cardiopulmonary bypass, based mostly on trial and error. The aim of this study was to determine the optimal inspired oxygen fraction during cardiopulmonary bypass. Ninety patients undergoing isolated coronary artery bypass operations were randomly allocated to one of 3 groups of 30 each. In group 1, cardiopulmonary bypass was started with an inspired oxygen fraction of 0.40, increased to 0.60 during rewarming. These settings were 0.40 and 0.50 in group 2, and 0.35 and 0.45 in group 3. Samples for blood gas analysis were collected at defined time periods during the operation. PaO2 was significantly higher in groups 1 and 2 compared to group 3. All patients in group 1 and 88% of patients in group 2 suffered at least one episode of hyperoxemia during cardiopulmonary bypass, compared to 30% of patients in group 3. The differences were significant, and we concluded that to avoid hyperoxemia, inspired oxygen fraction should be kept at 0.35 during cardiopulmonary bypass and increased to 0.45 during rewarming.

Cardiac surgery, a right target for hyperoxia?

In perioperative cardiac surgery period, supra-physiological arterial oxygen partial pressures is common practice, although there is no clear evidence of any benefit. Smit et al. have shown that a “conservative” approach did not improve hemodynamics, decrease oxidative stress or myocardial tissue damage, but was not associated with major deleterious event either. Here, we outline major oxygen friend or foes properties, which may partly explain the study results, and place the clinical trial from Smit et al. in a global context.

Cardiovascular effects of hyperoxia during and after cardiac surgery

During and after cardiac surgery with cardiopulmonary bypass, high concentrations of oxygen are routinely administered, with the intention of preventing cellular hypoxia. We systematically reviewed the literature addressing the effects of arterial hyperoxia. Extensive evidence from pre-clinical experiments and clinical studies in other patient groups suggests predominant harm, caused by oxidative stress, vasoconstriction, perfusion heterogeneity and myocardial injury. Whether these alterations are temporary and benign, or actually affect clinical outcome, remains to be demonstrated. In nine clinical cardiac surgical studies in low-risk patients, higher oxygen targets tended to compromise cardiovascular function, but did not affect clinical outcome. No data about potential beneficial effects of hyperoxia, such as reduction of gas micro-emboli or post-cardiac surgery infections, were reported. Current evidence is insufficient to specify optimal oxygen targets. Nevertheless, the safety of supraphysiological oxygen suppletion is unproven. Randomised studies with a variety of oxygen targets and inclusion of high-risk patients are needed to identify optimal oxygen targets during and after cardiac surgery.

Perioperative ventilatory strategies in cardiac surgery

Recent data promote the utilization of prophylactic protective ventilation even in patients without acute respiratory distress syndrome (ARDS), and especially after cardiac surgery. The implementation of specific perioperative ventilatory strategies in patients undergoing cardiac surgery can improve both respiratory and extra-pulmonary outcomes. Protective ventilation is not limited to tidal volume reduction. The major components of ventilatory management include assist-controlled mechanical ventilation with low tidal volumes (6-8 mL kg(-1) of predicted body weight) associated with higher positive end-expiratory pressure (PEEP), limitation of fraction of inspired oxygen (FiO2), ventilation maintenance during cardiopulmonary bypass, and finally recruitment maneuvers. In order for such strategies to be fully effective, they should be integrated into a multimodal approach beginning from the induction and continuing over the postoperative period.

Pulmonary complications of cardiopulmonary bypass

Pulmonary complications after the use of extracorporeal circulation are common, and they range from transient hypoxemia with altered gas exchange to acute respiratory distress syndrome (ARDS), with variable severity. Similar to other end-organ dysfunction after cardiac surgery with extracorporeal circulation, pulmonary complications are attributed to the inflammatory response, ischemia-reperfusion injury, and reactive oxygen species liberated as a result of cardiopulmonary bypass. Several factors common in cardiac surgery with extracorporeal circulation may worsen the risk of pulmonary complications including atelectasis, transfusion requirement, older age, heart failure, emergency surgery, and prolonged duration of bypass. There is no magic bullet to prevent or treat pulmonary complications, but supportive care with protective ventilation is important. Targets for the prevention of pulmonary complications include mechanical, surgical, and anesthetic interventions that aim to reduce the contact activation, systemic inflammatory response, leukocyte sequestration, and hemodilution associated with extracorporeal circulation.

Effect of intraoperative high inspired oxygen fraction on surgical site infection, postoperative nausea and vomiting, and pulmonary function: systematic review and meta-analysis of randomized controlled trials

BACKGROUND

Intraoperative high inspired oxygen fraction (FIO2) is thought to reduce the incidence of surgical site infection (SSI) and postoperative nausea and vomiting, and to promote postoperative atelectasis.

METHODS

The authors searched for randomized trials (till September 2012) comparing intraoperative high with normal FIO2 in adults undergoing surgery with general anesthesia and reporting on SSI, nausea or vomiting, or pulmonary outcomes.

RESULTS

The authors included 22 trials (7,001 patients) published in 26 reports. High FIO2 ranged from 80 to 100% (median, 80%); normal FIO2 ranged from 30 to 40% (median, 30%). In nine trials (5,103 patients, most received prophylactic antibiotics), the incidence of SSI decreased from 14.1% with normal FIO2 to 11.4% with high FIO2; risk ratio, 0.77 (95% CI, 0.59-1.00). After colorectal surgery, the incidence of SSI decreased from 19.3 to 15.2%; risk ratio, 0.78 (95% CI, 0.60-1.02). In 11 trials (2,293 patients), the incidence of nausea decreased from 24.8% with normal FIO2 to 19.5% with high FIO2; risk ratio, 0.79 (95% CI, 0.66-0.93). In patients receiving inhalational anesthetics without prophylactic antiemetics, high FIO2 provided a significant protective effect against both nausea and vomiting. Nine trials (3,698 patients) reported on pulmonary outcomes. The risk of atelectasis was not increased with high FIO2.

CONCLUSIONS

Intraoperative high FIO2 further decreases the risk of SSI in surgical patients receiving prophylactic antibiotics, has a weak beneficial effect on nausea, and does not increase the risk of postoperative atelectasis.

A novel measurement and delivery system for synchronizing oxygen gas flow with blood flow during cardiopulmonary bypass

Monitoring the blood pump and the oxygen gas flow meter are important maneuvers at the initiation of cardiopulmonary bypass (CPB). We present a novel system, designed to improve safety in the heart-lung machine by linking the control of blood flow and the oxygen gas flow meter. This system uses a mass flow controller to provide and control oxygen flow based on the ventilation-perfusion (V/Q) ratio, using the electronic signal of the blood flow. We tested the system, in vitro and in vivo, and examined the resulting level of blood oxygenation. When extracorporeal circulation was initiated, the oxygen flow was instantly linked to the circulating blood flow, providing an adequate V/Q ratio; the partial pressure of oxygen in the blood was maintained at a normal level. Although we have yet to confirm the safety of this system in clinical trials, the new safety assist device can automatically supply oxygen to the oxygenator at the beginning of CPB.

Pulmonary pathophysiology and lung mechanics in anesthesiology: a case-based overview

Anesthesia, surgical requirements, and patients' unique pathophysiology all combine to make the accumulated knowledge of respiratory physiology and lung mechanics vital in patient management. This article take a case-based approach to discuss how the complex interactions between anesthesia, surgery, and patient disease affect patient care with respect to pulmonary pathophysiology and clinical decision making. Two disparate scenarios are examined: a patient with chronic obstructive pulmonary disease undergoing a lung resection, and a patient with coronary artery disease undergoing cardiopulmonary bypass. The impacts of important concepts in pulmonary physiology and respiratory mechanics on clinical management decisions are discussed.

Pulmonary injury after cardiopulmonary bypass: beneficial effects of low-frequency mechanical ventilation

OBJECTIVE

Pulmonary dysfunction is a frequent postoperative complication after cardiac surgery with cardiopulmonary bypass, and atelectasis is thought to be one of the main causes. The aim of this study was to evaluate whether low-frequency ventilation and continuous positive airway pressure during cardiopulmonary bypass reduce postcardiopulmonary bypass lung injury.

METHODS

Eighteen Yorkshire pigs were subjected to 120 minutes of cardiopulmonary bypass (1 hour of cardioplegic arrest) followed by 90 minutes of recovery before being sacrificed. Six animals served as control with the endotracheal tube open to atmosphere during cardiopulmonary bypass. The remaining animals were divided into 2 groups of 6: One group received continuous positive airway pressure of 5 cm H(2)O, and one group received low-frequency ventilation (5/minutes) during cardiopulmonary bypass. Lung tissue biopsy and bronchoalveolar lavage samples were obtained before and 90 minutes after discontinuation of cardiopulmonary bypass for measurement of adenine nucleotide (adenosine-5'-triphosphate, adenosine diphosphate, adenosine monophosphate), lactate dehydrogenase, DNA levels, and histology. Hemodynamic data and arterial blood gases were also collected through the study.

RESULTS

The hemodynamic parameters were similar in the 3 groups. After cardiopulmonary bypass, the low-frequency ventilation group showed significantly better oxygen tension and alveolar arterial oxygen gradient, higher adenine nucleotide, lower lactate dehydrogenase levels, and reduced histologic damage in lung biopsy, as well as lower DNA levels in bronchoalveolar lavage compared with the control group. The continuous positive airway pressure group showed only significantly reduced lactate dehydrogenase levels compared with control.

CONCLUSION

Low-frequency ventilation during cardiopulmonary bypass in a pig experimental model reduces tissue metabolic and histologic damage in the lungs and is associated with improved postoperative gas exchange.

Oxygenation inhibits the physiological tissue-protecting mechanism and thereby exacerbates acute inflammatory lung injury

Acute respiratory distress syndrome (ARDS) usually requires symptomatic supportive therapy by intubation and mechanical ventilation with the supplemental use of high oxygen concentrations. Although oxygen therapy represents a life-saving measure, the recent discovery of a critical tissue-protecting mechanism predicts that administration of oxygen to ARDS patients with uncontrolled pulmonary inflammation also may have dangerous side effects. Oxygenation may weaken the local tissue hypoxia-driven and adenosine A2A receptor (A2AR)-mediated anti-inflammatory mechanism and thereby further exacerbate lung injury. Here we report experiments with wild-type and adenosine A2AR-deficient mice that confirm the predicted effects of oxygen. These results also suggest the possibility of iatrogenic exacerbation of acute lung injury upon oxygen administration due to the oxygenation-associated elimination of A2AR-mediated lung tissue-protecting pathway. We show that this potential complication of clinically widely used oxygenation procedures could be completely prevented by intratracheal injection of a selective A2AR agonist to compensate for the oxygenation-related loss of the lung tissue-protecting endogenous adenosine. The identification of a major iatrogenic complication of oxygen therapy in conditions of acute lung inflammation attracts attention to the need for clinical and epidemiological studies of ARDS patients who require oxygen therapy. It is proposed that oxygen therapy in patients with ARDS and other causes of lung inflammation should be combined with anti-inflammatory measures, e.g., with inhalative application of A2AR agonists. The reported observations may also answer the long-standing question as to why the lungs are the most susceptible to inflammatory injury and why lung failure usually precedes multiple organ failure.

The effect of high-frequency ventilation of the lungs on postbypass oxygenation: A comparison with other ventilation methods applied during cardiopulmonary bypass

OBJECTIVE

To compare the effect of high-frequency ventilation versus other ventilation methods applied during cardiopulmonary bypass on postbypass oxygenation.

DESIGN

Prospective, randomized study.

SETTING

University hospital.

PARTICIPANTS

Seventy-five patients undergoing coronary artery bypass graft surgery.

INTERVENTIONS

Patients were allocated to 5 equal groups of different ventilation methods during bypass. Groups 1 and 2 received high-frequency, low-volume ventilation with 100% and 21% oxygen, respectively. Groups 3 and 4 received 5 cm H(2)O of continuous positive airway pressure (CPAP) with either 100% or 21% oxygen. Patients from group 5 were disconnected from the ventilator during the bypass period.

MEASUREMENTS AND MAIN RESULTS

Spirometry data, blood gas analysis, oxygen saturation as measured by pulse oximetry, and end-tidal carbon dioxide were recorded 5 minutes before chest opening, 5 minutes before bypass, 5 minutes after bypass, 5 minutes after chest closure and 6, 12, 18, and 24 hours after surgery. There were no differences in compliance and mean airway pressures. Alveolar-to-arterial oxygen gradients increased, and PaO(2) decreased significantly (p < 0.05) in all groups 5 minutes after bypass and this trend continued in the postoperative period. Patients from group 3 had higher PaO(2) and lower alveolar-to-arterial oxygen gradients, 5 minutes after weaning from bypass (p < 0.05). Extubation times were similar in all groups.

CONCLUSIONS

The alveolar-arterial oxygen gradient was lower, and the PaO(2) was higher 5 minutes after bypass in patients receiving CPAP (100% O(2)) as compared with those ventilated with high-frequency ventilation.

Effects of off-pump coronary surgery on the mechanics of the respiratory system, lung, and chest wall: Comparison with extracorporeal circulation

OBJECTIVE

To compare the effects of cardiac surgery with and without extracorporeal circulation on the mechanics of the respiratory system, lung, and chest wall. We also determined the time course of those effects.

DESIGN

Prospective, controlled study.

SETTING

An eight-bed, cardiac-surgical intensive care unit at a university hospital.

PATIENTS

Two groups of patients scheduled for elective coronary bypass surgery were studied: ten patients with extracorporeal circulation and 13 patients without extracorporeal circulation.

INTERVENTIONS

Measurement of esophageal pressure after insertion of an esophageal balloon catheter to separate respiratory system mechanics into lung and chest wall components. Measurements were performed preoperatively after induction of anesthesia (control), immediately postoperatively at arrival in the intensive care unit (time 1), and after 3 hrs (time 2). In 12 of the 23 patients, measurements were also performed 6 hrs postoperatively (time 3).

MEASUREMENTS AND MAIN RESULTS

No significant differences concerning demographics or surgical procedure were noticed between the two groups. Respiratory system, chest wall, and lung mechanics were obtained using the technique of rapid airway occlusion during constant-flow inflation. In both the group with and without extracorporeal circulation there was a significant increase in static and dynamic elastance of the respiratory system and lung at times 1 and 2, which tended to decrease again at time 3; chest wall elastance significantly increased at times 2 and 3 in the group without extracorporeal circulation, whereas the increase in chest wall elastance in the group with extracorporeal circulation occurred earlier (also at time 1). Additional resistance of the respiratory system and lung remained unchanged; chest wall resistance, however, significantly increased in both groups. Work of breathing significantly increased in both groups at times 1 and 2. There was a significant reduction in the Pao2/Fio2 ratio in both groups at times 2 and 3. No significant differences between the groups at any moment were noticed.

CONCLUSIONS

Coronary bypass surgery with and without extracorporeal circulation results in dramatic impairment of respiratory system mechanics. Based on respiratory system mechanics, early extubation after coronary artery bypass grafting should be performed with caution, no matter whether the off-pump or cardiopulmonary bypass technique is used.