the most recent national hospital discharge survey reported over 400,000 patients underwent cardiac surgery in the united states in 2010 . in cardiac patients , respiratory problems following extubation are the most significant cause of postoperative comorbidity , despite considerable improvement in postoperative care and monitoring . in a recent study of 7,105 cardiac patients , 216 were readmitted to the intensive care unit ( icu ) due to respiratory complications . this represented 39% of all icu readmissions in that study . in recent study of the 7.8% of patients readmitted to the icu following cardiac surgery , respiratory failure was reported to be the cause for 39% of them [ 2 , 4 ] . current trends in cardiac anesthesia recommend early extubation , but that is not always feasible and prolonged endotracheal intubation and mechanical ventilation place cardiac patients at an increased risk of respiratory complications . previous data show that the airway can constrict immediately after extubation as a result of mechanical irritation to the upper airway and prolonged supine position . cardiopulmonary bypass patients are at an even greater risk since they can develop temporary pulmonary ischemia that decreases the production of surfactant and stiffens respiratory tissue . timely detection and prevention of respiratory compromise in this population has become a priority . despite the inherent dangers caused by respiratory complications , current in - hospital monitoring is still lacking . a non - invasive technology that could reliably measure respiratory function after extubation and give early warning signs of respiratory compromise before harmful events occurred could markedly improve patient care and safety . subjective clinical assessment and respiratory rate ( rr ) monitoring are both inconsistent and unreliable indicators of respiratory sufficiency , and current clinical practice relies heavily on continuous pulse oximetry or capnography . though used in almost every clinical setting , pulse oximetry can be unreliable and studies have shown nearly two - thirds of alarms triggered by the oximeter to be false [ 6 , 7 ] . furthermore , oxygen saturation is a lagging indicator , as it decreases only after a significant drop in ventilatory adequacy and readings are often confounded by the use of supplemental oxygen leading to possible delay in interventions and risk to patient safety . one of the difficulties with capnography in the non - intubated patients is patient compliance and maintaining proper placement of the sampling cannula . both oximetry and capnography are indirect measurements of respiratory performance , as they do not directly monitor the adequacy of respiration , but are rather surrogate measures of respiratory status . while a skilled clinician may measure a patient s rr by counting chest wall movements , an obstructed airway or chest muscle contraction could result in this same movement rather than true respiratory effort . in the obese population , observing a chest rise and fall is increasingly challenging and even if one can accurately count the rr , one would not be able to easily distinguish between adequate and inadequate breathing . the currently available measures are limited and often do not provide enough clinically relevant information to trigger healthcare provider response and early intervention to prevent respiratory compromise . recent development of a non - invasive respiratory volume monitor ( rvm ) that provides quantitative , continuous , real - time measurements of mv , tv and rr in non - intubated patients has addressed a number of these challenges . with the recommended electrode placement and calibration algorithms , strong correlations ( 0.96  0.16 , mean  95% ci for regular and erratic breathing ) between rvm and spirometric measurements the rvm can quantify ventilation in non - intubated patients and help identify patients at risk for respiratory complications following cardiac surgery and in other clinical settings [ 11 - 15 ] . the goal of this study was to demonstrate that an alternate electrode placement , to accommodate the sternal dressing , would not affect the rvm correlations to the ventilator and to show the utility of using rvm to monitor mv , tv , and rr in cardiac surgery patients after median sternotomy . it was hypothesized rvm would provide accurate , continuous measurements of mv , tv , and rr in this cohort before and after extubation . fourteen cardiothoracic patients ( four females and 10 males , average age 69  11.8 years , average bmi 29  6.0 kg / m , eight coronary artery bypasses , four valve replacements , and two myomectomies , table 1 ) were enrolled in an irb approved protocol for monitoring respiratory status in the tufts cardiothoracic unit ( ctu ) . respiratory traces were recorded from thoracic electrodes using an impedance - based rvm system ( exspiron , respiratory motion , inc . , all cardiothoracic procedures were performed under general anesthesia and all patients arrived in the ctu intubated and sedated . upon arrival in the ctu , the exspiron electrode padset was positioned to the right of the sternal dressing , approximately at the right mid - clavicular line , instead of the standard midsternal placement . the device was calibrated while the patient was on the ventilator ( puritan - bennett 840 , covidien , mansfield , ma ) during synchronous intermittent mandatory ventilation mode ( simv ) with setting at 6 - 8 ml / kg ibw as determined by the anesthesiologist . respiratory data ( mv , tv and rr ) were acquired only every 15 s from the ventilator , as the ventilator did not support continuous respiratory trace streaming . rvm - based mv , tv , and rr were calculated from continuous 30-s segments . rvm data were recorded continuously from ctu arrival until 24 h after extubation or ctu discharge , whichever occurred first . arterial blood gases were collected before extubation and again approximately 1 - 2 h after extubation in a subset of patients ( n = 12 ) as deemed necessary by the surgical care team . rvm traces were continuously recorded in the ctu for 27  3.7 h ( 6 - 49 h ) . after extubation , patients remained in the ctu and were monitored by the rvm for another 18  2.5 h ( 2 - 29 h ) . twelve of 14 of the patients had ventilator reading for analysis . during mechanical ventilation , tvs measured by the rvm strongly correlated with tvs reported by the ventilator ( r = 0.97 ) . this correlation was maintained during both mandatory ventilation ( simv ) and during spontaneous breathing with pressure support ( ps ) modes . patient average error in tv measurements between rvm and the ventilator was -2.6% , the average precision was 23.9% and average accuracy was 26.2% . an example of simultaneous rvm and ventilator recordings for 30 min is shown in figure 1 . during this time , the ventilator measured an average mv of 6.3 l / min , a tv of 520 ml , and a rate of 12 breaths / min , while rvm measured an average mv of 5.9 l / min ( 7% error ) , an average tv of 490 ml ( 6% error ) , and an average rr of 12.2 breaths / min ( 3% error ) . measured mv , tv , and rr over a 30-min period using rvm ( red ) and the puritan bennett ventilator in simv mode ( black ) for one example patient . average errors between rvm and the ventilator for mv , tv , and rr are less than 10% .   before and after extubation , the rvm recorded respiratory traces continuously , in real - time , and captured a distinct pattern of post - extubation respiratory variation . based on body surface area , the average predicted mv was calculated to be 7.6  0.6 l / min for the 14 patients . during 2 min of breathing on simv mode on the ventilator , the average baseline ventilation was slightly above the predicted at 8.2  0.4 l / min ( 1095% of predicted ) . analysis of the exspiron data from the 14 studied patients showed mv was significantly reduced 30 min after extubation to 779% of baseline ( 8.2  0.4 to 6.4  0.9 l / min , p < 0.05 ) and tv was significantly reduced to 719% of baseline ( 520  40 to 370  40 ml , p < 0.01 ) relative to the pre - extubation baseline mv ( table 2 , fig . mv began to increase and by 60 min after extubation , average mv and tv had returned to baseline levels ( mv : 11816% of baseline ( 9.6  1.2 l / min ) ; tv : 10516% of baseline ( 490  50 ml ) ; p > 0.4 ) . thirty minutes after extubation , rr was 1138% of baseline ( 17.1  1.5 to 17.5  1.0 breaths / min , p = 0.4 ) . it is important to note that rr is the only direct and continuous respiratory parameter currently monitored after extubation . sao2 monitoring was recorded every hour by the nursing staff and revealed no significant drops in blood oxygen saturation after extubation ( fig . ( a ) mv , tv , and rr as percent of baseline before and after extubation . a nadir in ventilation occurs 30 min after extubation with mv significantly reduced to 779% of baseline ( * p < 0.05 ) and tv significantly reduced to 719% of baseline ( * p < 0.01 ) , while rr is not significantly different at 1138% of baseline . sixty minutes after extubation , mv has returned to 11816% of baseline and tv has returned to 10516% of baseline . ( b ) absolute values of mv , tv , and rr measured before and after extubation . mv falls significantly from 8.2  0.4 l / min at baseline to 6.4  0.9 l / min 30 min after extubation and tv was significantly reduced from 520  40 ml at baseline to 370  40 ml after extubation . the minimum spo2 value recorded at each time point among all subjects is shown as a blood gas measurements were taken on average 56  25 min before extubation and 106  27 min after extubation . spo2 remained unchanged from before to after extubation ( 98  0.4 vs. 98  0.8 , p = 0.21 , table 3 ) . there was a slight significant increase in bicarbonate from before to after extubation , but was of no clinical importance ( 21.3  0.5 vs. 22.3  0.4 , p = 0.04 ) . example respiratory traces from a representative patient ( 60-year - old male , bmi 29 ) . as the patient awakes from anesthesia and progresses toward extubation , faster and shallower spontaneous breaths predominate with an occasional ventilator breath ( b ) . just after extubation , the patient s breathing pattern is more irregular ( c , d ) and tvs and mvs decrease ( d , e ) . tv and mv measurements increase over time after successful extubation ( f , g , h ) .   while intubated , rvm - based mv , tv and rr correlated well with ctu ventilator measurements in patients after median sternotomy . after extubation , rvm continuously reported mv , tv and rr , providing a continuous , non - invasive , quantitative assessment of respiratory competence . a pattern of mv and tv decrease during the first 30 min after extubation was noted in all patients . subsequent recovery to pre - extubation levels within 1 h as confirmed by paco2 from arterial blood gas was noted in all patients . rvm showed distinct changes in mv and tv after extubation , while rr showed no significant variations . although no patients in this study required reintubation , based on data from other studies collected after extubation in premature infants , we propose that patients who continue to have a decrease in mv with no rise to baseline within 1 h after extubation will require intervention such as non - invasive ventilation or reintubation . further studies to evaluate the use of rvm in postoperative cardiac surgical patients for early detection of respiratory failure for development of reintubation protocols are ongoing . this study demonstrated that , while intubated , rvm - based mv , tv and rr correlated well with ventilator measurements ( r = 0.97 ) . in addition , the change in the padset placement to accommodate the sternal wound and dressing did not affect the device measurements . this confirms that the rvm can provide accurate measurements of mv , tv and rr in patient s post - median sternotomy using the modified positioning of the electrode padset . after extubation , rvm continuously reported mv , tv and rr , providing a real - time , non - invasive , quantitative assessment of respiratory competence . a pattern of mv and tv decrease during the first 30 min after extubation subsequent recovery to pre - extubation levels within 1 h as confirmed by paco2 from arterial blood gas was noted in all patients . this shows that rvm is capable of providing patterns and trends in ventilation status not available to healthcare providers with other technology . respiratory complications are one of the leading causes of readmission to the icu for cardiac surgery patients . this study shows the benefits of this new technology and its application in the ctu and possibly other locations . pulse oximetry is the leading technology used to assess respiratory status in non - intubated patients ; however , spo2 levels do not begin to decline until well after respiratory decompensation has begun and is known for being prone to inaccuracies [ 13 , 17 ] . intermittent blood gas measurements were taken intermittently and on average over 1 h after extubation , well after rvm confirmed a recovery in mv . rvm showed distinct changes in mv and tv after extubation , while rr , generally measured with cardiac monitoring , showed no significant variations . the time it takes a patient to return to baseline mv is inversely proportional to successful extubation , therefore demonstrating the importance of obtaining mv measurements . additionally , determination of mv after extubation using rvm could potentially aid in quicker assessment of the necessity and timing of reintubation , or provide an early indication of when institute non - invasive ventilation ( niv ) , in the form of continuous positive airway pressure or biphasic positive airway pressure . although no patients in this study required reintubation , based on data from other studies collected after extubation in premature infants , it is hypothesized that patients who continue to have a decrease in mv with no rise to baseline within 1 h after extubation will require intervention such as non - invasive ventilation or reintubation . recently , the american society of anesthesiology has voiced the opinion that quantitative monitoring of respiratory status is superior over qualitative methods . qualitative clinical signs such as chest excursion and auscultation of breath sounds are useful , but they can only be obtained periodically and are often inaccurate . carbon dioxide monitoring is also a qualitative sign of respiration in non - intubated patients and its accuracy is dependent on many factors . however , non - invasive mv and tv have been previously unavailable in non - intubated patients . the rvm permits a more comprehensive , quantitative assessment of respiratory status . with this functionality , rvm - derived respiratory data can bring forth reproducible and comparable data across patients over a wide range of breathing patterns . this can promote better evaluation of respiratory status in the ctu and other healthcare settings . while intubated , rvm - based mv , tv and rr correlated well with ctu ventilator measurements in patients after median sternotomy . after extubation , rvm continuously reported mv , tv and rr , providing a continuous , non - invasive , quantitative assessment of respiratory competence . a pattern of mv and tv decrease during the first 30 min after extubation was noted in all patients . subsequent recovery to pre - extubation levels within 1 h as confirmed by paco2 from arterial blood gas was noted in all patients . rvm showed distinct changes in mv and tv after extubation , while rr showed no significant variations . although no patients in this study required reintubation , based on data from other studies collected after extubation in premature infants , we propose that patients who continue to have a decrease in mv with no rise to baseline within 1 h after extubation will require intervention such as non - invasive ventilation or reintubation . further studies to evaluate the use of rvm in postoperative cardiac surgical patients for early detection of respiratory failure for development of reintubation protocols are ongoing . this study demonstrated that , while intubated , rvm - based mv , tv and rr correlated well with ventilator measurements ( r = 0.97 ) . in addition , the change in the padset placement to accommodate the sternal wound and dressing did not affect the device measurements . this confirms that the rvm can provide accurate measurements of mv , tv and rr in patient s post - median sternotomy using the modified positioning of the electrode padset . after extubation , rvm continuously reported mv , tv and rr , providing a real - time , non - invasive , quantitative assessment of respiratory competence . a pattern of mv and tv decrease during the first 30 min after extubation was noted . subsequent recovery to pre - extubation levels within 1 h as confirmed by paco2 from arterial blood gas was noted in all patients . this shows that rvm is capable of providing patterns and trends in ventilation status not available to healthcare providers with other technology . respiratory complications are one of the leading causes of readmission to the icu for cardiac surgery patients . this study shows the benefits of this new technology and its application in the ctu and possibly other locations . pulse oximetry is the leading technology used to assess respiratory status in non - intubated patients ; however , spo2 levels do not begin to decline until well after respiratory decompensation has begun and is known for being prone to inaccuracies [ 13 , 17 ] . intermittent blood gas measurements were taken intermittently and on average over 1 h after extubation , well after rvm confirmed a recovery in mv . rvm showed distinct changes in mv and tv after extubation , while rr , generally measured with cardiac monitoring , showed no significant variations . the time it takes a patient to return to baseline mv is inversely proportional to successful extubation , therefore demonstrating the importance of obtaining mv measurements . additionally , determination of mv after extubation using rvm could potentially aid in quicker assessment of the necessity and timing of reintubation , or provide an early indication of when institute non - invasive ventilation ( niv ) , in the form of continuous positive airway pressure or biphasic positive airway pressure . although no patients in this study required reintubation , based on data from other studies collected after extubation in premature infants , it is hypothesized that patients who continue to have a decrease in mv with no rise to baseline within 1 h after extubation will require intervention such as non - invasive ventilation or reintubation . recently , the american society of anesthesiology has voiced the opinion that quantitative monitoring of respiratory status is superior over qualitative methods . qualitative clinical signs such as chest excursion and auscultation of breath sounds are useful , but they can only be obtained periodically and are often inaccurate . carbon dioxide monitoring is also a qualitative sign of respiration in non - intubated patients and its accuracy is dependent on many factors . however , non - invasive mv and tv have been previously unavailable in non - intubated patients . , the rvm permits a more comprehensive , quantitative assessment of respiratory status . with this functionality , rvm - derived respiratory data can bring forth reproducible and comparable data across patients over a wide range of breathing patterns . this can promote better evaluation of respiratory status in the ctu and other healthcare settings .
backgroundpatients who have undergone cardiac surgery are generally mechanically ventilated postoperatively . 
 early postoperative extubation is currently recommended in anesthesia guidelines . 
 no current technology can accurately , non - invasively , measure respiratory competence after extubation . 
 pulse oximetry has been helpful , but this is a late indicator of respiratory compromise . a novel , non - invasive , respiratory volume monitor ( rvm ) has been shown to deliver accurate continuous , real - time minute ventilation ( mv ) , tidal volume ( tv ) and respiratory rate ( rr ) measurements and provide an objective measure of respiratory competence . 
 the rvm will accurately reflect mv , tv and rr in cardiac surgery patients before and after extubation.methodsrvm traces were recorded from patients before and after cardiac surgery . 
 continuous monitoring began on admission to the unit and was ended at 24 h after extubation . 
 rvm - based mv , tv and rr were calculated from 30-s segments . 
 mv , tv and rr were also continuously recorded from the ventilator prior to extubation . 
 the rvm was calibrated to each patient using the readings from the ventilator.resultsduring mechanical ventilation , the rvm measured tvs strongly correlated with the ventilator tvs ( r = 0.97 ) . 
 following extubation , the patient s breathing became more erratic and tvs and mvs decreased . within 1 
 h , all patients studied showed a marked recovery of mv and tv.conclusionsrvm-based mv , tv and rr correlated well with similar data collected from ventilators . 
 after extubation , rvm shows promise as a means to monitor respiratory competence of non - intubated patients , and has implications for use in other settings and improving patient safety .