Rapid shallow breathing index.

Predicting successful liberation of patients from mechanical ventilation has been a focus of interest to clinicians practicing in intensive care. Various weaning indices have been investigated to identify an optimal weaning window. Among them, the rapid shallow breathing index (RSBI) has gained wide use due to its simple technique and avoidance of calculation of complex pulmonary mechanics. Since its first description, several modifications have been suggested, such as the serial measurements and the rate of change of RSBI, to further improve its predictive value. The objective of this paper is to review the utility of RSBI in predicting weaning success. In addition, the use of RSBI in specific patient populations and the reported modifications of RSBI technique that attempt to improve the utility of RSBI are also reviewed.

Predicting successful liberation process from mechanical ventilation (MV) has always been an important clinical issue.[12] Both unnecessary delayed and premature liberation from MV have been shown to be associated with undesirable effects on patient outcome, prolonged MV and increased the length of stay in the Intensive Care Unit (ICU).[34] Premature discontinuation places additional stress on the respiratory and cardiovascular systems,[5] while unnecessary delays can lead to diaphragmatic atrophy,[6] venous thromboembolism, delirium, and pneumonia; all of which are associated with increased mortality and morbidity.[789] Therefore, the decision of delayed discontinuation must be balanced against a possible premature discontinuation as it has been estimated that medical patients spend 42% of the total time on MV in the discontinuation process.[10] Many of the integrated weaning indices look promising, but no index has proven to be ideal.[11] Until the description rapid shallow breathing index (RSBI) by Yang and Tobin in 1991, clinicians depended mainly on weaning predictors such as vital capacity, maximum inspiratory pressure, and minute ventilation. Yang and Tobin described RSBI as the ratio of respiratory rate (RR) to tidal volume (VT), with a threshold value of >105 breaths/min/L being highly predictive of weaning failure, while RSBI <105 breaths/min/L is associated with weaning success. Our review examines the existing literature on the utility of RSBI in predicting successful weaning. We review the utility of RSBI in specific patient populations and also examined the reported modifications of RSBI that attempted to improve the value of RSBI.

Yang and Tobin, in their original prospective cohort study on RSBI, evaluated 100 mechanically ventilated medical patients.[12] The measurement of VT was performed using a hand-held spirometer attached to the endotracheal (ET) tube while the patient breathed room air for 1-min without any ventilator assistance. The study found that an RSBI >105 breaths/min/L was associated with weaning failure, while an RSBI <105 breaths/min/L predicted weaning success with a sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of 97%, 64%, 78%, and 95%, respectively. The findings of Yang and Tobin were supported by several subsequent studies conducted by Epstein[13] and Jacob et al.[14] in adults, and by Thiagarajan et al.[15] in the pediatric population. Frutos-Vivar et al. had identified that the RSBI as one of the best predictor associated with extubation failure, with a positive fluid balance and pneumonia as the other factors.[4]

Technical Considerations in Rapid Shallow Breathing Index

RSBI is now incorporated in many ventilator algorithms and are shown on the ventilator display as one of the ventilator parameters. It has been demonstrated that RSBI calculated from values obtained by direct ventilometry and the one obtained through the values available in the display of the mechanical ventilator are highly correlated.[16]

It is important to recognize the influence of ventilator setting on RSBI. A survey of respiratory therapists found great variation in weaning techniques and was observed that some respiratory therapists use continuous positive airway pressure (CPAP), and some use pressure support ventilation (PSV) to measure the RSBI, all of which could affect the measurements obtained.[17] Shingala et al. compared RSBI on PSV and RSBI on spontaneous breathing with T-piece and found that RSBI on PSV was a better predictor for extubation, easier to obtain, and less stressful for the patient.[18] Another study[19] examined RSBI values under various ventilatory support settings prior to extubation. The study compared RSBI in PSV, CPAP, and T-piece in 36 patients. RSBI was significantly smaller during PSV, and CPAP compared to T-piece. While RSBI was <105 breaths/min/L in all enrolled patients on both PSV and CPAP modes, this changed during T-piece trial, as 13 out of 36 patients had an RSBI of >105 breaths/min/L. This observation was confirmed in other studies.[2021] A recent study concluded that a threshold RSBI value of 75 breaths/min/L in PSV and 100 breaths/min/L in T-piece was more accurate for predicting successful weaning.[22]

In addition, it is important to recognize that sepsis, fever, supine position, anxiety, and restrictive lung diseases increase the respiratory frequency and hence affect RSBI. Some other factors such as narrow ET tube, female gender, and suctioning are shown to increase RSBI, and the need of various thresholds are recommended.[232425]

Skeptics of Rapid Shallow Breathing Index

Tanios et al. conducted a randomized controlled trial of the role of weaning predictors in clinical decision-making. The study enrolled 304 patients who were randomized to two groups; in one group the RSBI was measured but not used in the decision to wean, and in the other group, RSBI was measured and used, with a threshold of 105 breaths/min/L for weaning decision-making. Patients passing a 2-h spontaneous breathing trial (SBT) were eligible for an extubation attempt.[26] The study found that the median duration for weaning time was significantly shorter in the group where the weaning predictor was not used (2.0 vs. 3.0 days, P = 0.04) with no difference between the two groups in the incidence of extubation failure, in-hospital mortality rate, tracheostomy, or unplanned extubation. Other studies re-evaluated the most predictive cut-off threshold. Danaga et al. found that the classical cut-off value of RSBI (105 breaths/min/L) predicted only 20% of the cases that were ready for extubation while a cut-off value of 76.5 breaths/min/L provided substantial improvement in sensitivity (66%), with an acceptable loss of specificity (74%).[27]

Rapid Shallow Breathing Index in Specific Populations

While RSBI is appropriate for most ICU patients, there are certain patient populations in whom the use of RSBI may or may not accurately predict successful weaning.

Rapid shallow breathing index in chronic obstructive pulmonary disease patients

A study on weaning chronic obstructive pulmonary disease (COPD) patients found that RSBI measured early during an SBT cannot accurately predict the successful outcome in the patient population.[28] This is probably related to the ineffective inspiratory efforts, which does not trigger the ventilator leading to false positive results of RSBI <105 breaths/min/L. Therefore, a study found that 56% of the COPD patients with RSBI <80 breaths/min/L actually failed the weaning trials.[29]

Rapid shallow breathing index in cardiac patients

The value of RSBI in post cardiac surgery patient was confirmed by a study by Oribabor et al.[30] The study showed that the utilization of the RSBI as the sole criteria for weaning has led to significantly short mean extubation times in cardiac surgery patients without an increase in reintubation rates. In cardiac patients, in general, it is important to note the effect of positive pressure ventilation. Since positive pressure can improve cardiac function in congestive heart failure patients,[31] and this can also reduce RSBI,[32] it is important to keep in mind the effect of PSV and CPAP on RSBI values. This has been confirmed in a study of postcoronary artery bypass graft patients, as RSBI values were lower in CPAP (5 cm) than T-piece trials.[21]

Rapid shallow breathing index in neurosurgical patients

Neurosurgical patients represent a special group for weaning because the main reason for intubation is usually airway protection and not abnormal lung physiology. Not surprising therefore that RSBI is not a good predictor of successful weaning. A prospective cohort comprising 92 neurosurgical patients concluded that an RSBI <105 breaths/min/L was not associated with successful extubation, among the 15 participants who needed reintubation, only one patient exhibited an RSBI >105 breaths/min/L.[33] Another observational cohort comprising 119 traumatic brain injury (TBI) patients concluded no association between RSBI categories (<105 breaths/min/L and >105 breaths/min/L) and successful extubation in TBI patients.[34]

Rapid shallow breathing index in patients with tracheostomy

A prospective cohort of 191 patients with tracheostomy and on prolonged MV concluded RSBI as a good predictor of 1-h SBT tolerance, with 97 breaths/min/L as the RSBI threshold with a maximum accuracy of 81.7%.[35]

Rapid shallow breathing index in pediatric patients

RSBI was described in the adult population, and the cut-off points have been generated for that population. In the pediatric population, the performance and outcomes of RSBI have been debated and questioned.[363738] The results of a pediatric survey reflected the limited and conflicting literature on RSBI in children.[39]

Rapid shallow breathing index in burn patients

A study on burn patients identified that RSBI is predictive of successful extubation, although cough peak flow (CPF) and ET secretions in patients who passed SBT were highly predictive.[40] Therefore, CPF and ET secretions should be used in conjunction with RSBI in this population, and generally in any population with a poor cough and increased secretions.

Patients on prolonged mechanical ventilation

In a retrospective cohort study of patients receiving prolonged MV, RSBI was measured daily, with weaning per protocol. Initial, mean, and final RSBI; RSBI ≤105; rate of change; and variability were assessed. The study found that isolated RSBI measurements do not accurately predict successful weaning from, but RSBI trends may have prognostic value.[41]

Patients on non-invasive ventilation (NIV)

The RSBI may also have a role in predicting successful weaning from noninvasive ventilation (NIV). An initial assisted RSBI of >105 breaths/min/L in NIV patients was associated with the need for intubation and increased in-hospital mortality.[42] Another study evaluated the use of RSBI in non-intubated COPD patients and concluded that higher RSBI values in these patients with COPD exacerbations predicted the need for NIV.[43]

Modifications in Rapid Shallow Breathing Index

There have been increasing interest in maximizing the predictive value of RSBI. The use of serial RSBI and RSBI rate has been proposed.

Serial rapid shallow breathing index

The use of serial RSBI comes from the observation that breathing pattern in some patients may be stable at the beginning of SBT but deteriorate later. This deterioration is ascribed to poor respiratory muscle endurance or worsening of pulmonary mechanics that may not be present during the initiation of weaning.[44] Hence, some studies later focused on the serial assessment of RSBI and at various intervals. Chatila et al. reported that RSBI measured at 30 min of an SBT was a better predictor of weaning outcome than RSBI at the start of weaning initiation.[45] Krieger et al. summarized that serial measurements of RSBI may be more useful in the weaning process.[25] Another study about serial RSBI measurement and weaning outcome in critically ill patients concluded that RSBI measured at the completion of SBT was superior to that measured at the start of weaning.[46] However, the value of serial RSBI has not been shown consistently. In patients, who already had a successful SBT with initial RSBI <105 breaths/min/L, serial RSBI measurements during 120 min of SBT were unable to detect extubation failure.[47] Shah et al. concluded that RSBI does not change significantly during a 90 min SBT during serial measurement done at 1, 30, 60, and 90 min of SBT.[48]

Rapid shallow breathing index rate

RSBI rate refers to the rate of change in RSBI in serial measurement. Segal et al. studied serial RSBIs during weaning and looked into RSBI rate hypothesizing the rate of change in RSBI is more predictive of successful weaning, as respiratory failure is a dynamic phenomenon. In this prospective cohort, 2 h of SBT was given, and the variables were measured periodically during SBT. RSBI rate was calculated by the formula: (RSBI-2 – RSBI-1)/RSBI-1 × 100 and the identified threshold was 20%. Of 30 patients, 21 were successfully extubated, 3 were reintubated within 24 h, and six remained intolerant to the SBT. The study found that the RSBI rate <20% has a sensitivity of 90.4% and specificity of 100% in predicting weaning success. It had a PPV of 100% and an NPV of over 81%.

In another prospective observational study of Segal et al.,[49]63 out of 72 patients were successfully extubated (24 h postextubation window period), by using RSBI rate as a weaning predictor. RSBI was assessed in every 30 min of the 2-h trial. Initial RSBI was similar in extubation success and extubation failure groups (77.0 ± 4.8, 77.0 ± 4.8, P ≥ 0.05). RSBI remained the same in subsequent intervals of the successfully extubated group, whereas, the values of RSBI were in the increasing trend (either due to increased RR or decreased VT) in extubation failure group. They concluded that the percent change of RSBI during SBT was a better predictor of successful extubation than a single determination of RSBI.

In the above trial, 24 were COPD patients and 10 out of 24 (41.66%) continued ventilator support. Of note, breathing trials were performed using T-piece, where it has been identified already that RSBI values will be on a higher side in T-piece group, compared to that of RSBI values in the ventilator-supported group.[18]

Conclusion

RSBI is an important predictor of weaning outcome. Serial RSBI and RSBI rate have better predictive value than a single RSBI measurement. However, interpretation of RSBI values must take in consideration certain technical aspects, such as the ventilator settings as well as the patient population. RSBI should not be used universally in all populations to predict successful extubation. In certain patient populations, particularly in whom the primary problem is related to airway protection, increased secretions and poor cough, liberation from MV should not rely solely on RSBI, but on other parameters such as cough adequacy­ and a number of secretions [Tables 1–3].

Table 1

Studies on RSBI in general medical-surgical ICU patients

Table 3

Studies about variants in RSBI, including serial RSBI and RSBI rate

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

Table 2

Studies on RSBI in special ICU populations