The Indian Anaesthetists’ Forum

ORIGINAL ARTICLE
Year
: 2016  |  Volume : 17  |  Issue : 2  |  Page : 43--47

Accuracy of tension and saturation-based oxygen indices in the assessment of disease severity and its progress in adults with acute respiratory distress syndrome


Ghada F El-Baradey, Nagat S El-Shmaa 
 Department of Anesthesia and Surgical ICU, Faculty of Medicine, Tanta University, Tanta, Egypt

Correspondence Address:
Nagat S El-Shmaa
Department of Anesthesia and Surgical ICU, Faculty of Medicine, Tanta University, Tanta
Egypt

Abstract

Objective: The goal of this study was to assess the reliability of different tension and saturation-based oxygen indices with traditional oxygenation index (OI) in the assessment of disease severity and its progress in adult patients with acute respiratory distress syndrome (ARDS) when compared to the standard arterial oxygen tension (PaO2)/inspired fraction of oxygen (FiO2) (PF) ratio included in the Berlin definition of ARDS; the primary objective was assessed by the correlation of different OIs to the standard PF index. Design: This was a prospective, observational study. Setting: This study was carried out in Intensive Care Unit (ICU) in a university hospital. Patients and Methods: Sixty adult patients with different grades of ARDS severity according to Berlin definition were enrolled in this study. Measurements: The following indices were measured: (1) PF, (2) OI = FiO2 × mean airway pressure [MAP] × 100/PaO2 , (3) oxygen saturation index (OSI) = FiO2 × MAP × 100/oxygen saturation by pulse oximeter. The primary outcome measurement was the correlation of different OIs to the standard PF index. The secondary outcome measurements were sensitivity and specificity of each index. Results: There were a strong significant negative correlation between OI and OSI with the PF (r = −0.9 and −0.91, respectively) and a significant positive correlation between OI and OSI (r = 0.93). Total mortality rate was 36% (22 patients). The PF, OI, and OSI were sensitive and specific (sensitivity: 0.63, 0.77, and 0.81, respectively, and specificity: 0.76, 0.86, and 0.78, respectively). Conclusion: OSI can be used as a noninvasive index for the assessment of ARDS severity in adults as it correlates significantly with PF ratio and OI with high sensitivity and specificity to predict ICU mortality.



How to cite this article:
El-Baradey GF, El-Shmaa NS. Accuracy of tension and saturation-based oxygen indices in the assessment of disease severity and its progress in adults with acute respiratory distress syndrome.Indian Anaesth Forum 2016;17:43-47


How to cite this URL:
El-Baradey GF, El-Shmaa NS. Accuracy of tension and saturation-based oxygen indices in the assessment of disease severity and its progress in adults with acute respiratory distress syndrome. Indian Anaesth Forum [serial online] 2016 [cited 2017 Mar 30 ];17:43-47
Available from: http://www.theiaforum.org/text.asp?2016/17/2/43/193103


Full Text

 INTRODUCTION



Acute respiratory distress syndrome (ARDS) severity was assessed by arterial oxygen tension (PaO2)/inspired fraction of oxygen (FiO2) index (PF), which was first described by Horovitz et al. in 1974 to compare arterial oxygenation at different FiO2 indexes.[1] These indexes did not take into account the factors which affect PF, as functional residual capacity changes when positive end-expiratory pressure (PEEP) was applied, and the inspiratory time changes or lung mechanics. All these made it an insensitive indicator of progress in disease severity after changing ventilator parameter by applying PEEP, high FiO2 , or inverse inspiratory-to-expiratory time ratio (I: E ratio). At that time, PF ratio will improve while actually disease severity did not improve.[2]

As PEEP may improve lung recruitment and compliance, mean airway pressure (MAP) is affected by the level of PEEP, thus it may be an indicator of lung recruitment and compliance. Thus, it is not only an indicator of oxygenation but also an indicator of lung compliance and recruitment through PEEP. Oxygenation index (OI = FiO2 × MAP × 100/PaO2), therefore, reflects the functional status of lung. OI is more sensitive than the traditional PF ratio in assessing the oxygen exchanging status and severity of the lung injury because it includes PF ratio and pulmonary alveolar macrophage which is related to compliance.[3]

As oxygenation can be continuously and noninvasively assessed using pulse oximetry, this makes use of oxygen saturation-based indices to assess the severity of ARDS, easily with no need for arterial line or arterial blood sampling. Thus, use of the oxygen saturation index (OSI = MAP × FiO2 × 100/oxygen saturation by pulse oximeter [SpO2]) may be an alternate method for assessing the severity of ARDS.[4] The goal of the study was to assess the reliability of different tension and saturation-based oxygen indices with traditional OI in the assessment of disease severity and its progress in adult patients with ARDS when compared to the standard PF ratio included in the Berlin definition of ARDS; the primary objective was assessed by the correlation of different OIs to the standard PF index.

 Patients and Methods



This prospective, observational study was carried out in surgical Intensive Care Unit (ICU) of a university hospital from May 2012 to March 2015 after obtaining approval from Ethical Committee, and a written informed consent was obtained from the nearest relatives, on sixty adult patients on mechanical ventilation with different grades of ARDS severity according to the Berlin definition in 2011.

Timing: Within 1 week of a known clinical insult or new or worsening respiratory symptomsChest imaging: Bilateral opacities consistent with pulmonary edema not fully explained by cardiac failure or fluid overload. If there is no clear cause or precipitating factor for ARDS, echocardiography (ECHO) is doneMild ARDS: 200 mmHg< PF ≤300 mmHg with PEEP or continuous positive airway pressure (CPAP) ≥5 cm H2 O.

Moderate ARDS: 100 mmHg< PF ≤200 mmHg with PEEP or CPAP ≥5 cm H2 O.

Severe ARDS: PF ≤100 mmHg with PEEP or CPAP ≥5 cm H2 O.[5]

Inclusion criteria

Patients of age more than 18 years, applied mechanical ventilation through an orotracheal tube or tracheostomy, and after 24 h of fulfilling Berlin definition to confirm the criteria of ARDS and excluding other causes of hypoxemia and pulmonary infiltrates were included in the study.

Exclusion criteria

Patients with neuromuscular disease, left ventricular dysfunction (on ECHO), patients with obstructive lung disease or with a history of bronchial asthma, intracranial hypertension, hemodynamically unstable patients or on high-dose vasopressor or inotropes, pneumothorax, barotrauma as interstitial emphysema, or subcutaneous emphysema, and pregnant patients were excluded from the study.

All patients were mechanically ventilated on volume-controlled mode using "Inspiration" LS ventilator series e-Vent ventilator (Hilditch Group Ltd Gloucester Road Trading Estate Malmesbury, Wiltshire, UK), with initial tidal volume (Vt) 6 ml/kg predicted body weight (PBW) (in female = [0.65 × height in cm] − 50.74), (in male = [0.73 × height in cm] − 59.42) and PEEP at 5 cm H2 O increase by 2 cm H2 O targeting inspiratory plateau pressure <30 cm H2 O and FiO2 100%, and then decrease gradually to achieve oxygenation goal of PaO2 55-80 mmHg or SpO2 88%-95%. If oxygenation goal was not achieved, I:E ratio was inversed with monitoring to auto-PEEP and PaCO2 .

If the plateau pressure was >30 cm H2 O with a Vt of 6 ml/kg PBW, a stepwise Vt reduction of 1 ml/kg PBW to as low as 4 ml/kg/PBW was allowed. Respiratory rate was adjusted to maintain adequate minute ventilation and pH between 7.30 and 7.45. All patients were relaxed or deeply sedated during the measurement of MAP.

Once patients were diagnosed as ARDS, the following indices were measured daily:

PFOI = FiO2 × MAP × 100/PaO2OSI = FiO2 × MAP × 100/SpO2 .

Where, PaO2 : Arterial oxygen tension, FiO2 : Inspired fraction of oxygen, MAP: Mean airway pressure, SpO2 : Oxygen saturation by pulse oximeter.

The oxygen saturation and the MAP were recorded immediately before arterial blood sample aspiration for arterial blood gases.

The primary outcome measurement was the correlation of different OIs to the standard PF index.

The secondary outcome measurements were sensitivity and specificity of each index.

Statistical analysis

Based on a similar investigation, a sample size of sixty patients was calculated for 90% power, α = 0.05, β = 0.1, and anticipated effect size = 0.40 by using sample size software (G*Power Version 3.00.10, Germany). Descriptive and analytic statistics were performed on IBM compatible computer by using the SPSS software version 11.0.1 for Windows (SPSS Inc., Chicago, IL, USA) under Windows XP operating system. Data were presented in the form of mean ± standard deviation. Correlation between different OIs was calculated using correlation coefficient (Pearson's formula). The calculated values were as follows: Sensitivity (true positive/true positive + false negative), specificity (true negative/true negative + false positive), positive predictive value (true positive/true positive + false positive), negative predictive value (true negative/true negative + false negative), and diagnostic accuracy = (true positive + true negative)/(true positive + true negative + false positive + false negative).

 RESULTS



Patients were similar as regard to age, gender, APACHE II score, duration of mechanical ventilation and causes of ARDS [Table 1]. There were a strong significant negative correlation between OI and OSI with the PF (r = −0.9, −0.91 respectively) as demonstrated in [Table 2] and [Figure 1], [Figure 2] and a significant positive correlation between OI and OSI (r = 0.93) [Figure 3]. Total mortality rate was 36% (22 patients). The PF, OI, and OSI are sensitive and specific [Table 3].{Figure 1}{Table 1}{Table 2}{Figure 2}{Figure 3}

 DISCUSSION



ARDS is characterized by severe diffuse alveolar injury, resulting in hypoxemia. Severity of ARDS assessed using PF is most commonly used due to its simplicity and reliability in both spontaneously breathing and mechanically ventilated patients.[6] However, usage of PF in the follow-up of ARDS severity and progress after changing ventilator parameter by applying high PEEP, FiO2 , or inverse I:E ratio may be misleading. At that time, PF ratio will improve while disease severity did not improve.

Thus, addition of MAP to PF ratio as in OI will incorporate PEEP, I:E time, tidal volume, and/or peak airway pressure according to the mode of mechanical ventilation. Hence, OI reflects lung-gas exchange status and function. The only limitation for this index is that it cannot be used in nonintubated or spontaneously ventilated patients as MAP will be zero.[7]

Many previous studies found a strong correlation between PaO2 and SpO2 . Miranda et al., who studied the relationship between PF and SatO2/FiO2 and found that SatO2 /FiO2 can be used in the assessment of disease severity in children with a cutoff of 200 for SatO2 /FiO2, had a sensitivity of 97.5% for classifying patients with PF values lower or higher than 200.[8]

Nowadays, there is an increased interest in noninvasive methods for oxygenation assessment with the advantage of being bedside, easy, can be used frequently, and with no need for frequent blood sampling.

In this study, there were a strong significant negative correlation between OI and OSI with the PF (r = −0.9 and −0.91, respectively) and a significant positive correlation between OI and OSI (r = 0.93) with sensitivity (77% and 81%, respectively) and specificity (86% and 78%, respectively).

Van Haperen et al. demonstrated that OI >8.1 was usually in agreement with PF <200 and OI 5.3-8.1, frequently with PF <200 in patients with ARDS.[9]

In agreement with our results, Rawat et al. found that calculated OSI correlates with OI in neonates with hypoxic respiratory failure.[4]

In addition, Thomas et al. demonstrated the use of OSI as a noninvasive method to define ARDS in pediatrics who had significant sensitivity and specificity.[10]

Anthony et al., in agreement with our results, found that OSI had a strong linear association with OI. Thus, they conclude OSI to be a good substitute for OI in critically ill adults with ARDS.[11]

Limitations of the current study should be considered. First, most of the previous studies were done in pediatrics, so further studies on adult population are needed. Second, MAP measurement related to mandatory breath cannot be used in nonintubated or spontaneously ventilated patients as MAP will be zero. Third, the small number of patients (n = 60) enrolled for the study. Therefore, the statistical power may not have been adequate. In addition, there was no controlled group.

 CONCLUSION



OSI can be used as a noninvasive index for the assessment of ARDS severity in adults as it correlates significantly with PF ratio and OI with high sensitivity and specificity to predict ICU mortality.

Acknowledgments

The authors would like to thank the nurses at Intensive Care Unit of Tanta University Hospital, Tanta, for their assistance in conducting the study.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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