Non-invasive ventilation for acute hypoxemic respiratory failure: intubation rate and risk factors

Patients

All consecutive patients admitted during a three-year period (June 2008 to June 2011) and who received NIV as initial ventilatory support for AHRF were included. AHRF was defined as recent dyspnea with a respiratory rate >25 breaths/minute and/or sternocleidomastoid muscle activation with pulmonary infiltrates on chest X-ray, and a PaCO₂ below or equal to 45 mmHg. We excluded patients who were intubated before ICU admission or intubated upon ICU admission without prior NIV, and patients for whom NIV was used with a “do not intubate” order. However, the outcome for those who were directly intubated for acute respiratory failure without prior NIV, and who met clinical criteria for moderate or severe ARDS was also collected. The study was conducted after the implementation of a nurse-driven NIV protocol which included prospective daily collection of clinical data and ventilatory parameters on a specific NIV monitoring form. When the NIV form was unavailable or incomplete, data were retrieved from the patient’s records.

Non-invasive ventilation protocol and definitions

All stages of the protocol had been developed within a multidisciplinary working group including ICU physicians, nurses and respiratory therapists. The protocol aimed at empowering nurses to adjust the ventilatory settings and to improve the patient's tolerance to NIV following a simple decision algorithm. A daily NIV prescription by the physician indicated the duration of NIV sessions and targeted expiratory tidal volume (around 6 to 8 ml/kg) and oxygen saturation (SpO₂) (≥94%).

Pressure-support (PS) ventilation was started using a pressure-support level of 8 cmH₂O, a positive end-expiratory pressure (PEEP) level of 5 cmH₂O, an inspiratory trigger of 3 L/minute, and a maximal inspiratory time of one second. The nurses then adjusted the ventilatory parameters, including pressure-support level and FiO₂ (fraction of inspired oxygen), according to the protocol. Pressure-support level was gradually increased by 2 cmH₂O steps to reach the target expiratory tidal volume and PEEP level was then adjusted as prescribed. FiO₂ was gradually adjusted by 5% steps to reach the targeted SpO₂. Non-invasive ventilation was applied intermittently for periods of at least two hours, with a minimal duration of six hours per day and was maintained until signs of respiratory distress improved. An algorithm was used by nurses in case of leaks, which involved first repositioning of the mask; second, reducing the PEEP level at 2 cmH₂O; third, reducing the pressure-support level by steps of 2 cmH₂O until the minimal expiratory volume was reached; and fourth, changing the mask interface.

A mobile cart containing all types and sizes of interfaces was available at the bedside during initiation of NIV. NIV was performed via a non-vented full-face mask (FreeMotion™ RT041, Fisher & Paykel, Auckland, New Zealand or Ultra Mirage™, Resmed, CA, USA), with an ICU ventilator using a dedicated NIV mode (Evita XL, Dräger, Lübeck, Germany, or Engström Carestation, GE Healthcare, Fairfield, CT, USA), equipped with a heated humidifier (MR850, Fisher & Paykel).

The following criteria were used for endotracheal intubation: loss of consciousness or psychomotor agitation hindering nursing care and requiring sedation; persistent hypotension (defined by systolic arterial blood pressure below 90 mmHg or mean arterial blood pressure below 65 mmHg) despite fluid resuscitation, or need for vasopressors; or two of the following criteria: frank worsening of respiratory distress under NIV, respiratory rate above 40 breaths per minute, SpO₂ remaining below 90% despite FiO₂ 100%, dependence to NIV for more than 12 hours, or pH <7.35. NIV failure was defined by the need for endotracheal intubation.

Data collection

From the NIV monitoring forms, we analyzed the number and duration of NIV sessions, ventilator settings (pressure support level, positive end-expiratory pressure, FiO₂), ventilatory parameters (SpO₂, respiratory rate, expiratory tidal volume), hemodynamic parameters (heart rate, blood pressure and level of consciousness assessed using the Richmond Agitation-Sedation Scale (RASS) [20], with altered consciousness defined as a RASS <0. NIV tolerance and number of leaks were recorded on a 4-point scale, then dichotomized into “acceptable” (scored 2 to 3) or “poor” (scored 0 to 1) tolerance, and “minor” (scored 0 to 1) or “major” (scored 2 to 3) leaks, respectively. Blood gases were routinely measured one hour after initiation of NIV. Clinical data (respiratory rate, SpO₂, blood pressure, heart rate, Glasgow coma score) and blood gases at admission before NIV initiation were retrospectively collected from the medical chart. We also recorded the occurrence of shock at admission or at initiation of NIV (defined by hypoperfusion signs and administration of at least 30 ml/kg fluids, dobutamine or vasopressors).

Patients were stratified according to the presence of clinical criteria for ARDS. The severity of ARDS was stratified using the recent Berlin definition [19], according to the value of oxygenation recorded within the first hour after NIV initiation, and classified as mild (201 ≤ PaO₂/FiO₂ (partial pressure of oxygen/fraction of inspired oxygen) ≤ 300 mmHg), moderate (101 ≤ PaO₂/FiO₂ ≤ 200 mmHg) or severe (PaO₂/FiO₂ ≤ 100 mmHg).

Statistical analysis

Dichotomous variables are reported as number (percentage), and were compared using the chi-square or Fisher’s exact tests. Continuous variables are expressed as mean (± standard deviation) or as median and interquartile range (IQR, (25^th to 75^th percentiles)) after testing their normal distribution using the Shapiro-Wilk test. Groups were compared using the unpaired Student’s t-test or Wilcoxon rank-sum and Kruskall-Wallis tests, when appropriate. Odds ratios (OR) with 95% confidence intervals (CI) were used to describe differences between subgroups for NIV failure or death.

Survival without intubation was tested using Kaplan-Meier estimates and compared with the log-rank test. To evaluate independent factors associated with NIV failure, variables with a univariate P-value <0.10 were entered in a Cox proportional hazards model with time to intubation as the dependent variable, censoring data at ICU discharge. Among related variables, the most significant or clinically relevant was entered into the model in order to minimize the effect of colinearity. Because it was measured at 24 hours after admission, the general severity score SAPS 2 was not included in this analysis. Variables included in the model are reported with their corresponding hazard ratio (HR) and 95% CI. We considered two-tailed P- values <0.05 as significant. Statistical analyses were performed using the statistical software package STATA version 10.1 (Stata Corp., TX, USA).

Patients

Among 430 patients who received NIV during the study period, 188 had non-hypercapnic acute respiratory failure. After excluding patients with cardiogenic pulmonary edema and those without pulmonary infiltrates, 113 had de novo acute hypoxemic respiratory failure (Figure 1). Eighty-two patients had clinical criteria for ARDS at the time of NIV initiation, including 16 with mild (20%), 47 with moderate (57%) and 19 (23%) with severe ARDS. ARDS was due to bacterial pneumonia (n = 21), viral pneumonia (n = 7), pneumocystis jirovecii (n = 4), pneumonia without microbiological documentation (n = 24), aspiration (n = 5), alveolar hemorrhage (n = 6), drug induced pneumonia (n = 5), extra-pulmonary sepsis (n = 8), transfusion acute lung injury (n = 1), and fat embolism (n = 1). The 31 remaining patients without clinical criteria for ARDS (non-ARDS) had pneumonia (n = 17), atelectasis (n = 5), aspiration (n = 4), intra-alveolar hemorrhage (n = 2), pleural effusion (n = 2) or extra-pulmonary sepsis (n = 1). Overall, 50 patients (44%) were immunocompromised (Table 1), because of hematologic malignancy (n = 22), organ transplant (n = 10), HIV infection (n = 6), vasculitidis or steroid therapy (n = 4) or active/metastatic solid cancer (n = 8).

Rates of NIV failure and in-ICU mortality

The rate of intubation was 61% (50/82) in ARDS and 35% (11/31) in non-ARDS patients (P = 0.015). This rate did not differ between patients without ARDS or those with mild ARDS (P = 0.71), but increased with increasing clinical severity of ARDS from 31% (5/16) in mild, 62% (29/47) in moderate, to 84% (16/19) in severe ARDS (P = 0.0016) (Figure 2). Patients with moderate or severe ARDS were twice as likely to fail NIV (45/66, 68%) than those with no ARDS or with mild ARDS (16/47, 34%); (OR = 4.15, 95% CI: 1.78 to 9.70; P = 0.0004) Survival analysis showed that intubation rates differed markedly (P <0.00001, Log-rank test) between patients with no or mild ARDS and those with moderate or severe ARDS (Figure 3).

Overall in-ICU mortality rate was 25% (28/113), and tended to be higher in patients with ARDS (24/82, 29%) than others (4/31, 13%, P = 0.07) (Figure 2). The mortality rate of patients with moderate or severe ARDS was also twice as high (21/66; 32%) as those with no or mild ARDS (7/47; 15%) (OR = 2.7; 95% CI: 1.003 to 7.09; P = 0.041).

Among intubated patients, the overall in-ICU mortality rate was 46% (28/61). Thirty-three patients (54%) were intubated within the first 24 hours while the 28 patients remaining (46%) were intubated beyond 24 hours. The delay between NIV initiation and intubation had no influence on outcome with a similar time to intubation in survivors and non-survivors (Figure 4). Among patients with moderate or severe ARDS, in-ICU mortality was similar in patients who were intubated after failure of NIV as compared to patients who were directly intubated without prior NIV (Figure 5).

Factors associated with NIV failure

Cox regression analysis showed that the risk of intubation was significantly associated with active cancer, a lower Glasgow coma score, shock, moderate/severe ARDS and a lower PEEP level (Table 2).

NIV failure rate in patients with acute hypoxemic respiratory failure

In patients receiving NIV for AHRF, we found an overall rate of intubation of 54%, which is substantially higher than the 25 to 35% rate reported in randomized controlled trials evaluating NIV in AHRF [17, 18]. However, in these two studies nearly 20 to 30% of the patients received NIV for cardiogenic pulmonary edema. Moreover, patients enrolled in such randomized studies are selected and, consistent with our results, intubation rates up to 60% have been reported in a series of unselected patients with AHRF of non-cardiac origin [6–8].

In their analysis of 147 ARDS patients receiving NIV as first-line therapy, Antonelli et al. [9] reported an intubation rate of only 46%. In this study, a high SAPS 2 (> 34) and low PaO₂/FiO₂ ratio (≤ 175 mmHg) after NIV initiation were the two risk factors of NIV failure, with an intubation rate of 78% (25/32) when both risk factors were present [9]. Although our overall intubation rate was higher, the intubation rate of patients presenting with the combination of these two criteria was strictly similar (79%, 27/34) and their mortality rate was likewise similar. However, using the SAPS 2 is clinically impractical since this score is computed only after 24 hours of admission, therefore taking into account the potential complications of intubation in patients who failed NIV within the first 24 hours.

Time to intubation and impact on outcome

It has been suggested that NIV failure in patients with AHRF is independently associated with poor outcome as compared to patients intubated without prior NIV [6]. Therefore, it is essential to assess intubation rates and the impact of NIV failure on outcome in different subsets of the population with AHRF. It was recently suggested at an international conference that NIV may be a first line treatment in mild ARDS [21]. Our study supports this contention, as the intubation rate in patients with mild ARDS did not differ from that recorded in non-ARDS patients. In our patients with moderate or severe ARDS, however, the intubation rate was much higher (68%). Nevertheless, the mortality rate of patients failing NIV did not differ according to the time to intubation, in contrast with previous studies of patients with community acquired pneumonia [22] or receiving NIV during the post-extubation period [23]. We were unable to identify a time beyond which maintaining NIV may worsen outcome, and we believe that intubation should be decided according to standard criteria regardless of NIV duration.

It could also be argued that in our study NIV was used in moderately ill patients, while more severe patients would have been intubated without prior NIV. However, the rate of severe ARDS (19/82, 23%) in our population is close to that reported in the recent Berlin definition [19]. Mortality rates observed in intubated patients with ARDS are usually higher in observational studies than in randomized controlled studies and can reach 45% in unselected cohorts of patients [24, 25]. Thus, our mortality rate of 48% in ARDS patients who failed NIV and required intubation is in line with these cohort studies of intubated patients.

Predictive factors for NIV failure

As previously reported [6], immunosuppression had no influence on the success or failure of NIV; however, all eight patients in the subgroup having active or metastatic cancer failed NIV, and this factor remained significantly associated with NIV failure after adjustment (Table 2); thus, using NIV in this subgroup should be carefully considered. Not surprisingly, a low GCS and, to a lesser extent, shock were associated with NIV failure. Although neither controlled studies [17, 18] nor surveys [5, 7] found that the occurrence of shock was a risk factor of intubation, two others studies found that shock was associated with NIV failure [8, 22].

Several studies found that hypoxemia was independently associated with NIV failure [7–9, 22]. Our results confirm that stratification of patients according to the clinical severity of ARDS using the recent Berlin definition was clearly associated with the risk of NIV failure, with a low risk in patients with mild ARDS, increasing to 84% in those who had a PaO₂/FiO₂ ≤100 mmHg at initiation of NIV. However, a cut-off of 150 mmHg (a value close to that reported by Antonelli et al. [9]) appeared to more accurately segregate patients who failed from those who succeeded NIV. Therefore, whereas almost all patients with severe ARDS are likely to fail NIV, some patients with “moderate” ARDS might still benefit from a NIV trial.

Limitations

Our study was conducted in a single unit with a long-standing experience in the practice of NIV and, therefore, our results may not be applicable to other centers with less extensive experience. Experience and nurse-driven protocols may improve NIV tolerance, and we report a poor tolerance rate of only 13% after one hour of NIV. In line with previous studies [5], poor tolerance was associated with NIV failure in univariate analysis but not after adjustment for other variables associated with NIV failure. However, whereas rate of NIV failure could be significantly reduced for hypercapnic patients in experienced centers [26], our rate of intubation was not lower in this series than in surveys including less experienced centers [5, 7]. Another limitation is the retrospective nature of the study. However, prospective data collection of ventilatory parameters under NIV was available for a vast majority of our patients and, because of the availability of computerized medical charts for all patients, all those receiving NIV for AHRF could be analyzed.