Noninvasive ventilation (NIV) has become a foundational intervention in emergency and critical care medicine for patients presenting with acute respiratory failure. By delivering ventilatory support without endotracheal intubation, NIV offers meaningful advantages over invasive mechanical ventilation, including preservation of airway clearance, swallowing function, and verbal communication, while reducing the risk of ventilator-associated pneumonia and the need for sedation [1,2].

In patients with acute-on-chronic respiratory failure (ACRF)—a condition encompassing COPD exacerbations, obesity-hypoventilation syndrome, neuromuscular disease, bronchiectasis, obstructive sleep apnea, and chronic thoracic sequelae—NIV is regarded as the standard of care, with robust evidence particularly in COPD-associated hypercapnic failure [3,4]. Multiple society guidelines endorse NIV as a first-line intervention for hypercapnic ACRF [5,6].

Despite these benefits, NIV failure rates ranging from 40 to 54% have been reported in patients with hypoxemic respiratory failure [3–6]. Critically, NIV failure—especially when recognized late—is associated with substantially increased mortality compared to early intubation [7,8]. Patients who deteriorate under NIV and require rescue intubation experience worse outcomes than those intubated electively at an earlier stage [5,9]. This underscores the imperative to identify patients at high risk of NIV failure as early as possible, so that timely invasive ventilation can be instituted.

To address this clinical need, the HACOR (Heart rate, Acidosis, Consciousness, Oxygenation, Respiratory rate) score was developed and validated by Duan et al. [10] as a bedside predictive tool for NIV failure in hypoxemic patients. This score integrates five physiological parameters—each readily measurable at the bedside—into a composite scale ranging from 0 to 25, where higher scores correspond to greater risk of NIV failure. The HACOR score was originally validated in COPD patients; however, since the pathophysiological mechanisms of ACRF are broadly shared across COPD and non-COPD etiologies, we hypothesized that the HACOR score would demonstrate comparable predictive validity in a mixed emergency population.

The present study was therefore designed to validate the HACOR score in patients with ACRF presenting to a tertiary care emergency department, to assess its diagnostic performance at one hour of NIV, and to examine its relationship to NIV failure outcomes.

This was a prospective observational study conducted at the Emergency Department of Max Smart Super Speciality Hospital, Saket, and New Delhi, India— a tertiary referral center with approximately 9,000 ED visits per year.

All adult patients (age ≥18 years) presenting to the ED with acute-on-chronic respiratory failure, defined as arterial pH <7.35 and PaCO₂ >45 mmHg, who were commenced on NIV and were compliant with therapy, were considered for enrollment. Patients were excluded if they were younger than 18 years, if NIV was contraindicated, if they had no prior history of chronic respiratory disease, if pH was ≥7.35 on arrival, or if they arrived already intubated. Patients who left against medical advice (LAMA) were also excluded.

Of 409 patients presenting with respiratory failure, 120 had no prior respiratory history. Of the remaining 289, 56 had pH ≥7.35 before NIV, and 7 arrived intubated. The final analytic cohort comprised 226 patients.

Data Collection and HACOR Score Calculation

Arterial blood gas (ABG) analysis was performed at the following time points: (1) at patient arrival, (2) one hour after NIV initiation, (3) twelve hours after NIV initiation, and (4) twenty-four hours after NIV initiation. The HACOR score was calculated at baseline and at one hour of NIV using the following five parameters, each scored on a weighted scale with a maximum total score of 25:

• Heart Rate (HR)
• Acidosis (arterial pH)
• Consciousness (Glasgow Coma Scale, GCS)
• Oxygenation (PaO₂/FiO₂ ratio)
• Respiratory Rate (RR)

NIV failure was defined as the requirement for endotracheal intubation at any point following NIV initiation.

Statistical Analysis

Data were analyzed using SPSS version 22.0 (IBM Inc., USA). Continuous variables were expressed as mean ± standard deviation (SD) or median with interquartile range (IQR), as appropriate. Normality was assessed using the Shapiro–Wilk test. Normally distributed continuous variables were compared using the unpaired Student’s t-test; non-normally distributed variables were compared using the Mann–Whitney U test. Categorical variables were analyzed using the Chi-squared test or Fisher’s exact test. Sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and accuracy were calculated for HACOR >5 at one hour of NIV. The odds ratio (OR) and 95% confidence intervals (CI) were calculated according to Altman (1991). A p-value <0.05 was considered statistically significant.

A total of 226 patients met inclusion criteria and were enrolled. The mean age was 64.5 ± 9.43 years. The majority of patients (71.23%) were older than 60 years of age. Regarding sex distribution, 168 patients (74.30%) were male and 58 patients (25.70%) were female.

Table 1: Age Distribution of Study Participants (n=226)

Table 2: Sex Distribution (n=226)

COPD was the most common underlying diagnosis, present in 92 patients (40.07%), followed by ‘others’ (33.6%), obstructive sleep apnea (OSA) in 38 patients (16.8%), and bronchiectasis in 18 patients (7.9%).

Table 3: Distribution of Underlying Respiratory Disorders (n=226)

At baseline, the majority of patients had a HACOR score of 2–3 (43.35%), followed by 4–5 (40.25%). Only 2 patients (0.88%) had a HACOR score exceeding 5 at admission.

Table 4: HACOR Score Distribution at Admission (n=226)

At one hour of NIV initiation, the score distribution shifted notably, with 36 patients (15.9%) achieving a HACOR score >5—a critical threshold associated with high NIV failure risk.

Table 5: HACOR Score at One Hour of NIV Initiation (n=226)

Of 226 patients, 29 (12.83%) experienced NIV failure requiring intubation. Among patients with a HACOR score >5 at one hour, 25 of 36 (69.4%) failed NIV, compared to only 4 of 190 (2.1%) with a score ≤5.

Table 6: Contingency Table – HACOR Score >5 vs. NIV Failure

Table 7: Diagnostic Performance of HACOR Score >5 at 1 Hour of NIV

The HACOR score >5 at one hour of NIV demonstrated high specificity (97.84%) and an exceptional odds ratio of 105.68 (95% CI: 31.26–357.31, p <0.0001), confirming a strong and statistically significant association with NIV failure

This prospective observational study validates the HACOR score as a reliable, early predictor of NIV failure in patients with acute-on-chronic respiratory failure presenting to a tertiary emergency department. Our findings are consistent with and extend the original derivation study of Duan et al. [10], which reported a diagnostic accuracy of 81.8–86.0% for a HACOR score >5 at one hour of NIV.

The HACOR score’s clinical appeal lies in its simplicity. All five component variables—heart rate, arterial pH, GCS score, PaO₂/FiO₂ ratio, and respiratory rate—are routinely obtained at the bedside and in emergency settings without requiring complex laboratory investigations or physiological monitoring equipment. This is particularly advantageous in high-acuity environments where rapid, reliable risk stratification can directly guide clinical decisions.

A specificity of 97.84% is particularly meaningful in this clinical context. A highly specific tool reduces unnecessary intubation in patients who would have responded to NIV, thereby sparing them the morbidity of invasive ventilation. Simultaneously, the odds ratio of 105.68 establishes that patients with HACOR >5 are over 100 times more likely to fail NIV, providing strong quantitative justification for escalating care in this subgroup. The high NPV of 94.27% further confirms that patients with a HACOR score ≤5 can be safely managed with continued NIV with high confidence.

Our study enrolled a heterogeneous population spanning COPD, OSA, bronchiectasis, and other chronic respiratory conditions. The applicability of the HACOR score across non-COPD etiologies of ACRF is an important finding, as prior validation was predominantly confined to COPD populations [10,14]. This broadens the clinical utility of the score to the full spectrum of patients presenting with hypercapnic respiratory failure in an emergency setting.

The predominance of older patients (>60 years in 71.23% of the cohort) and male sex (74.3%) in this study reflects established epidemiological patterns in ACRF. Interestingly, consistent with prior literature [28], we found no significant difference in NIV failure rates across age groups, suggesting that age per se is not a reliable determinant of NIV outcome and that clinical scoring tools such as HACOR are more appropriate for individualized risk stratification.

The pathophysiology of ACRF in COPD is characterized by progressive hypercapnia due to ventilation–perfusion mismatch and diminished hypoxic ventilatory drive, leading to respiratory acidosis [14]. NIV in this setting improves gas exchange by augmenting tidal volume, reducing inspiratory muscle load, and counteracting intrinsic PEEP. When these benefits fail to materialize—as reflected by a persistently high or rising HACOR score at one hour—clinicians should recognize this as a signal for imminent NIV failure and plan for definitive airway management.

The study also reinforces the importance of early identification over delayed intubation. Multiple studies have demonstrated that late intubation in NIV-failure patients is associated with substantially higher ICU mortality compared to early intubation when high-risk features are recognized [5,9]. The HACOR score provides an objective, reproducible framework to make this decision systematically rather than relying on clinician gestalt, which may vary based on experience and risk tolerance.

The HACOR score is a practical, bedside-applicable, and highly accurate tool for predicting NIV failure in patients with acute-on-chronic respiratory failure in emergency settings. A HACOR score >5 at one hour of NIV initiation is a strong, independent predictor of NIV failure, with an odds ratio exceeding 105 and an overall diagnostic accuracy of 93.21%. The score demonstrates clinical utility across both COPD and non-COPD etiologies of hypercapnic respiratory failure. Routine implementation of the HACOR score in emergency departments is recommended to facilitate early, objective risk stratification of NIV failure, guide timely decisions regarding intubation, and ultimately reduce morbidity and mortality in this high-risk patient population. 

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