This prospective observational study was conducted in the paediatric intensive care unit (PICU) of the Department of Paediatrics at Mahatma Gandhi Medical College and Hospital, Jaipur, over a period from 1st April 2023 to 31st August 2024. The study included all pediatric patients aged between 1 month and 18 years who were admitted to the PICU with a primary diagnosis of lower respiratory tract infection (LRTI). Exclusions were made for children with concurrent cardiac conditions with or without congestive heart failure, cases of foreign body aspiration, congenital anomalies of the upper gastrointestinal or respiratory system, chronic pulmonary diseases, immunocompromised states, or those already receiving mechanical ventilation prior to PICU admission. Sample size calculations were based on a prevalence rate of 48% from a previous study by Ramesh et al.8 (2021), employing the formula n=Z^2×p×(1−p)/d^2, which indicated a need for a sample size of 96 children; this was increased to 106 to accommodate a 10% non-response rate. Ultimately, 120 participants were included through purposive sampling. After securing informed consent from parents or guardians, a thorough clinical history and examination were undertaken. Following stabilization, arterial blood gas (ABG) samples were collected to assess pH and partial pressure of carbon dioxide (pCO2), and the pCO2/pH ratio was computed. Various clinical parameters, including respiratory rate, presence of wheezing, use of accessory muscles, peripheral oxygen saturation, and feeding challenges, were documented, and the PRESS score was determined. Patients were admitted to the PICU with extensive clinical data recorded, and their outcomes were tracked and documented. Statistical Analysis Data were analyzed using SPSS v21. Descriptive statistics were applied for baseline variables. Pearson correlation was used to assess associations between the pCO₂/pH ratio and clinical parameters. Logistic regression was performed to evaluate mortality risk. Predictive accuracy was assessed with ROC analysis. A p-value <0.05 was considered significant.

This prospective observational study was conducted in the paediatric intensive care unit (PICU) of the Department of Paediatrics at Mahatma Gandhi Medical College and Hospital, Jaipur, over a period from 1st April 2023 to 31st August 2024. The study included all pediatric patients aged between 1 month and 18 years who were admitted to the PICU with a primary diagnosis of lower respiratory tract infection (LRTI). Exclusions were made for children with concurrent cardiac conditions with or without congestive heart failure, cases of foreign body aspiration, congenital anomalies of the upper gastrointestinal or respiratory system, chronic pulmonary diseases, immunocompromised states, or those already receiving mechanical ventilation prior to PICU admission. Sample size calculations were based on a prevalence rate of 48% from a previous study by Ramesh et al.8 (2021), employing the formula n=Z^2×p×(1−p)/d^2, which indicated a need for a sample size of 96 children; this was increased to 106 to accommodate a 10% non-response rate. Ultimately, 120 participants were included through purposive sampling. After securing informed consent from parents or guardians, a thorough clinical history and examination were undertaken. Following stabilization, arterial blood gas (ABG) samples were collected to assess pH and partial pressure of carbon dioxide (pCO2), and the pCO2/pH ratio was computed. Various clinical parameters, including respiratory rate, presence of wheezing, use of accessory muscles, peripheral oxygen saturation, and feeding challenges, were documented, and the PRESS score was determined. Patients were admitted to the PICU with extensive clinical data recorded, and their outcomes were tracked and documented. Statistical Analysis Data were analyzed using SPSS v21. Descriptive statistics were applied for baseline variables. Pearson correlation was used to assess associations between the pCO₂/pH ratio and clinical parameters. Logistic regression was performed to evaluate mortality risk. Predictive accuracy was assessed with ROC analysis. A p-value <0.05 was considered significant.

Patient Characteristics Among 120 patients, 67 (55.8%) were female, and 53 (44.2%) were male. The median age was 14.5 months (IQR 8–42). Most children (45.8%) were under one year of age. Clinical Outcomes Mortality was observed in 20 children (16.7%), while 100 (83.3%) survived. The mean hospital stay was 8.7 ± 3.4 days, and the mean duration of ventilatory support was 4.8 ± 3.5 days. pCO₂/pH Ratio The average pH of 7.20 ± 0.22 and an average pCO2 of 46.29 ± 11.14. These average levels of pH and pCO2 suggest a trend towards acidosis, underscoring the seriousness of the condition. The mean pCO₂/pH ratio was 6.46 ± 1.72. A higher ratio was significantly associated with worse clinical outcomes Table 1: Association of pCo2/ pH ratio with Hospital stay, ventilatory support, PRESS score Parameter Correlation with pCO₂/pH ratio p-value Hospital stays 0.268 <0.05 Ventilatory support days 0.440 <0.001 PRESS score 0.437 <0.001 the ratio of pCO2 to pH was positive associated with the total number of days spent in the hospital, with coefficient 0.268, which was statistically significant (p<0.05). The pCO2/pH ratio were positively correlate with the PRESS score, yielding a correlation coefficient of 0.437, and it was highly significant (p<0.001). Similarly, the pCO2/pH ratio were also positively correlate with the duration of ventilatory support days, with a correlation coefficient of 0.440 (p<0.001). Table 2. Actual vs Predicted Outcome by pCO₂/pH Ratio Model Parameters Actual Outcome Survived Died Outcome predicted by pCO2/pH ratio Survived 98 6 Died 2 14 Percentage correct prediction 98% 70% The analysis found that the pCO2/pH ratio accurately identified 14 deaths and 98 survivals. However, it incorrectly diagnosed 2 individuals who died but were classified as survivors and misidentified 6 survivors who actually died. Thus, it accurately predicted 98% of survivals and 70% of deaths Table 3. Logistic Regression Analysis Parameter Odds Ratio p-value pCO₂/pH ratio 10.39 <0.001 The odds ratio for the pCO2/pH ratio in relation to predicting mortality was found to be 10.39. This indicates that for every one-unit increase in the pCO2/pH ratio, the likelihood of mortality rises by a logarithmic factor of 10.39, which is statistically significant (p<0.001) Table 4. Predictive Accuracy of pCO₂/pH Ratio Parameters Values 95% CI Lower Upper Sensitivity 0.98 0.94 0.997 Specificity 0.7 0.483 0.868 PPV 0.942 0.887 0.977 NPV 0.875 0.662 0.978 Positive Likelihood Ratio 3.267 1.672 6.384

This study demonstrates that the pCO₂/pH ratio is a strong independent predictor of morbidity and mortality in children with severe LRTIs. A higher ratio correlated with prolonged hospital stay, extended ventilatory support, higher PRESS scores, and increased mortality. The mortality rate of 16.7% observed in this cohort aligns with reports from other tertiary care centers managing severe pediatric respiratory infections (9,10). The strong correlation between the pCO₂/pH ratio and PRESS scores highlights its utility in complementing established clinical severity indices (11). The logistic regression analysis confirmed that each unit increase in the pCO₂/pH ratio was associated with a ten-fold rise in the odds of mortality. ROC analysis showed excellent discriminatory power, with an AUC of 0.981. The optimal cutoff value of 7.22 provided high sensitivity (98%) and moderate specificity (70%), making it a useful tool for early identification of high-risk patients. In a study by Wrotek et al. (2021) on capillary blood gas parameters predicting the need for intensive care in children with bronchiolitis, they analysed 534 patients and found that partial carbon dioxide pressure was significantly elevated in those requiring ICU transfer (median 44.8 mmHg vs. 36.2 mmHg, p < 0.01), while other analysed CBG parameters were not statistically significant. Their ROC analysis demonstrated that pCO2 provided the best predictive value for ICU transfer, with an AUC of 0.773 (95% CI: 0.638–0.907, p < 0.01), while other parameters remained insignificant. A cutoff value of 41.8 mmHg (95% CI: 35.6–41.8) exhibited 67% sensitivity (95% CI: 38.4–88.2%) and 82% specificity (95% CI: 78.6–85.7%), along with a positive predictive value of 10.8% (95% CI: 7.4–15.3%) and a negative predictive value of 98.7% (95% CI: 97.4–99.4%). Wrotek et al. (2022) conducted another study on capillary blood gas in influenza patients, predicting the risk of lower respiratory tract infections and found that pCO2 showed the highest area under the curve for predicting LRTI in children under 6 months (AUC = 0.75, 95% CI: 0.63–0.87, p < 0.01), but was not significant in older age groups. A study by Vo et al. (2021) assessed pCO2 levels in capillary blood gas samples taken upon hospital admission from 134 bronchiolitis patients and recorded significantly elevated mean pCO2 levels in those experiencing respiratory decompensation (48 mmHg versus 44 mmHg). A significant correlation between pCO2 levels and respiratory decompensation was established, demonstrating an increased odds ratio (OR = 1.07). These findings suggest that integrating the pCO₂/pH ratio into routine PICU assessment may improve triage and guide timely escalation of care. Strengths and Limitations Strengths include the prospective design and comprehensive data collection. Limitations include the single-center setting and relatively small sample size, which may affect generalizability. Further multicenter studies are warranted to validate these results.

The pCO₂/pH ratio is a reliable, easily obtainable prognostic marker for children with severe LRTIs. A ratio above 7.22 identifies patients at higher risk of morbidity and mortality, enabling clinicians to prioritize intensive management strategies. Routine use of this ratio may enhance outcome prediction in pediatric critical care.

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