Correlation of EtCO₂- and P(A-a)O₂-Based Dead Space Calculations in Mechanically Ventilated ICU Patients

NCT06947486 | Completed

• 1 other identifier: 2025/3
• Study Type: observational
• Target: 41
• Locations: 1 country
• Sites: 1

Brief Summary

This study aims to evaluate two different methods for calculating physiological dead space in adult patients undergoing invasive mechanical ventilation in the intensive care unit (ICU). Physiological dead space refers to the portion of air that is ventilated but does not participate in gas exchange due to impaired perfusion or ventilation-perfusion mismatch. Traditionally, dead space is calculated using the end-tidal carbon dioxide (EtCO₂) method, which estimates the difference between arterial and exhaled CO₂ values. However, this method may be influenced by circulatory failure or abnormal CO₂ distribution. An alternative method using the alveolar-arterial oxygen gradient \[P(A-a)O₂\] has been proposed, as it may provide a more stable measurement under critical conditions by relying on oxygenation efficiency rather than CO₂ elimination. In this prospective observational study, patients receiving mechanical ventilation in a tertiary ICU will be monitored. Physiological dead space will be calculated using both the EtCO₂-based method and the P(A-a)O₂-based method. Various respiratory and clinical parameters, including arterial blood gases, ventilator settings, and severity scores, will be recorded. The correlation between the two methods will be assessed, and their relationship with ICU mortality will be analyzed. The results of this study may help determine whether the P(A-a)O₂ method can be used as a reliable alternative for estimating dead space in ICU patients and whether it has prognostic value in predicting patient outcomes.

Conditions

Physiological Dead Space; Ventilation-Perfusion Mismatch; Mechanical Ventilation; Enghoff Correction

Keywords

Physiological Dead Space; EtCO₂; P(A-a)O₂ Gradient; Mechanical Ventilation; Volumetric Capnography; Ventilation-Perfusion Mismatch; Enghoff Modification; Kuwabara Equation; Intensive Care Unit

Outcome Measures

Primary Outcomes (1)

• Correlation between EtCO₂- and P(A-a)O₂-based dead space calculations

  Description: Pearson or Spearman correlation coefficient calculated between dead space values obtained from EtCO₂ (Enghoff modification) and P(A-a)O₂-based gradient methods in mechanically ventilated ICU patients.

  Within the first 72 hours of ICU admission

Secondary Outcomes (3)

• Association of each dead space method with ICU mortality

  From ICU admission until ICU discharge or death

• Agreement between EtCO₂- and P(A-a)O₂-based dead space estimates

  Within first 3 days of ICU stay

• Relationship between dead space values and gas exchange/ perfusion variables

  Daily for up to 72 hours

Study Arms (1)

Mechanically Ventilated ICU Patients

Adult patients (≥18 years old) admitted to a tertiary ICU and receiving invasive mechanical ventilation for at least 24 hours. Participants are prospectively monitored with capnographic and arterial blood gas measurements, and physiological dead space is calculated using both EtCO₂- and P(A-a)O₂-based methods. Patients are followed until ICU discharge or death.

Eligibility Criteria

• Age: 18 Years+
• Sex: all
• Healthy Volunteers: No
• Age Groups: Adult (18-64), Older Adult (65+)
• Sampling Method: Non-Probability Sample

Study Population

The study population includes adult patients (aged 18 years and older) who are admitted to a tertiary academic intensive care unit and require invasive mechanical ventilation for respiratory failure. Eligible patients must be expected to remain intubated for at least 24 hours and undergo both volumetric capnography and arterial blood gas monitoring. Participants are enrolled prospectively and consecutively based on predefined inclusion and exclusion criteria. Patients with circulatory shock, severe metabolic or respiratory derangements, or technical limitations interfering with capnographic measurement are excluded. The study represents a critically ill cohort commonly encountered in ICU practice.

You may qualify if:

• Age ≥ 18 years
• Admission to the intensive care unit (ICU)
• Receiving invasive mechanical ventilation
• Expected ICU stay ≥ 24 hours
• Availability of both arterial blood gas and volumetric capnography measurements
• Central venous cathater for central venous blood analyses
• Informed consent obtained from legal representative

You may not qualify if:

• Refusal of participation by the patient or legal surrogate
• Hemoglobin \< 7 g/dL
• PaCO₂ \> 70 mmHg
• Lactate \> 4 mmol/L
• Capillary refill time \> 3 seconds
• Mean arterial pressure (MAP) \< 65 mmHg
• Mottling score ≥ 2
• PaCO₂ - PcCO₂ gradient \> 8 mmHg
• pH \< 7.20
• Body temperature \> 38°C
• BMI \> 40 kg/m²
• VCO₂ \> 4 mL/kg/min
• Technical limitations preventing accurate capnographic monitoring
• Patients who are extubated, transferred, or deceased within the first 24 hours

Sponsors & Collaborators

• Karadeniz Technical University: lead

Study Sites (1)

Karadeniz Technical University Faculty of Medicine, Department of Chest Diseases, Intensive Care Unit

Trabzon, 61080, Turkey (Türkiye)

Location

Study Design

• Study Type: observational
• Observational Model: COHORT
• Time Perspective: PROSPECTIVE
• Sponsor Type: OTHER
• Responsible Party: PRINCIPAL INVESTIGATOR
• PI Title: MD, Assist Prof.

Study Record Dates

• First Submitted: April 20, 2025
• First Posted: April 27, 2025
• Study Start: January 5, 2025
• Primary Completion: April 10, 2025
• Study Completion: April 18, 2025
• Last Updated: April 27, 2025

Record last verified: 2025-04

Data Sharing

• IPD Sharing: Will not share

Locations

TU (1)