BET 1: Everything in graduation: arterial/end-tidal CO2 gradient and the diagnosis of pulmonary embolism

A short cut review was carried out to establish the diagnostic characteristics of alveolar dead space fraction (AVDSf) in the diagnosis of pulmonary embolism (PE). This is calculated from the arterial and end-tidal CO2. Three papers were selected to answer the clinical question. The author, study type, relevant outcomes, results and weaknesses are tabulated. It is concluded that there is good evidence to support the use of AVDSf within a clinical prediction model to exclude a PE in patients when there is a low pretest probability. However, the specificity is not sufficient to support it as a ‘rule in’ test.

End-tidal to arterial PCO2 ratio: a bedside meter of the overall gas exchanger performance

Background The physiological dead space is a strong indicator of severity and outcome of acute respiratory distress syndrome (ARDS). The “ideal” alveolar PCO2, in equilibrium with pulmonary capillary PCO2, is a central concept in the physiological dead space measurement. As it cannot be measured, it is surrogated by arterial PCO2 which, unfortunately, may be far higher than ideal alveolar PCO2, when the right-to-left venous admixture is present. The “ideal” alveolar PCO2 equals the end-tidal PCO2 (PETCO2) only in absence of alveolar dead space. Therefore, in the perfect gas exchanger (alveolar dead space = 0, venous admixture = 0), the PETCO2/PaCO2 is 1, as PETCO2, PACO2 and PaCO2 are equal. Our aim is to investigate if and at which extent the PETCO2/PaCO2, a comprehensive meter of the “gas exchanger” performance, is related to the anatomo physiological characteristics in ARDS. Results We retrospectively studied 200 patients with ARDS. The source was a database in which we collected since 2003 all the patients enrolled in different CT scan studies. The PETCO2/PaCO2, measured at 5 cmH2O airway pressure, significantly decreased from mild to mild–moderate moderate–severe and severe ARDS. The overall populations was divided into four groups (~ 50 patients each) according to the quartiles of the PETCO2/PaCO2 (lowest ratio, the worst = group 1, highest ratio, the best = group 4). The progressive increase PETCO2/PaCO2 from quartile 1 to 4 (i.e., the progressive approach to the “perfect” gas exchanger value of 1.0) was associated with a significant decrease of non-aerated tissue, inohomogeneity index and increase of well-aerated tissue. The respiratory system elastance significantly improved from quartile 1 to 4, as well as the PaO2/FiO2 and PaCO2. The improvement of PETCO2/PaCO2 was also associated with a significant decrease of physiological dead space and venous admixture. When PEEP was increased from 5 to 15 cmH2O, the greatest improvement of non-aerated tissue, PaO2 and venous admixture were observed in quartile 1 of PETCO2/PaCO2 and the worst deterioration of dead space in quartile 4. Conclusion The ratio PETCO2/PaCO2 is highly correlated with CT scan, physiological and clinical variables. It appears as an excellent measure of the overall “gas exchanger” status.

Penggunaan Ventilatory Ratio dan Alveolar Dead Space Fraction sebagai Prediktor Mortalitas pada Pasien COVID-19 dengan Acute Respiratory Distress Syndrome

Pada pasien COVID-19 dengan ARDS terjadi gangguan oksigenasi dan ventilasi. Menurut kriteria Berlin ARDS, oksigenasi diukur dengan PaO2/FiO2, namun tidak mengukur ventilasi alveolar yang diukur dengan dead space yang dapat terjadi akibat kondisi, seperti kerusakan endotel, mikrotrombus, dan penggunaan ventilator yang berlebih. Tujuan penelitian ini menganalisis penggunaan ventilatory ratio (VR) dan dead space fraction (Vd/Vt) sebagai prediktor mortalitas pasien COVID-19 ARDS. Penelitian ini adalah analitik kohort retrospektif. Data dikumpulkan dari rekam medik pasien COVID-19 yang dirawat di RIK RSUD Dr. Soetomo periode Juni–September 2020 dengan teknik total sampling terhadap subjek yang memenuhi kriteria inklusi dan tidak termasuk eksklusi. Data yang dikumpulkan adalah nilai VR dan Vd/Vt (diambil dari data laboratorium), kondisi klinis pasien dan pengaturan ventilator 24 jam pertama setelah terintubasi. Penelitian ini didapatkan 77 dari 80 subjek yang memenuhi kriteria. Nilai VR berhubungan dengan mortalitas secara signifikan dengan nilai p 0,001; cut off 1,84; sensitivitas 84,2%; spesifisitas 85%; RR 30,22; CI 95%: 7,31–124,89. Vd/Vt dan mortalitas menunjukkan hubungan yang signifikan terhadap mortalitas dengan nilai p 0.001. Uji analisis Spearman VR dengan Vd/Vt didapatkan hasil korelasi yang kuat dengan koefisien korelasi 0,704 dan p 0,001. Simpulan, nilai VR dan Vd/Vt dapat digunakan sebagai prediktor mortalitas pasien COVID-19 dengan ARDS dan keduanya mempunyai korelasi yang kuat. VR dapat menggantikan Vd/Vt.