Use of the Alveolar Dead Space Fraction (Vd/Vt) and Plasma D‐dimers to Exclude Acute Pulmonary Embolism in Ambulatory Patients

Abstract

Objective: To evaluate the utility of a modified calculation of the alveolar dead space fraction (Vd/Vt), combined with plasma D-dimers, to aid in the exclusion of acute pulmonary embolism (PE). Methods: A prospective comparison of screening modalities was performed in a metropolitan teaching ED. Ambulatory patients evaluated for PE underwent simultaneous end-tidal CO, and arterial blood gas determinations, as well as venous latex-agglutination D-dimer quantification. The modified Bohr equation was used to calculate Vd/Vt as an index of alveolar dead space. Acute PE was diagnosed or excluded using appropriate combinations of clinical suspicion, ventilation-perfusion lung scanning, lower-extremity venous Doppler ultrasonography, pulmonary angiography, and comprehensive follow-up. Results: Of 170 subjects studied, PE was confirmed (PE+) in 26 (15%) and excluded (PE-) in 144 (85%). In the PE+ group, Vd/Vt was 0.31 & 0.13 (mean 2 SD), and in the PE-group, Vd/Vt was 0.06 t-0.10 (p c 0.05, t-test). Regarding false-negative rates, Vd/Vt was normal (i.e., c0.2) in 3/26 PE+ patients and D-dimer concentrations were normal (~0 . 5 pg/L) in 4/26 patients in the PE+ group. The combination of a normal Vd/Vt and D-dimer concentration was 100% sensitive (95% CI = 88-100%) in excluding PE. Falsepositive testing (either test positive) occurred in 49/14 subjects (specificity 65%, 95% CI = 52-73%). The age-adjusted alveolar-arterial O2 gradient was 33 5 38 torr in the PE+ group vs 13 2 37 torr in the PEgroup (p = 0.1 1). Conclusions: In ambulatory patients, the finding of Vd/Vt ~0 . 2 and D-dimers c0.5 pg/L lowers the probability of acute PE.

Ambulatory vital signs in the workup of pulmonary embolism using a standardized 3-minute walk test

“…11,15,18,[21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] The three most widely used clinical decision rules are the simplified Wells score, the revised Geneva score, and the pulmonary embolism rule-out criteria (PERC) rule. The simplified Wells score comprises seven variables and has been derived and validated in the North American population.…”

Pulmonary Dysfunction in Patients with Femoral Shaft Fracture Treated with Intramedullary Nailing

“…Despite this positive report, there is a scarcity of data documenting pulmonary gas exchange and/ I or early pulmonary outcomes in this patient population 22 . The rationale for measuring alveolar dead space to demonstrate a functional consequence of pulmonary embolization has been well documented [7][8][9]11,12,14 . In an experimental study involving glass-bead embolization, alveolar dead space increased proportionate to the degree of pulmonary vascular obstruction in mechanically ventilated goats 15 .…”

“…Furthermore, experimental studies on the effect of pulmonary embolism have demonstrated an increase in alveolar dead space secondary to a reduction in perfusion of the affected lung 16 . Measurement of alveolar dead space is a sensitive screening test (sensitivity of 88%) for acute pulmonary emboli in ambulatory patients 12 , and it has a high sensitivity and specificity for detection of pulmonary emboli in mechanically ventilated patients with intrinsic lung disease (asthma, chronic obstructive pulmonary disease, and so on) 9,11 . Thus, there is ample evidence suggesting that pulmonary gas exchangespecifically, alveolar dead space-should change if a large embolic burden is routinely induced by reaming and/or nailing of a fractured femoral shaft.…”

“…A large bolus of embolized marrow can block perfusion to a portion of the well-ventilated lung, resulting in a high ventilation-to-perfusion ratio or increased alveolar dead space [7][8][9][10][11][12][13][14][15][16][17] . The method used to prepare the medullary canal may determine the amount of bone marrow extrusion during intramedullary nailing 3,[18][19][20] .…”

“…However, few patients had an adverse pulmonary outcome, limiting the conclusions of the study. Measurement of alveolar dead space provides a clinical measure of pulmonary vascular occlusion, regardless of the etiology, because it accurately reflects the difference between measurements of alveolar and arterial carbon dioxide 8,[10][11][12][14][15][16]23 . In healthy patients, the ratio should approach 1.0, as carbon dioxide rapidly debonds from the hemoglobin molecule at the alveolar level.…”