Miscellaneous
Noninvasive Cardiac Output Measurement by Inert Gas Rebreathing in Suspected Pulmonary Hypertension

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The objective of this study was to evaluate inert gas rebreathing (IGR) reliability in cardiac output (CO) measurement compared with Fick method and thermodilution. IGR is a noninvasive method for CO measurement; CO by IGR is calculated as pulmonary blood flow plus intrapulmonary shunt. IGR may be ideal for follow-up of patients with pulmonary hypertension (PH), sparing the need of repeated invasive right-sided cardiac catheterization. Right-sided cardiac catheterization with CO measurement by thermodilution, Fick method, and IGR was performed in 125 patients with possible PH by echocardiography. Patients were grouped according to right-sided cardiac catheterization–measured mean pulmonary and wedge pressures: normal pulmonary arterial pressure (n = 20, mean pulmonary arterial pressure = 18 ± 3 mm Hg, pulmonary capillary wedge pressure = 11 ± 5 mm Hg), PH and normal pulmonary capillary wedge pressure (PH-NW, n = 37 mean pulmonary arterial pressure = 42 ± 13 mm Hg, pulmonary capillary wedge pressure = 11 ± 6 mm Hg), and PH and high pulmonary capillary wedge pressure (PH-HW, n = 68, mean pulmonary arterial pressure = 37 ± 9 mm Hg, pulmonary capillary wedge pressure = 24 ± 6 mm Hg). Thermodilution and Fick measurements were comparable. Fick and IGR agreement was observed in normal pulmonary arterial pressure (CO = 4.10 ± 1.14 and 4.08 ± 0.97 L/min, respectively), whereas IGR overestimated Fick in patients with PH-NW and those with PH-HW because of intrapulmonary shunting overestimation in hypoxemic patients. When patients with arterial oxygen saturation (SO2) ≤90% were excluded, IGR and Fick agreement improved in PH-NW (CO = 4.90 ± 1.70 and 4.76 ± 1.35 L/min, respectively) and PH-HW (CO = 4.05 ± 1.04 and 4.10 ± 1.17 L/min, respectively). In hypoxemic patients, we estimated pulmonary shunt as Fick − pulmonary blood flow and calculated shunt as: −0.2423 × arterial SO2 + 21.373 L/min. In conclusion, IGR is reliable for CO measurement in patients with PH with arterial SO2 >90%. For patients with arterial SO2 ≤90%, a new formula for shunt calculation is proposed.

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Methods

The present study involved 125 patients consecutively referred to our Pulmonary Hypertension Unit with an echocardiographic evaluation suggestive of PH. To confirm the diagnosis of PH, all subjects underwent right-sided cardiac catheterization, which included simultaneous CO determination by 3 techniques: thermodilution, direct Fick method, and IGR. Patients with severe chronic pulmonary disease at spirometry (forced expiratory volume <60% of predicted and vital capacity <60%), congenital heart

Results

From January 2008 to November 2012, 125 consecutive patients with an echocardiographic evaluation suggestive of PH were enrolled in the study. Thirty-seven patients had PH-NW, 68 had PH-HW, and 20 had normal pulmonary arterial pressure (Table 1). All patients underwent right-sided cardiac catheterization without complications. The hemodynamic features are detailed in Tables 2 and 3. In patients with PH, either with elevated or normal pulmonary wedge pressure, IGR overestimated CO (Table 3).

Discussion

The present study shows an overall agreement among CO determination by thermodilution, direct Fick method, and IGR in patients with PH without significant arterial hypoxemia. These results indicate that there is the possibility of a closer and better-tolerated hemodynamic follow-up in these patients. Indeed, although early treatment and combination therapies improved survival, PH remains a fatal disease.5 Many studies focused on finding accurate predictors of survival to develop an even more

Disclosures

The authors have no conflicts of interest to disclose.

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