Predicting dead space ventilation in critically ill patients using clinically available data.

OBJECTIVE: To develop and validate an equation to predict dead space to tidal volume ratio (Vd/Vt) from clinically available data in critically ill mechanically ventilated patients. DESIGN: Prospective, observational study using a convenience sample of patients whose arterial blood gas and respiratory gas exchange had been measured with indirect calorimetry. SETTING: Medical and surgical critical care units of a university medical center. PATIENTS: Adult, mechanically ventilated patients at rest with Fio2 < or =0.60 and no air leaks who had recent arterial blood gas recordings and end-tidal carbon dioxide concentration monitoring. INTERVENTIONS: Observational only. MEASUREMENTS AND MAIN RESULTS: Indirect calorimetry was used to determine carbon dioxide production and expired minute ventilation in 135 patients. Tidal volume and respiratory rate were recorded from the ventilator. End tidal carbon dioxide concentration, body temperature, arterial carbon dioxide partial pressure (Paco2), and other clinical data were recorded. Vd/Vt was calculated using the Enghoff modification of the Bohr equation (Paco2 - PECO2/Paco2). Regression analysis was then used to construct a predictive equation for Vd/Vt using the clinical data: Vd/Vt = 0.32 + 0.0106 (Paco2 - ETCO2) + 0.003 (RR) + 0.0015 (age) (R = 0.67). A second group of 50 patients was measured using the same protocol and their data were used to validate the equations developed from the original 135 patients. The equation was found to be unbiased and precise. CONCLUSIONS: Vd/Vt is predictable from clinically available data. Whether this predicted quantity is valuable clinically must still be determined.

Control of breathing during acute change in cardiac preload in a patient with partial cardiopulmonary bypass.

We recently had the opportunity to investigate the ventilatory effects of changing the rate of venous return to the heart (and thus pulmonary gas exchange) in a patient equipped with a venous-arterial oxygenated shunt (extracorporeal membrane oxygenation (ECMO) support). The presence of the ECMO support provided a condition wherein venous return to the right heart could be increased or decreased while maintaining total aortic blood flow and arterial blood pressure (ABP) constant. The patient, who had received a heart transplant 12 years ago, was admitted for acute cardiac failure related to graft rejection. The clinical symptomatology was that of right heart failure. We studied the patient on the 4th day of ECMO support, while she was breathing spontaneously. The blood flow diverted through the ECMO system represented 2/3 of the total aortic flow (4 l min(-1)). With these ECMO settings, the baseline level of ventilation was low (3.89+/-0.99 l min(-1)), but PET(CO2) was not elevated (37+/-2 mmHg). When Pa(CO2) in the blood coming from the ECMO was increased, no stimulatory effect on ventilation was observed. However, when the diversion of the venous return to the ECMO was stopped then restored, minute ventilation respectively increased then decreased by more than twofold with opposite changes in PET(CO2). These maneuvers were associated with large changes in the size of the right atrium and ventricle and of the left atrium. This observation suggests that the change in venous return affects breathing by encoding some of the consequences of the changes in cardiac preload. The possible sites of mediation are discussed.

Granular cell tumor of trachea.

Granular cell tumors of the tracheobronchial tree are rare benign lesions of neurogenic origin. These benign tumors mostly involve the skin, oral cavity, or esophagus. There is no consensus regarding treatment of granular cell tumors. Treatment varies from simple observation to different bronchoscopic interventions, such as laser therapy or fulguration to surgical resection.