Robert A. Klocke

Variability of indices of hypoxemia in adult respiratory distress syndrome

OBJECTIVE To determine the usefulness of indices of hypoxemia in assessing patients with the adult respiratory distress syndrome (ARDS). DESIGN Retrospective analysis of previously published data that describe the distributions of ventilation and pulmonary blood flow in ARDS. SETTING University research laboratory. PATIENTS Sixteen patients with ARDS. INTERVENTIONS The FIO2 was varied between 0.21 and 1.0 in a computer model of gas exchange, based on a 50-compartment model of ventilation/perfusion inhomogeneity plus true shunt and deadspace. The indices of hypoxemia that were calculated as a function of inspired oxygen concentration included PaO2/FIO2, arterial/alveolar ratio (PaO2/alveolar PO2), the alveolar-arterial PO2 difference (P[A-a]O2), respiratory index (P[A-a]O2/PaO2), and venous admixture. MEASUREMENTS AND MAIN RESULTS The PaO2/FIO2 ratio in patients with moderate shunts (< 30%) varied considerably with alteration in FIO2. At both extremes of FIO2, the PaO2/FIO2 in these patients was substantially greater than at intermediate FIO2. Patients with larger shunts (> 30%) had greater PaO2/FIO2 ratios at low FIO2, but the PaO2/FIO2 ratios decreased to relatively stable values at FIO2 values of > 0.5. In all patients, PaO2/FIO2 remained relatively stable at FIO2 values of > or = 0.5 and PaO2 values of < or = 100 torr (< or = 13.3 kPa). Other PO2-based indices exhibited less stability as FIO2 was varied. If hypoxemia resulted from true shunting, venous admixture was found to be stable at all FIO2 values. However, approximately one half of patients had clinically important hypoxemia resulting from mismatching of ventilation and blood flow. In these patients, venous admixture varied substantially with change in FIO2, and the degree of variation was proportional to the fraction of cardiac output perfusing gas exchange units with ventilation/perfusion ratios of < 0.1. CONCLUSIONS All indices of hypoxemia are affected by changes in FIO2 in patients with ARDS. PaO2/FIO2 ratio exhibits the most stability at FIO2 values of > or = 0.5 and PaO2 values of < or = 100 torr (< or = 13.3 kPa), and is a useful estimation of the degree of gas exchange abnormality under usual clinical conditions. Venous admixture varies substantially with alteration of FIO2 in patients who have clinically important ventilation/perfusion abnormalities. Under these circumstances, venous admixture is a poor indicator of the efficiency of pulmonary oxygen exchange, even if venous admixture is calculated from measured arterial and venous oxygen content values. Estimated venous admixture, based on an assumed arterial-venous oxygen content difference, is even more unreliable.

A survey of internal medicine residents and faculty about the duration of attendings’ inpatient rotations

AbstractOBJECTIVE: Some training programs are shortening the duration of attendings’ rotations from 4 weeks to 2 weeks. Our objective was to determine the effect of 2-week inpatient rotation on self-reported impact on medical education, patient care practices, and faculty performance by internal medicine residents and teaching faculty. DESIGN: Cross-sectional study using an anonymous mailed and emailed survey. SETTING: University-based internal medicine residency program in Buffalo, New York that recently introduced 2-week rotations. PARTICIPANTS: One hundred nineteen residents (99 responded, 83%) and 83 teaching faculty (76 responded, 92%). MEASUREMENTS: Perceived impact on medical education, patient care, and attending performance on 7-point Likert scales ranging from negative (−3) across neutral (0) to positive (+3) ratings. RESULTS: In general, residents and attendings felt that the short rotation negatively affects the attending’s ability to evaluate residents and some aspects of patient care, but that it has no negative impact on residents’ or medical students’ learning. Attendings thought the 2-week rotation positively affects their private life and overall productivity. Subgroup analysis indicated that residents who graduated from U.S. medical schools were more pessimistic about the 2-week rotation compared to their international counterparts. Attendings who had completed at least one short rotation had consistently higher ratings of the 2-week rotation. CONCLUSION: Residents and attendings’ perceptions suggest that the shorter attending inpatient rotation might have negative impact on medical education and patient care but positive effects on the attending’s work productivity and private life. This tradeoff requires further evaluation including objective medical education and patient care outcomes.

Pulmonary hypoxic vasoconstriction: How strong? how fast?☆

We have developed a minimally invasive technique for studying regional blood flow in conscious sheep, bypassing the complications of open-chest surgery, flow probes and tracer infusion. We quantitate regional perfusion continuously on the basis of regional clearance of methane (methane is produced in the sheep rumen, enters the circulation and is eliminated nearly completely (greater than 95%) in the lung). Tracheal intubation with a dual-lumen catheter isolates the gas exchange of the right apical lobe (RAL; less than 15% of the lung) from that of the remainder of the lung, which serves as a control (CL). We measure RAL and CL methane elimination by entraining expirates in constant flows, sampled continuously for methane. Results obtained with this technique and from regional oxygen uptake are in excellent agreement. We have found that hypoxic vasoconstriction is far more potent and stable during eucapnic hypoxia than during hypocapnic hypoxia. The time course of the vasoconstriction suggests that many of the data in the literature may have been obtained prior to steady state.

High-frequency oscillation during simulated altitude exposure

Ventilatory requirements using high-frequency oscillation (HFO) during simulated altitude exposure were investigated in control dogs and animals with oleic acid-induced lung injury. FIO2 values of 0.21 and 1.0 were supplied by bias flow to the normal and injured dogs, respectively. After a control period, animals were exposed to a simulated altitude of 8,000 ft (barometric pressure 564 torr), followed by a second control period at ground level. Both experimental groups had similar values of PaCO2 at ground level and during exposure to reduced barometric pressure. The tidal volume necessary to maintain eucapnia was higher in oleic acid-injured animals compared with the control group; cardiac output and functional residual capacity were lower. The alveolar-arterial oxygen difference was substantially larger in the oleic acid group. Adequate gas exchange can be maintained with HFO during exposure to altitude provided that ventilation and inspired PO2 are not reduced below normobaric levels.

Measurement of cardiac output using improved chromatographic analysis of sulfur hexafluoride (sf6)

A constant current variable frequency pulsed electron capture detector has been incorporated into the gas chromatographic analysis of trace amounts of sulfur hexafluoride (SF6) in water and blood. The resulting system offers a broader effective operating range than more conventional electron capture units and has been utilized for measurements of cardiac output employing constant-rate infusion of dissolved SF6. The SF6 technique has been validated against direct volumetric measurements of cardiac output in a canine right-heart bypass preparation and used subsequently for rapidly repeated measurements in conscious animals and man.