Oxygen kinetics: Pitfalls in clinical research revisited

Physiologic aspects of anemia

The most important adaptive responses from a physiological stance involved the cardiovascular system, consisting in particular of elevation of the cardiac output and its redistribution to favor the coronary and cerebral circulations, at the expense of the splanchnic vascular beds. The evidence regarding these physiological responses, especially in experimental studies that permit the control of many variables, is particularly powerful and convincing. On the other hand, there is a remarkable lack, in quality and quantity, of clinical studies addressing how normal physiological adaptive responses may be affected by a variety of diseases and conditions that often accompany and may complicate anemia, and interactions with other such compounding variables as age and different patient populations. For these reasons, it is not possible to offer guidelines on how to increase, maintain, or even to determine optimal DO2 in high-risk patients and how best transfusion strategies might be used under these conditions. From the brief review of physiological principles and the strong consensus in the literature, it is evident that cardiac function must be a central consideration in decisions regarding transfusion in anemia, because of the critical role it plays in assuring adequate oxygen supply of all vital tissues. Particular attention should be paid to the possible presence of CAD or incipient or cardiac failure, as these conditions may require careful transfusions to improve DO2 at levels that may not necessitate such interventions when cardiac disease is absent. Although the cerebral circulation also serves an obligate aerobic organ unable to tolerate significant hypoxia, there is little convincing evidence to support the notion that cerebral ischemia is aggravated by anemia and that this can be prevented by improved DO2 through rapid correction of anemia. Consequently, the arguments favoring transfusions in the presence of ischemic heart disease do not appear to apply to occlusive cerebrovascular disease. Because firm evidence is lacking on the interactions of concurrent diseases and anemia in various patient populations, understanding of the physiological consequences of anemia, and of the diseases concerned, is useful but not fully sufficient to provide firm and rational guidance to transfusion practice in specific complex clinical instances. A good deal of clinical and experimental investigation is required to support fully rational and comprehensive guidelines. In the meantime, prudent and conservative management, based on awareness of risks and sound understanding of the normal and pathological physiology, must remain the guiding principle.

Relationship between pulmonary oxygen consumption, lung inflammation, and calculated venous admixture in patients with acute lung injury

OBJECTIVE

To determine in patients with acute lung injury whether increased pulmonary oxygen consumption (VO2pulm), computed as the difference between oxygen consumption measured by indirect calorimetry (VO2meas) and calculated by the reverse Fick method (VO2Fick), would: (1) correlate with the degree of lung inflammation assessed by bronchoalveolar lavage (BAL); (2) lead to an overestimation of calculated venous admixture (Qva/Qt).

DESIGN

Prospective study.

SETTING

University hospital, medical intensive care unit.

INTERVENTION

None.

MEASUREMENTS AND RESULTS

In nine mechanically ventilated patients with acute lung injury (Apache II 12 +/- 5, lung injury score 2 +/- 0.6, mean +/- SD), whole-body VO2 (VO2wb) was determined simultaneously by indirect calorimetry and the reverse Fick technique, after which BAL was immediately performed. VO2meas was significantly higher than VO2Fick (128 +/- 24 and 102 +/- 18 ml/min per m2, respectively, p < 0.001). Median VO2pulm was 25.3 ml/min per m2 (range 1.98-51.5), thus representing 19 +/- 11% of VO2wb. Total BAL cellularity was increased in all patients (median 47, range 24-200 x 10(4)/ml), as was the total polymorphonuclear (PMN) count (median 78 range 5-93 x 10(4)/ml). Macrophage counts were in the normal range. There were raised BAL levels of interleukin-6 (IL-6) (median 945, range 23-1800 ng/ml) and elastase (median 391, range 5-949 ng/ml). Median protein levels were 270 micrograms/ml (range 50-505). There was no correlation between VO2pulm and BAL cellularity, PMNs, elastase, IL-6, or protein. Qva/Qt was 31.7 +/- 8%. Qva/Qt, corrected for the presence of VO2pulm, (Qva/Qtcorr), was 30.3 +/- 8% (P < 0.01 vs Qva/Qt), a 4.2% overestimation due to VO2plum. There was no correlation between Qva/Qt or Qva/Qtcorr and VO2pulm.

CONCLUSIONS

In mechanically ventilated patients with acute lung injury, VO2pulm was increased and led to a 19% underestimation of VO2wb determined by the reverse Fick method, as well as to a 4.2% overestimation of calculated Qva/Qt. Lung inflammatory activity was increased, as assessed by BAL cellularity, IL-6 and elastase levels. However, there was no correlation between VO2pulm and the intensity of pulmonary inflammation.

Effects of rapid permissive hypercapnia on hemodynamics, gas exchange, and oxygen transport and consumption during mechanical ventilation for the acute respiratory distress syndrome

OBJECTIVE

To measure the effects of rapid permissive hypercapnia on hemodynamics and gas exchange in patients with acute respiratory distress syndrome (ARDS).

DESIGN

Prospective study.

SETTING

18-bed, medical intensive care unit, university hospital.

PATIENTS

11 mechanically ventilated ARDS patients.

INTERVENTION

Patients were sedated and ventilated in the controlled mode. Hypercapnia was induced over a 30-60 min period by decreasing tidal volume until pH decreased to 7.2 and/or P50 increased by 7.5 mmHg. Settings were then maintained for 2 h.

RESULTS

Minute ventilation was reduced from 13.5 +/- 6.1 to 8.2 +/- 4.1 l/min (mean +/- SD), PaCO2 increased (40.3 +/- 6.6 to 59.3 +/- 7.2 mmHg), pH decreased (7.40 +/- 0.05 to 7.26 +/- 0.05), and P50 increased (26.3 +/- 2.02 to 31.1 +/- 2.2 mmHg) (p < 0.05). Systemic vascular resistance decreased (865 +/- 454 to 648 +/- 265 dyne.s.cm-5, and cardiac index (CI) increased (4 +/- 2.4 to 4.7 +/- 2.4 l/min/m2) (p < 0.05). Mean systemic arterial pressure was unchanged. Pulmonary vascular resistance was unmodified, and mean pulmonary artery pressure (MPAP) increased (29 +/- 5 to 32 +/- 6 mmHg, p < 0.05). PaO2 remained unchanged, while saturation decreased (93 +/- 3 to 90 +/- 3%, p < 0.05), requiring an increase in FIO2 from 0.56 to 0.64 in order to maintain an SaO2 > 90%. PvO2 increased (36.5 +/- 5.7 to 43.2 +/- 6.1 mmHg, p < 0.05), while saturation was unmodified. The arteriovenous O2 content difference was unaltered. Oxygen transport (DO2) increased (545 +/- 240 to 621 +/- 274 ml/min/m2, p < 0.05), while the O2 consumption and extraction ratio did not change significantly. Venous admixture (Qva/Qt) increased (26.3 +/- 12.3 to 32.8 +/- 13.2, p < 0.05).

CONCLUSIONS

These data indicate that acute hypercapnia increases DO2 and O2 off-loading capacity in ARDS patients with normal plasma lactate, without increasing O2 extraction. Whether this would be beneficial in patients with elevated lactate levels, indicating tissue hypoxia, remains to be determined. Furthermore, even though hypercapnia was well tolerated, the increase in Qva/Qt, CI, and MPAP should prompt caution in patients with severe hypoxemia, as well as in those with depressed cardiac function and/or severe pulmonary hypertension.