Four patients with respiratory failure who required mechanical ventilation and had chest roentgeno-graphic findings of a unilateral infiltrate were included in the study (Table 1). All of the patients had a history of smoking. In all patients with an interstitial process, turning the “good” side to the dependent position (down) improved oxygenation (Table 2). This improvement in oxygenation was not associated with a significant change in PaCO2. Cardiac output decreased in three of the four patients when the abnormal side was nondependent (up), without significant change in mean pulmonary arterial or mean wedge pressure.
Table 3 and Figures 1 and 2 show the ventilation-perfusion and retention-excretion data for all patients with the “good” side either dependent (down) or nondependent (up). The improvement in oxygenation noted in patients 1 and 3 was mainly the result of a decrease in right-to-left intrapulmonary shunt. In patient 2, the oxygenation improved despite a small increase in right-to-left shunt by a decrease in very low ventilation-perfusion ratios and an improvement in overall ventilation-perfusion matching. Patient 4 had no measurable right-to-left shunt in either position, but perfusion of areas with low ventilation-perfusion ratios (0.01 to 0.1) decreased when the “good” side was dependent (down).
The proposed mechanisms which define gas exchange in patients with unilateral pulmonary disease are primarily based on studies of normal lungs involving awake or anesthetized-paralyzed volunteers. Most investigators have speculated that the improvement in oxygenation noted when the abnormal lung is nondependent (up) is the result of improvement in ventilation-perfusion equality or shunt or both; however, the specific mechanisms remain unclear. In this study of patients with unilateral pulmonary disease, we verified the underlying causes responsible for improved oxygenation noted when the abnormal lung is up; however, the series is small and may not explain all mechanisms of improved gas exchange in patients with unilateral pulmonary disease. Dream of becoming a doctor may become true if you read the information published there – All news that you need at Canadian health care website.
Studies on the influence of body position on gas distribution in normal lungs have shown that differences between the awake and anesthetized-paralyzed state are attributable to changes in the mechanical properties of the respiratory system. Inspiratory gas distribution in awake spontaneously breathing subects lying in the lateral decubitus position is preferential to the dependent (down) lung at volumes above functional residual capacity, but with anesthesia-paral-ysis and mechanical ventilation, preferential gas distribution is to the nondependent (up) lung. This is because of the differences in impedance between the two hemithoraces. In the awake state the vertical pleural pressure gradient favors gas distribution to the dependent zone because of higher compliance and larger excursion of the diaphragm. With mechanical ventilation after anesthesia-paralysis, diaphragmatic movement is passive and has greater displacement in the nondependent zone, and the dependent hemi-thorax is less compliant. In this study, patients were mechanically ventilated but did not receive anesthesia-paralysis; therefore, the chest wall and diaphragm would play a major role in gas distribution relative to the difference in impedance between the two hemi-thoraces.
In contrast to gas distribution, blood flow to the lungs is mainly determined by gravitational forces, with the greatest flow to the dependent regions. Blood flow may also be influenced by nongravity-related (actors, but these are still not completely defined. Distribution of pulmonary perfusion can be influenced by changes in cardiac output and by alveolar pressure. Although hypoxemia has been shown to influence regional blood flow distribution, gravitational forces are not overcome by hypoxemia in the lateral decubitus position, and flow is not directed to the good lung when it is nondependent (up).
The distribution of ventilation-perfusion ratios by the multiple inert-gas technique has shown, not surprisingly, that the causes for improved gas exchange with change in body position are not clearly defined by clinical findings or laboratory data. Quantitation of the ventilation-perfusion distribution showed no consistent changes in pattern between patients; that is, the changes in gas exchange were not solely attributable to either improvement in the matching of ventilation and perfusion or to a decrease in right-to-left intrapulmo-nary shunting.
All patients had a bimodal ventilation-perfusion pattern when the good side was down. This is in accord with previous observations in patients whose lungs were mechanically ventilated. In two patients the biomodal pattern disappeared when the “good” side was up. In these patients (patients 2 and 3), a broad-based pattern of ventilation-perfusion ratios existed which may be the result of chronic obstructive pulmonary disease in the “good” lung.
Nevertheless, the pattern of ventilation-perfusion distribution (that is, bimodal vs broad-based pattern when the “good” side was up) did not relate to the causes responsible for the improvement in gas exchange. The major cause for improved oxygenation was a decrease in right-to-left shunting in patients 1 and 3 and improved ventilation-perfusion matching in patients 2 and 4. In patient 2, the percentage of shunt increased slightly when the “good” side was dependent; however, combined shunt and very low ventila-tion-perfusion ratio (V/Q) (<0.01) decreased from 16.8 percent to 14.9 percent. Changes in cardiac output or dead space did not contribute to the changes in gas exchange.
In conclusion, we verified the causes by which improved oxygenation occurs when the “good” side is dependent (down) in patients with unilateral pulmonary disease. Distribution of ventilation-perfusion ratios determined by the multiple inert-gas technique shows variability between patients but indicates that either improved right-to-left shunt or improved ven-tilation-perfusion matching is predominant. This variability may result from the nonhomogeneity of pulmonary disease among patients; however, both mechanisms respond the same clinically with oxygenation, improving when the “good” lung is dependent (down).
Table 1—Characteristics of Patients with Respiratory Failure Who Required Mechanical Ventilation and Findingp on Chest Roentgenogram
|1/83||Aspiration, right lung||Infiltrate, right lung||IMV 8||0.52||0|
aneurysmInfiltrate, left lungA/C 80.5133/76Postoperative left upper lobectomy, retained secretionsInfiltrate, left lungA/C 140.5054/68Postoperativerepair,thoraco
aneurysmInfiltrate, atelectasis, left lungIMV 80.403
Table 2—Arterial Blood Gas Levels, Ventilation, and Hemodynamic Variables
|Case and Pbsition of “Good” Side||PaO*, mm HG||PaC02, mm HG||0.L/min||Ve,L/min||Mean Pressure, mm Hg|
Table 3—Effect of Body Position on Va/Q Distribution Relative to Blood Flow and Ventilation
|Case and Position of “Good” Side||PaOj/FIOj||Blood Flow||Ventilation|
|Qs/Q.percent||MeanVa/Q||In SD||Vd/Vt,percent||MeanVa/Q||In SD|
Figure 1. Distribution of ventilation-perfusion ratios for four patients with respiratory failure and unilateral pulmonary disease on chest roentgenogram, with “good” side nondependent (up) (a) or dependent (down) (b).
Figure 2. Retention and excretion vs blood-gas partition coefficient for six inert gases for four patients with respiratory failure and unilateral pulmonary disease on chest roentgenogram, with “good” side nondependent (up) (a) or dependent (down) (b).