The effect of positioning on Sa02 is summarized in Figure 2. A mean Sa02 value determined for the 15-minute period during each position was calculated by averaging the three measurements obtained in each position. The mean Sa02 in the sitting position was 95.0±0.3 percent (SE) and fell slightly to 94.3±0.4 percent in the supine position (not significant). When positioned such that the pleural efiusion was uppermost, the mean Sa02 was 94.7 ±0.7 percent (not significant). The Sa02 values were the lowest when subjects were positioned with the pleural efiusion dependent (“down”) (mean Sa02, 93.4 ±0.7 percent). Analysis of variance demonstrated a significant difference in mean Sa02 in the position of “pleural efiusion down,” compared to both the sitting position and “pleural efiusion up” (p<0.05).
To determine if there were any transient changes in Sa02 within the 15-minute monitoring period in each position, we examined the three Sa02 values that were recorded per position. Analysis of variance demonstrated no significant differences between the Sa02 values recorded at 5,10, and 15 minutes for each of the four positions tested (Fig 3). Transient dips in Sa02 immediately following a positional change occurred only once (as discussed subsequently).
We found no consistent relationship between the size of the efiusion and alterations in Sa02 during different positions.
At no time during the study did any patient exhibit signs or symptoms suggestive of hypoxemia or respiratory distress. Large but transient decreases in SaO£ occurred in only one subject immediately following positional changes. During this episode, Sa02 fell from 92 percent and 85 percent when the subject was placed in the position of pleural efiusion down. Within ten seconds the Sa02 rose to 90 percent.
Mean heart rate was 87 ±5 beats per minute (SE) in the sitting position, 85 ±5 beats per minute while in the supine position, 87 ±5 beats per minute when positioned with “pleural efiusion side down,” and 85 ±5 beats per minute when with the “pleural effusion up.” These differences were not statistically significant (f= 1.13). Canadian Neighbor Pharmacy online is the most comfortable way either to order drugs or get to know the latest medical publications.
Mean respiratory rate was 26 ± 2 breaths per minute in the sitting position, 26 ± 2 breaths per minute after assuming the supine position, 24 ± 2 breaths per minute with the “pleural efiusion down,” and 24 ±2 breaths per minute with the “pleural efiusion up.” No significant difference between these respiratory rates were found (f=3.84).
Blood pressure did not significantly change in any body position.
This study indicates that a small decrease in SaOs occurs in patients with normal SaOs when positioned with the pleural efiusion down. Patients with lowered SaO£ might experience greater foils in Sa02 with positional changes; however, this was not evaluated in this study. The decrease of less than 2 percent in mean Sa02 was statistically significant when compared to that found in the sitting position and with the “pleural efiusion up;” however; these differences are not considered clinically significant This view is substantiated by the fact that no increase in heart rate or respiration rate was found with the changes in SaOs.
One previous study examined the effect of positioning on oxygenation in patients with unilateral pleural effusions. That study evaluated PaOa 15 minutes after assuming both the right and left lateral positions. These investigators reported a small decrease in PaOs (5.1 mm Hg) when their subjects were positioned such that the pleural efiusion side was dependent; however, because they evaluated oxygenation only at 15 minutes after assuming that position» a transient larger change in P&02 might have been overlooked. The results of our study appear to dispel that notion, for within each of the four positions tested, transient and large changes in SaOs occurred only once. Furthermore, Sonnenblick et al“ reported that the larger the efiusion, the smaller the percent change in Pa02. In the present study, we could find no relationship between the size of the efiusion and change in Sa02. The reason for this discrepancy is most likely due to the relative stability of SaOa when compared to Pa02. Thus, a relatively wide range of changes in Pa02 produced smaller changes in SaOs. In the present study, these smaller changes in Sa02 do not correlate with the size of the pleural efiusion.
We chose to monitor SaOs, rather than PaOs, because this allowed us to continuously monitor oxygenation of the blood of patients with pleural efiusion in various positions. We believe that the benefits of this noninvasive and accurate oxygen monitoring technique outweigh the information obtained by intermittent measurements of arterial blood gas levels.
We wondered if factors other than the pleural efiiisions could have influenced the SaOa in the patients. Seven out of ten subjects had their efiusion after surgery (six had cardiac surgery and one gastroesophagostomy); however, none of the subjects underwent unilateral thoracotomy, which might result in decreased ventilation on the affected side. All had midline incisions and no radiographic evidence of unilateral pulmonary involvement.
In summary, we conclude that only small decrements in SaOs occur when patients with normal oxygenation are positioned such that the side with the larger pleural efiusion is dependent We were unable to demonstrate a relationship between the size of the efiusion and amount of change in Sa02with positional changes. Those with oxyhemoglobin desaturation while upright might demonstrate a different postural effect, but this was not evaluated in the present study. Additional studies need to be conducted to assess the effect of positional changes on more critically ill patients with pleural efiusions. It would be important to better understand the response of patients who are hypoxemic in the “favorable” position.
Figure 2. Mean SaOs (± SE) was significantly lower when subjects were positioned with pleural effusion down, compared to SaOa in sitting position or with pleural efiusion up.
Figure 3. There were no significant differences between mean Sa02 values recorded at 5, 10, and 15 minutes for each of four positions tested. NS, Not significant.