Margareta Mure

Dramatic effect on oxygenation in patients with severe acute lung insufficiency treated in the prone position.

OBJECTIVE To confirm the positive effect of prone positioning on oxygenation in patients with acute lung insufficiency. DESIGN Clinical follow-up study. SETTING The intensive care unit at a tertiary care academic hospital. PATIENTS Thirteen patients suffering from severe acute lung insufficiency caused by trauma, septicemia, aspiration, and burn injury. Eleven of the patients had severe hypoxia (oxygenation indices [PaO2/FIO2] < or = 80 torr [< or = 10.7 kPa]). Patients > 70 yrs of age were excluded from the study. INTERVENTIONS Treatment in the prone position without changing other ventilatory settings than FIO2 when saturation increased. MEASUREMENTS AND MAIN RESULTS Twelve of the 13 patients responded to treatment in the prone position. The patient that did not respond improved her gas exchange when nitric oxide was instituted. She died, however, from a Gram-negative septicemia. No patient needed extracorporeal membrane oxygenation. Apart from the settings of FIO2 when saturation increased, the ventilatory settings were unchanged. In the prone position, the oxygenation index increased (p < .0002) and the alveolar-arterial oxygen gradient, P(A-a)O2, decreased dramatically (p < .0001). CONCLUSIONS The prone position significantly improves impaired gas exchange due to severe acute lung insufficiency. It is suggested that this treatment is used before more complex modalities.

Posture primarily affects lung tissue distribution with minor effect on blood flow and ventilation

We used quantitative single photon emission computed tomography to estimate the proportion of the observed redistribution of blood flow and ventilation that is due to lung tissue shift with a change in posture. Seven healthy volunteers were studied awake, breathing spontaneously. Regional blood flow and ventilation were marked using radiotracers that remain fixed in the lung after administration. The radiotracers were administered in prone or supine at separate occasions, at both occasions followed by imaging in both postures. Images showed greater blood flow and ventilation to regions dependent at the time of imaging, regardless of posture at radiotracer administration. The results suggest that a shift in lung parenchyma has a major influence on the imaged distributions. We conclude that a change from the supine to the prone posture primarily causes a change in the vertical distribution of lung tissue. The effect on the vertical distribution of blood flow and ventilation within the lung parenchyma is much less.

Lung Ventilation and Perfusion in Prone and Supine Postures with Reference to Anesthetized and Mechanically Ventilated Healthy Volunteers

Background:The literature on ventilation (V) and lung perfusion (Q) distributions during general anesthesia and controlled mechanical ventilation in supine and prone position is contradictory. The authors aimed to investigate whether V, Q, and ventilation to perfusion ratio (V/Q ratio) matching in anesthetized and mechanically ventilated volunteers are gravity dependent irrespective of posture. Methods:Seven healthy volunteers were studied at two different occasions during general anesthesia and controlled mechanical ventilation. One occasion studied ventral to dorsal V and Q distributions in the supine posture and the other in the prone posture. Imaging was performed in supine posture at both occasions. A dual radiotracer technique and single photon emission computed tomography were used. V and Q were simultaneously tagged with 99mTc-Technegas (Tetley Manufacturing Ltd., Sydney, Australia) and 113mIn-labeled macroaggregates of human albumin (TechneScan LyoMAA, Mallinckrodt Medica, Petten, The Netherlands), respectively. Results:No differences in V between postures were observed. Q differed between postures, being more uniform over different lung regions in prone posture and dependent in supine posture. The contribution of the vertical direction to the total V/Q ratio heterogeneity was larger in supine (31.4%) than in prone (16.4%) (P = 0.0639, two-tailed, paired t test) posture. Conclusions:During mechanical ventilation, prone posture favors a more evenly distributed Q between lung regions. V distribution is independent of posture. This results in a tendency toward lower V/Q gradients in the ventral to dorsal direction in prone compared with supine posture.

Regional lung blood flow and ventilation in upright humans studied with quantitative SPECT

We used quantitative Single Photon Emission Computed Tomography (SPECT) to study the effect of the upright posture on regional lung blood flow and ventilation. Nine (upright) plus seven (prone and supine) healthy volunteers were studied awake, breathing spontaneously. Regional blood flow and ventilation were marked in sitting upright, supine and prone postures using (113m)In-labeled macroaggregates and inhaled Technegas ((99m)Tc); both remain fixed in the lung after administration. All images were obtained while supine. In comparison with horizontal postures, both blood flow and ventilation were greater in caudal regions when upright. The redistribution was greater for blood flow than for ventilation, resulting in decreasing ventilation-to-perfusion ratios down the lung when upright. We conclude that gravity redistributes regional blood flow and ventilation in the upright posture, while the influence is much less in the supine and prone postures.

Pulmonary Blood flow does not redistribute in dogs with reposition from supine to left lateral position

Background Recent studies have questioned the classical gravitational model of pulmonary perfusion. Because the lateral position is commonly used during surgery, the authors studied the redistribution of pulmonary blood flow in the left lateral decubitus position using a high spatial resolution technique. Methods Distributions of pulmonary blood flow were measured using intravenously injected 15‐[micro sign]m diameter radioactive‐labeled microspheres in eight halothane‐anesthetized dogs, which were studied in the supine and left lateral decubitus positions in random order. Lungs flushed free of blood were air‐dried at total lung capacity and sectioned into 1,498–2,396 (1.7 cm3) pieces per animal. Radioactivity was measured by a gamma counter, and signals were corrected for piece weight and normalized to mean flow. Results Blood flow to the dependent left lung did not increase, and blood flow to the nondependent right lung did not decrease in the lateral position. The left lung received 39.3 +/‐ 7.0% and 39.2 +/‐ 8.8% (mean +/‐ SD) of perfusion in the supine and left lateral positions, respectively. Detailed assessment of the spatial distributions of pulmonary blood flow revealed the lack of a gravitational gradient of blood flow in the lateral position. The distributions of blood flow did not differ in the supine and left lateral decubitus positions. Conclusions Perfusion to each lung did not change with movement from the supine to the left lateral position. These findings contradict the prediction of increased dependent lung and decreased nondependent lung blood flow based on the gravitational model. It was concluded that the distribution of blood flow in the lateral position in dogs is dominated by pulmonary vascular structure.

Positive End-expiratory Pressure Redistributes Regional Blood Flow and Ventilation Differently in Supine and Prone Humans

Background:Animal studies have demonstrated an interaction between posture and the effect of positive end-expiratory pressure (PEEP) on regional ventilation and lung blood flow. The aim of this study was to explore this interaction in humans. Methods:Regional lung blood flow and ventilation were compared between mechanical ventilation with and without PEEP in the supine and prone postures. Six normal subjects were studied in each posture. Regional lung blood flow was marked with 113mIn-labeled macroaggregates and ventilation with Technegas (99mTc). Radiotracer distributions were mapped using quantitative single-photon emission computed tomography. Results:In supine subjects, PEEP caused a similar redistribution of both ventilation and blood flow toward dependent (dorsal) lung regions, resulting in little change in the V/Q correlation. In contrast, in prone subjects, the redistribution toward dependent (ventral) regions was much greater for blood flow than for ventilation, causing increased V/Q mismatch. Without PEEP, the vertical ventilation-to-perfusion gradient was less in prone postures than in supine, but with PEEP, the gradient was similar. Conclusions:During mechanical ventilation of healthy volunteers, the addition of PEEP, 10 cm H2O, causes redistribution of both lung blood flow and ventilation, and the effect is different between the supine and prone postures. Our results suggest that the addition of PEEP in prone might be less beneficial than in supine and that optimal use of the prone posture requires reevaluation of the applied PEEP.

Dosing Oxygen: A Tricky Matter or a Piece of Cake?

S omething is burning in my chest,” the healthy volunteer complains after breathing 100% oxygen for 24 h. “There is an inflammation in the trachea. It is all red,” the bronchoscopist concludes. Classical observations explained in a classical paper on “The Toxicity of Oxygen” by Winter and Smith (1). Just as there are side effects with various invasive interventions and drug treatments, there are also complications with oxygen. Tissue oxygen toxicity is one example, and development of pulmonary atelectasis after acute use of 100% oxygen is another. This issue of Anesthesia & Analgesia presents two interesting investigations concerned with the acute use of high-inspired oxygen concentrations and postoperative complications in the near postanesthesia period (2,3). The studies demonstrate a negative influence on lung function after tracheal extubation preceded by a short period of 100% oxygen breathing—an observation of significant clinical importance because leading postoperative complications today, particularly in the elderly, are pulmonary. One of the studies is based on an experimental design evaluating ventilation/perfusion matching using the multiple inert gas technique. It shows less efficient gas exchange after 100% oxygen breathing before extubation. The other study was performed in patients using the computed tomography scanner to detect increased pulmonary atelectasis at 100% oxygen. Certainly, the two investigations suggest that postoperative pulmonary complications could, at least to some extent, be reduced if routines for 100% oxygen just before extubation were modified. The two studies are not concerned with the longterm (up to 24 h) use of 100% oxygen, when more than 70% of individuals develop respiratory problems (1). They do not involve retinopathy of prematurity, nor do they highlight patients with chronic obstructive lung disease and hypercarbia. They concern the frequent acute use of 100% oxygen for only brief periods during which time no signs of tissue toxicity have been described. However, there have been several reports on the development of pulmonary atelectasis after breathing 100% oxygen before endotracheal intubation at the induction of anesthesia (4,5). These series put forward careful conclusions suggesting lower inspired oxygen concentrations than before practiced to prevent atelectasis formation early during anesthesia and surgery. Alternatively, effective recruitment of lung tissue is advocated immediately after the intubation or whenever 100% oxygen has been used (6). Similar problems are now described for emergence from anesthesia. Patients do get atelectasis from a few minutes of 100% oxygen breathing. To compensate by doing a recruitment maneuver, up to 40 cm H2O, in a recently extubated patient is definitely not a piece of cake. It seems logical just to change the routines; the knowledge is there! Yet, the tradition is long lived, and we still frequently practice 100% oxygen before extubation. Why is this routine so persistent? First, anesthesiologists actively cause prolonged pharmacologicinduced apnea for which we have to take full responsibility. Second, experience tells us that we sometimes have a patient that just cannot be ventilated, in which case the extra minutes provided by hyperoxygenation are appreciated. Because the functional residual capacity of the lung is the volume that contains the oxygen reserve, the following reasoning might be helpful: Let us assume a functional residual capacity volume of 2500 mL. At normal air breathing this container holds 500 mL of oxygen. After breathing 100% oxygen for approximately 5 min, you have approximately 2500 mL of oxygen. At an oxygen consumption of 200 mL/min, the oxygen supply is used up after about 2 min breathing air and 10 min later after hyperoxygenation. It is these extra 10 min that support the safetyfirst strategy, justifying maintained routines, during the induction of anesthesia. At the other end, i.e., emergence from anesthesia, there are fewer chances of being completely surprised by a patient that cannot be ventilated. Accepted for publication July 31, 2002. Address correspondence and reprint requests to Sten G.E. Lindahl, MD, PhD, FRCA, Department of Anesthesiology and Intensive Care Medicine, Karolinska Hospital and Institute, SE-171 76 Stockholm, Sweden. Address e-mail to sten.lindahl@kirurgi.ki.se.

High continuous positive airway pressure level induces ventilation/perfusion mismatch in the prone position.

ObjectiveGas exchange in patients with adult respiratory distress syndrome is influenced by posture. The combined effect of continuous positive airway pressure and posture has not been investigated. We studied the effect of normal spontaneous breathing, and that of continuous positive airway pressure, on ventilation/perfusion distributions in healthy volunteers while they were in supine and prone positions. SettingNuclear medicine department in a university hospital. DesignExperimental study. SubjectsSixteen healthy volunteers. InterventionsIn the supine or prone position, the subjects inhaled a technetium-labeled aerosol (technetium-99m diethylenetriamine pentaacetic acid) through a tight-fitting mask. Single photon emission computed tomography images of the lungs were obtained. The subjects then received an intravenous injection of technetium-99m-labeled macroaggregates of albumin, and an identical single photon emission computed tomography imaging was performed. In the group that received continuous positive airway pressure, an end-expiratory pressure of 10 cm H2O was applied during both inhalation and injection. Measurements and Main Results During spontaneous breathing, ventilation/perfusion distribution assessed by regression analysis was uniform (i.e., not significantly different from zero) both in supine and prone positions, with a slope of −1.5 ± 3.5%/cm supine and 1.5 ± 3.5%/cm prone. During continuous positive airway pressure breathing in the supine position, ventilation/perfusion had a slope of −3.4 ± 2.4 compared with 8.3 ± 1.1%/cm in the prone position according to analysis of spatial resolution. ConclusionThere was a less favorable ventilation/perfusion ratio in the prone position when the subjects were exposed to continuous positive airway pressure of 10 cm H2O.

Protective effect of prone posture against hypergravity-induced arterial hypoxaemia in humans

Patients with acute respiratory distress syndrome have increased lung tissue weight and therefore an increased hydrostatic pressure gradient down the lung. Also, they have a better arterial oxygenation in prone (face down) than in supine (face up) posture. We hypothesized that this effect of the direction of gravity also existed in healthy humans, when increased hydrostatic gradients were induced by hypergravity. Ten healthy subjects were studied in a human centrifuge while exposed to 1 or 5 G in anterio‐posterior (supine) or posterio‐anterior (prone) direction. We measured blood gases using remote‐controlled sampling and gas exchange by mass spectrometry. Hypergravity led to marked impairments of arterial oxygenation in both postures and more so in supine posture. At 5 G, the arterial oxygen saturation was 84.6 ± 1.2 % (mean ±s.e.m.) in supine and 89.7 ± 1.4 % in prone posture (P < 0.001 for supine vs. prone). Ventilation and alveolar PO2 were increased at 5 G and did not differ between postures. The alveolar‐to‐arterial PO2 difference increased at 5 G to 8.0 ± 0.2 kPa and 6.6 ± 0.3 kPa in supine and prone postures (P= 0.003). Arterial oxygenation was less impaired in prone during hypergravity due to a better‐preserved alveolo‐arterial oxygen transport. We speculate that mammals have developed a cardiopulmonary structure that favours function with the gravitational vector in the posterio‐anterior direction.

Inhalation anesthesia increases V/Q regional heterogeneity during spontaneous breathing in healthy subjects.

Background:The underlying mechanism for the increased alveolar-arterial oxygen tension difference resulting from almost all forms of general anesthesia is unknown. We hypothesized that inhalation anesthesia influences the intrapulmonary distribution of ventilation (V) and perfusion (Q), leading to less advantageous V/Q matching. Methods:Ten healthy volunteers were studied in supine position on two separate occasions, once awake and once during mild anesthesia (sevoflurane inhalation) with maintained spontaneous breathing. On both occasions, the distribution of V and Q were simultaneously imaged using single photon emission computed tomography. V was tagged with [99mTc]-labeled carbon particle aerosol and Q with [113mIn]-labeled macroaggregates of human albumin. Atelectasis formation during anesthesia was prevented using low concentrations of oxygen in inhaled air. Results:Mean V and Q distributions in the ventral-to-dorsal direction, measured in 20 equally spaced volumes of interest and in three regions of interest of equal volume, did not differ between conditions. Anesthesia, when compared with the awake state, significantly decreased the total heterogeneity of the Q distribution (P = 0.002, effect size 1.16) but did not alter V (P = 0.37, effect size 0.41). The corresponding V/Q total heterogeneity was higher under anesthesia (P = 0.002, effect size 2.64). Compared to the awake state, the V/Q frequency distribution under anesthesia became wider (P = 0.009, 1.76 effect size) with a tendency toward low V/Q ratios. Conclusion:Inhalation anesthesia alone affects Q but not V, suggesting that anesthesia has a direct effect on the active regulatory mechanism coordinating Q with V, leading to less favorable V/Q matching.