Prone position improves lung mechanical behavior and enhances gas exchange efficiency in mechanically ventilated chronic obstructive pulmonary disease patients

Chest wall loading in the ICU: pushes, weights, and positions

Clinicians monitor mechanical ventilatory support using airway pressures—primarily the plateau and driving pressure, which are considered by many to determine the safety of the applied tidal volume. These airway pressures are influenced not only by the ventilator prescription, but also by the mechanical properties of the respiratory system, which consists of the series-coupled lung and chest wall. Actively limiting chest wall expansion through external compression of the rib cage or abdomen is seldom performed in the ICU. Recent literature describing the respiratory mechanics of patients with late-stage, unresolving, ARDS, however, has raised awareness of the potential diagnostic (and perhaps therapeutic) value of this unfamiliar and somewhat counterintuitive practice. In these patients, interventions that reduce resting lung volume, such as loading the chest wall through application of external weights or manual pressure, or placing the torso in a more horizontal position, have unexpectedly improved tidal compliance of the lung and integrated respiratory system by reducing previously undetected end-tidal hyperinflation. In this interpretive review, we first describe underappreciated lung and chest wall interactions that are clinically relevant to both normal individuals and to the acutely ill who receive ventilatory support. We then apply these physiologic principles, in addition to published clinical observation, to illustrate the utility of chest wall modification for the purposes of detecting end-tidal hyperinflation in everyday practice.

Regional lung viscoelastic properties in supine and prone position in a porcine model of acute respiratory distress syndrome

Regional viscoelastic properties of thoracic tissues in acute respiratory distress syndrome (ARDS) and their change with position and positive end-expiratory pressure (PEEP) are unknown. In an experimental porcine ARDS, dorsal and ventral lung (R₂,L and E₂,L) and chest wall (R₂,cw and E₂,cw) viscoelastic resistive (R) and elastic (E) parameters were measured at 20, 15, 10, and 5 cmH₂O PEEP in supine and prone position. E₂ and R₂ were obtained by fitting the decay of pressure after end-inspiratory occlusion to the equation: P_viscmax (t) =R₂ e^-t/τ₂, where t is the length of occlusion and τ₂ time constant. E₂ was equal to R₂/τ₂. R₂,cw and E₂,cw were measured from esophageal, dorsal, and ventral pleural pressures. Global R₂,L and E₂,L were obtained from the global transpulmonary pressure (airway pressure-esophageal pressure), and regional R₂,L and E₂,L from the dorsal and ventral airway pressure-pleural pressure difference. Lung ventilation was measured by electrical impedance tomography (EIT). Global R₂,cw and E₂,cw did not change with PEEP or position. Global R₂,L [median(Q1-Q3)] was 37.1 (11.0-65.1), 5.1 (4.3-5.5), 12.1 (8.4-19.5), and 41.0 (26.6-53.5) cmH₂O/L/s in supine, and 15.3 (9.1-41.9), 7.9 (5.7-11.0), 8.0 (5.1-12.1), and 12.9 (6.4-19.4) cmH₂O/L in prone from 20 to 5 cmH₂O PEEP (P = 0.06 for PEEP and P = 0.06 for position). Dorsal R₂,L significantly and positively correlated with the amount of collapse measured with EIT. Global and regional lung and chest wall viscoelastic parameters can be described by a simple rheological model. Regional E₂ and R₂ were uninfluenced by PEEP and position except for PEEP on dorsal E₂,L and position on dorsal E₂,cw.NEW & NOTEWORTHY In a porcine model of acute respiratory distress syndrome, data were successfully fitted to a rheological model of the nonlinear behavior of viscoelastic properties of lung and chest wall at different positive end-expiratory pressure (PEEP) in the supine and prone position. Prone position tended to decrease lung viscoelastic resistive component. PEEP had a significant effect on dorsal lung viscoelastic elastance. Finally, lung viscoelastic resistance correlated with the amount of lung collapse assessed by electrical impedance tomography.

Cerebral Oxygenation Under General Anesthesia Can Be Safely Preserved in Patients in Prone Position: A Prospective Observational Study

BACKGROUND

The effects of prone position (PP) on cerebral tissue metabolism are not well known. The aim of this investigation was to evaluate regional cerebral oxygen desaturation in patients undergoing lumbar spine surgery in PP during routine anesthesia management.

MATERIALS AND METHODS

Between July 2013 and October 2013, 50 consecutive patients undergoing lumbar spine surgery under general anesthesia in PP were enrolled. The anesthetic technique was standardized. Using near-infrared spectroscopy, bilateral regional cerebrovascular oxygen saturation was recorded during the surgery.

RESULTS

After 30 and 60 minutes of prone repositioning, significant decreases in bilateral regional cerebral oxygen saturation were observed compared with the values in the supine position (from 76.24% to 73.18% at 30 min and 72.76% at 60 min on the right side and from 77.06% to 73.76% at 30 min and 72.92% at 60 min on the left side; P<0.05). These changes were not clinically important and returned to supine values after 90 minutes of prone positioning. Decreases in cerebral oxygen saturation were accompanied by reductions in heart rate and mean arterial pressure (P<0.05). Older age and higher perioperative risk had a significant effect on the reduction of cerebral oxygen values (P<0.05).

CONCLUSIONS

The results of our study show that margin of safety against impaired cerebral oxygenation can be maintained in PP. Preventing bradycardia and arterial hypotension is crucial. Older patients and those at higher perioperative risk need more meticulous attention.

Prone positioning for intramedullary nailing of subtrochanteric fractrures, the techniques of intraoperative fluoroscopy and reduction: A technique note

The treatment of subtrochanteric fractures is a challenge for orthopaedic trauma surgeons. Three positions have been described previously: supine on a fracture table, supine on a flat radiolucent table, and the lateral decubitus position on a flat radiolucent table. Each one has its advantages and limitations. In this article we describe a prone position for intramedullary nailing of subtrochanteric femoral fractures. This position has the advantages including: 1) an easy approach to reduce and maintain the reduction of fracture by adjusting only the leg plate on injured side, 2) perfect intraoperation fluoroscopic imaging on both anteroposterior view and lateral view, and 3) an easy approach to establish an appropriate entry point even in obese patients.

The Effects of Prone with Respect to Supine Position on Stress Relaxation, Respiratory Mechanics, and the Work of Breathing Measured by the End-Inflation Occlusion Method in the Rat

PURPOSE

The working hypothesis is that the prone position with respect to supine may change the geometric configuration of the lungs inside the chest wall, thus their reciprocal mechanical interactions, leading to possible effects on stress relaxation phenomena and respiratory mechanics.

METHOD

The effects of changing body posture from supine to prone on respiratory system mechanics, particularly on stress relaxation, were investigated in the rat by the end-inflation occlusion method.

RESULTS

In the prone with respect to supine position, an increment of the frictional resistance of the airway (from 0.13 ± 0.01 to 0.19 ± 0.02 cm H2O/l sec(-1), p < 0.05) and a decrement of the stress relaxation-linked pressure dissipation (from 0.51 ± 0.05 to 0.45 ± 0.05 cm H2O/l sec(-1), p < 0.01) were found. Respiratory system elastance and total resistive pressure dissipation did not change significantly. Accordingly, a significant increase of the frictional "ohmic" mechanical inspiratory work of breathing and a decrease of the visco-elastic work of inspiration were demonstrated, while no significant changes occurred for the total mechanical work of breathing and its total resistive and elastic components.

CONCLUSION

It is concluded that postural changes affect the visco-elastic characteristics of the respiratory system and the related stress relaxation phenomena by influencing the disposition and relation of the lungs inside the chest wall and their relative geometrical configuration, and the interaction phenomena of the constitutive parenchymal structures, i.e., elastin and collagen fibers. Since the prone position resulted in no serious or disadvantageous respiratory system mechanical derangement, it is suggested it may be usefully applied in nursing or for therapeutic goals.

Prone positioning in hypoxemic respiratory failure: meta-analysis of randomized controlled trials

PURPOSE

Prone positioning is used to improve oxygenation in patients with hypoxemic respiratory failure (HRF). However, its role in clinical practice is not yet clearly defined. The aim of this meta-analysis was to assess the effect of prone positioning on relevant clinical outcomes, such as intensive care unit (ICU) and hospital mortality, days of mechanical ventilation, length of stay, incidence of ventilator-associated pneumonia (VAP) and pneumothorax, and associated complications.

METHODS

We used literature search of MEDLINE, Current Contents, and Cochrane Central Register of Controlled Trials. We focused only on randomized controlled trials reporting clinical outcomes in adult patients with HRF. Four trials met our inclusion criteria, including 662 patients randomized to prone ventilation and 609 patients to supine ventilation.

RESULTS

The pooled odds ratio (OR) for the ICU mortality in the intention-to-treat analysis was 0.97 (95% confidence interval [CI], 0.77-1.22), for the comparison between prone and supine ventilated patients. Interestingly, the pooled OR for the ICU mortality in the selected group of the more severely ill patients favored prone positioning (OR, 0.34; 95% CI, 0.18-0.66). The duration of mechanical ventilation and the incidence of pneumothorax were not different between the 2 groups. The incidence of VAP was lower but not statistically significant in patients treated with prone positioning (OR, 0.81; 95% CI, 0.61-1.10). However, prone positioning was associated with a higher risk of pressure sores (OR, 1.49; 95% CI, 1.17-1.89) and a trend for more complications related to the endotracheal tube (OR, 1.30; 95% CI, 0.94-1.80).

CONCLUSIONS

Despite the inherent limitations of the meta-analytic approach, it seems that prone positioning has no discernible effect on mortality in patients with HRF. It may decrease the incidence of VAP at the expense of more pressure sores and complications related to the endotracheal tube. However, a subgroup of the most severely ill patients may benefit most from this intervention.

Effect of body position on lung sounds in healthy young men

BACKGROUND

The effect of body position on the generation of abnormal respiratory sounds (eg, snoring and stridor) is well recognized. Postural effects on normal lung sounds have been studied in less detail but need to be clarified if respiratory acoustic measurements are to be used effectively in clinical practice.

METHODS

Lung sounds and airflow were recorded in six healthy male subjects. Two acoustic sensors were placed over corresponding sites of the right and left chest, first anteriorly and then on the back. Subjects were studied in sitting, supine, prone, and lateral decubitus positions. Lung sound intensity (LSI) was determined at flows of 0.4 to 0.6 L/s and 0.8 to 1.2 L/s within frequency bands of 150 to 300 Hz and 300 to 600 Hz.

RESULTS

LSI was greater over the dependent lungs in the lateral decubitus positions. In the sitting position, LSI was greater on the left compared with the right posterior lung at the same airflow within the same frequency bands. Compared with sitting, neither the supine nor prone positions caused a significant change in LSI.

CONCLUSIONS

Our study confirms previously reported asymmetries of normal lung sounds. The insignificant change of flow-specific LSI between the upright and horizontal positions in healthy subjects is encouraging for the clinical use of respiratory acoustic measurements. Further studies should address postural effects on lung sounds in patients with acute lung injury and other lung pathologies.

Viscoelastic properties of lungs and thoracic wall of anesthetized mechanically ventilated piglets

OBJECTIVE

To investigate the viscoelastic properties of lungs and thoracic wall in piglets.

STUDY DESIGN

Prospective experimental study.

ANIMALS

Six piglets weighting 30 kg.

METHODS

Animals were tracheotomized, anesthetized and mechanically ventilated under controlled conditions. After control measurements of the mechanical properties of the lung of the pigs had been taken, acute lung injury (ALI) was induced by saline lavage. Lung and thoracic wall tissue resistance (DeltaR), which reflects viscoelastic properties and/or time constant inequalities, were determined by using a rapid airway occlusion technique during constant flow inflation (V), at constant tidal volume. was varied from 0.1-0.2 to 1.2 L second(-1) on a single breath. Multiple data sets of DeltaR of lung (DeltaR(L)) and thoracic wall (DeltaR(w)) to inspiratory time (T(I) = V(T)/V) were fitted to a model whose prediction equation was DeltaR = R(2)[1 -exp(-T(I)/tau(2))], where R(2) and tau(2) are the 'viscoelastic' resistance and time constant, respectively. Subscripts (L) and (W) are used to represent lung and thoracic wall, respectively (R(2L), R(2W), tau(2L), tau(2W)). Two more sets of physiological measurements were then taken--the first under zero end-expiratory pressure (ZEEP) and the second under a positive end-expiratory pressure (PEEP) of 10 cmH(2)O.

RESULTS

Data of DeltaR adequately fitted to the prediction equation in all instances. In control, R(2,L) was 15.3 (10.7-22.6) cmH(2)O L(-1) second(-1) (median, interquartile range), tau(2,L) 3.3 (1.9-5.5) seconds, R(2,w) 6.5 (2.2-10.3) cmH(2)O L(-1) second(-1) and tau(2,w) 2.9 (1.1-4.3) seconds. In ALI, R(2,L) significantly increased to 129.6 (105.9-171.3) cmH(2)O L(-1) second(-1) on ZEEP but not significantly decreased to 48.9 (17.8-109.6) cmH(2)O L(-1) second(-1) with PEEP. The corresponding values of tau(2,L) were 7.1 (5.1-11.6) and 4.4 (3.1-5.5) seconds. The values pertaining to thoracic wall did not change significantly among conditions.

CONCLUSIONS AND CLINICAL RELEVANCE

Viscoelastic properties of the lung and thoracic wall in piglets can be described by a viscoelastic model. Values of parameters of this model were markedly increased in ALI and decreased with PEEP.

Does gas exchange response to prone position predict mortality in hypoxemic acute respiratory failure?

OBJECTIVE

To determine whether gas exchange response to a first prone position session can predict patient outcome in hypoxemic acute respiratory failure.

METHODS

Data from a previous multicenter randomized controlled trial were retrospectively analyzed for relationship between PaO(2)/FIO(2) ratio and PaCO(2) changes during the first 8-h prone position session to day 28 mortality rate; 370 prone position sessions were analyzed. Arterial blood gas was measured in supine position before proning and in prone position at the end of the session. Gas exchange improvement was defined as increase in the PaO(2)/FIO(2) ratio of more than 20% (PaO(2)R) or decrease in PaCO(2) of more than 1 mmHg (PaCO(2)R).

MAIN RESULTS

The 28-day mortality rate was 26.5% in PaO(2)R-PaCO(2)R, 31.7% in PaO(2)R-PaCO(2)NR, 38.9% in PaO(2)NR-PaCO(2)R, and 43% in PaO(2)NR-PaCO(2)NR (log-rank 14.02, p = 0.003). In a Cox proportional hazards model the gas exchange response was a significant predictor to patient outcome with a 82.5% increase in risk of death in the case of PaO(2)NR-PaCO(2)R or PaO(2)NR-PaCO(2)NR, relative to the gas exchange improvement response (odds ratio 1.825). However, after adjusting for the difference in oxygenation between day 2 and day 1 the gas exchange response does no longer reach significance.

CONCLUSION

In patients with hypoxemic acute respiratory failure initial improvement in gas exchange in the first PP session was associated with a better outcome, but this association disappeared when the change in oxygenation from day 1 to day 2 was taken into account, suggesting that underlying illness was the most important predictor of mortality in this patient population.

Effects of thoraco-pelvic supports during prone position in patients with acute lung injury/acute respiratory distress syndrome: a physiological study

Introduction

This study sought to assess whether the use of thoraco-pelvic supports during prone positioning in patients with acute lung injury/acute respiratory distress syndrome (ALI/ARDS) improves, deteriorates or leaves unmodified gas exchange, hemodynamics and respiratory mechanics.

Methods

We studied 11 patients with ALI/ARDS, sedated and paralyzed, mechanically ventilated in volume control ventilation. Prone positioning with or without thoraco-pelvic supports was applied in a random sequence and maintained for a 1-hour period without changing the ventilation setting. In four healthy subjects the pressures between the body and the contact surface were measured with and without thoraco-pelvic supports. Oxygenation variables (arterial and central venous), physiologic dead space, end-expiratory lung volume (helium dilution technique) and respiratory mechanics (partitioned between lung and chest wall) were measured after 60 minutes in each condition.

Results

With thoraco-pelvic supports, the contact pressures almost doubled in comparison with those measured without supports (19.1 ± 15.2 versus 10.8 ± 7.0 cmH₂O, p ≤ 0.05; means ± SD). The oxygenation-related variables were not different in the prone position, with or without thoraco-pelvic supports; neither were the CO₂-related variables. The lung volumes were similar in the prone position with and without thoraco-pelvic supports. The use of thoraco-pelvic supports, however, did lead to a significant decrease in chest wall compliance from 158.1 ± 77.8 to 102.5 ± 38.0 ml/cmH₂O and a significantly increased pleural pressure from 4.3 ± 1.9 to 6.1 ± 1.8 cmH₂O, in comparison with the prone position without supports. Moreover, when thoraco-pelvic supports were added, heart rate increased significantly from 82.1 ± 17.9 to 86.7 ± 16.7 beats/minute and stroke volume index decreased significantly from 37.8 ± 6.8 to 34.9 ± 5.4 ml/m². The increase in pleural pressure change was associated with a significant increase in heart rate (p = 0.0003) and decrease in stroke volume index (p = 0.0241).

Conclusion

The application of thoraco-pelvic supports decreases chest wall compliance, increases pleural pressure and slightly deteriorates hemodynamics without any advantage in gas exchange. Consequently, we stopped their use in clinical practice.

Static pressure volume curves and body posture in acute respiratory failure

OBJECTIVE

In acute respiratory distress syndrome the body posture effects on pressure-volume (PV) curves are still unclear. We examined the effects of prone position on inflation PV curves and their potential relationships with postural alterations in gas exchange.

DESIGN AND SETTING

Prospective study with patients serving as their own controls in a university-affiliated 30-bed intensive care unit.

PATIENTS AND PARTICIPANTS

Thirteen anesthetized, paralyzed, semirecumbent, mechanically ventilated patients with early/severe/diffuse ARDS.

INTERVENTIONS

Sequential body posture changes: preprone semirecumbent, prone, and postprone semirecumbent.

MEASUREMENTS AND RESULTS

In each posture hemodynamics, gas exchange, and lung volumes were determined before/during removal and after restoration of positive end-expiratory pressure (PEEP=10.2+/-0.6 cmH2O). At zero PEEP PV curves of respiratory system, lung, and chest wall were constructed. Prone position vs. preprone semirecumbent resulted in significantly reduced pressure at lower inflection point of lung PV curve (2.2+/-0.2 vs. 3.7+/-0.5 cmH2O) and increased volume at upper inflection point (0.87+/-0.03 vs. 0.69+/-0.05 l). Postural reduction in lower inflection point pressure of lung PV curve was the sole independent predictor of pronation-induced increases in PaO2/FIO2 (R2=0.76). PaO2/FIO2 increases were also significantly related with increases in functional residual capacity (R2=0.60).

CONCLUSIONS

In early/severe/diffuse ARDS prone position reduces lower inflection point pressure and increases upper inflection point UIP volume of the lung PV curve. Lower inflection point pressure reductions explain oxygenation improvements, which are also associated with a postural increase in functional residual capacity.

An evaluation of McCoy balloon laryngoscopy in patients with moderate-to-major endotracheal intubation difficulty

We hypothesized that combined McCoy-balloon laryngoscopy may facilitate airway management relative to McCoy or balloon laryngoscopy. In 10 anesthetized/paralyzed patients with prior intubation difficulty scale scores of >5, McCoy-balloon laryngoscopy versus conventional/balloon/McCoy laryngoscopies resulted in greater laryngeal aperture exposure (2.3 +/- 0.6 versus 0.6 +/- 0.2/1.4 +/- 0.4/1.5 +/- 0.6 cm2, respectively), lower intubation difficulty scale score (0.00 (0.00-0.00) versus 6.00 (6.00-8.25)/1.50(0.00-4.00)/2.00(0.75-5.00), respectively, median [interquartile range]), and 9%-74% shorter time to intubation confirmation (P < 0.05-0.001 for all). Balloon and McCoy laryngoscopies improved laryngoscopic/intubating conditions relative to conventional laryngoscopy. In patients with moderate-to-major conventional airway management difficulty, McCoy-balloon laryngoscopy further improves laryngoscopic/intubating conditions.

IMPLICATIONS

This study shows that, in patients with moderate-to-major conventional airway management difficulty, combined McCoy-balloon laryngoscopy results in improved laryngoscopic/intubating conditions when compared with the conventional, McCoy, and balloon laryngoscopic techniques. McCoy-balloon laryngoscopy combines the merits of McCoy and balloon laryngoscopy and can be recommended for patients with moderate-to-major intubation difficulty.

The effects of nebulized salbutamol, external positive end-expiratory pressure, and their combination on respiratory mechanics, hemodynamics, and gas exchange in mechanically ventilated chronic obstructive pulmonary disease patients

We hypothesized that combined salbutamol and external positive end-expiratory pressure (PEEPe) may present additive benefits in chronic obstructive pulmonary disease (COPD) exacerbation. In 10 anesthetized, mechanically ventilated, and bronchodilator-responsive COPD patients exhibiting moderate intrinsic PEEP (PEEPi), we assessed respiratory system (rs) mechanics, hemodynamics, and gas exchange at (a) baseline (zero PEEPe [ZEEPe]), (b) 30 min after 5 mg of nebulized salbutamol administration (ZEEPe-S), (c) 30 min after setting PEEPe at baseline PEEPi level (PEEPe), and (d) 30 min after 5 mg of nebulized salbutamol administration with PEEPe maintained unchanged (PEEPe-S). Return of determined variable values to baseline values was confirmed before PEEPe application. Relative to ZEEPe, (a) at ZEEP-S, PEEPi (4.8 +/- 0.7 versus 7.0 +/- 1.1 cm H(2)O), functional residual capacity change (115.6 +/- 23.1 versus 202.1 +/- 46.0 mL), minimal rs (airway) resistance (9.3 +/- 1.4 versus 11.8 +/- 2.2 cm H(2)O.L(-1).s(-1)), and additional rs resistance (5.2 +/- 1.4 versus 7.2 +/- 1.3 cm H(2)O.L(-1).s(-1)) were reduced (P < 0.01), and hemodynamics were improved; (b) at PEEPe, PEEPi (3.7 +/- 1.3 cm H(2)O) was reduced (P < 0.01), and gas exchange was improved; and (c) at PEEPe-S, PEEPi (2.0 +/- 1.2 cm H(2)O) was minimized, and rs mechanics (static rs elastance included), hemodynamics, and gas exchange were improved. Conclusively, in carefully preselected COPD patients, bronchodilation/PEEPe exhibits additive benefits.

Short-term effects of prone position in chronic obstructive pulmonary disease patients with severe acute hypoxemic and hypercapnic respiratory failure

OBJECTIVE

To assess the short-term effects of prone positioning (PP) in chronic obstructive pulmonary disease (COPD) patients with severe hypoxemic and hypercapnic respiratory failure requiring invasive mechanical ventilation.

DESIGN AND SETTING

Prospective observational study in the general intensive care unit of a university-affiliated hospital.

PATIENTS

11 consecutive COPD patients with persistent hypoxemia (PaO2/FIO2 < or = 200 mmHg with FIO2 > or = 0.6) and hypercapnia requiring invasive mechanical ventilation. Patients with adult respiratory distress syndrome or left ventricular failure were excluded. Mean age was 73+/-11 years, mean weight 86+/-31 kg, mean SAPS II 53+/-10, and ICU mortality 36%.

INTERVENTIONS

Patients were turned every 6 h.

MEASUREMENTS AND RESULTS

A response to PP (20% or greater PaO2/FIO2 increase) was noted in 9 (83%) patients. Blood gases were measured in the PP and supine (SP) positions 3 h after each turn, for 36 h, yielding six measurement sets (SP1, PP1, SP2, PP2, SP3, and PP3). PaO2/FIO2 was significantly better in PP: 190+/-26 vs. 113+/-9 mmHg for PP1/SP1, 175+/-22 vs. 135+/-16 mmHg for PP2/SP2, and 199+/-24 vs. 151+/-13 mmHg for PP3/SP3. After PP1 PaO2/FIO2 remained significantly improved, and the PaO2/FIO2 improvement from SP1 to SP2 was linearly related to PaO2/FIO2 during PP1 (r=0.8). The tracheal aspirate volume improved significantly from SP1 to PP1. PaCO2 was not significantly affected by position.

CONCLUSIONS

PP was effective in treating severe hypoxemia in COPD patients. The first turn in PP was associated with increased tracheal aspirate.