Peak volume history and peak pressure-volume curve pressures independently affect the shape of the pressure-volume curve of the respiratory system

OBJECTIVE: To determine the specific effect of peak volume history pressure on the inflation limb of the pressure-volume curve and peak pressure-volume curve pressure on the deflation limb of the pressure-volume curve. DESIGN: Prospective assessment of pressure-volume curves in saline, lung lavage injured sheep. SETTING: Large animal laboratory of a university-affiliated hospital. SUBJECTS: Eight female Dorset sheep. INTERVENTIONS:: The effect of two volume history pressures (40 and 60 cm H2O) and three pressure-volume curve peak pressures (40, 50, and 60 cm H2O) were randomly compared. MEASUREMENTS AND MAIN RESULTS: Peak volume history pressure affected the inflation curve beyond the lower inflection point but did not affect the inflection point (Pflex). Peak pressure-volume curve pressure affected the deflation curve. Increased peak volume history pressure increased inflation compliance (p <.05). Increased peak pressure-volume curve pressure increased the point of maximum compliance change on the deflation limb and deflation compliance and decreased compliance between peak pressure and the point of maximum curvature on the deflation limb (p <.05). CONCLUSION: Peak volume history pressure must be considered when interpreting the inflation limb of the pressure-volume curve of the respiratory system beyond the inflection point. The peak pressure achieved during the pressure-volume curve is important during interpretation of deflation compliance and the point of maximum compliance change on the deflation limb.     

The Theracute alternating pressure replacement mattress

The use of alternating pressure mattresses is still acknowledged as an effective adjunct in the prevention and treatment of pressure damage; however, with the plethora of equipment now available, there is an increasing need to provide information that supports the practitioners' decisions ensuring individual patients' clinical needs are met. This article reviews a new addition to the Talley Medical range of alternating pressure mattresses, the Theracute mattress replacement system.     

The open lung during small tidal volume ventilation: concepts of recruitment and "optimal" positive end-expiratory pressure.

OBJECTIVES: To test the hypotheses that during small tidal volume ventilation (5 mL/kg) deliberate volume recruitment maneuvers allow expansion of atelectatic lung units and that a high positive end-expiratory pressure (PEEP) above the lower inflection point of the pressure/volume (PV) curve is not necessarily required to maintain recruited lung volume in acute lung injury. DESIGN: Prospective, randomized, controlled animal study. SETTING: An animal laboratory in a university setting. SUBJECTS: Adult New-Zealand rabbits. INTERVENTIONS: We studied a) the relationship of dynamic loops during intermittent positive pressure ventilation to the quasi-static PV curve, and b) the effect of lung recruitment on oxygenation, end-expiratory lung volume (EELV), and dynamic compliance in two groups (n = 4 per group) of lung-injured animals (lung lavage model): 1) the sustained inflation group, which received ventilation after a recruitment maneuver (sustained inflation); and 2) the control group, which received ventilation without any lung recruitment. MEASUREMENTS AND MAIN RESULTS: In the presence of PV hysteresis, a single sustained inflation to 30 cm H2O boosted the ventilatory cycle onto the deflation limb of the PV curve. This resulted in a significant increase in EELV, oxygenation, and dynamic compliance despite equal PEEP levels used before and after the recruitment maneuver. Furthermore, after a single sustained inflation, oxygenation remained high over 4 hrs of ventilation when a PEEP above the critical closing pressure of the lungs, defined as "optimal" PEEP, was used and was significantly higher compared with that in the control group ventilated at equal PEEP without preceding lung recruitment. CONCLUSIONS: The observation that ventilation occurs on the deflation limb of the tidal cycle-specific PV curve allows placement of the ventilatory cycle, by means of a recruitment maneuver, onto the deflation limb of the PV envelope of the optimally recruited lung. This strategy ensures sufficient lung volume recruitment to maintain the lungs during the tidal cycle while using relatively low airway pressures.     

Combined effect of catheter and tubing size on fluid flow

Several studies have evaluated the effect of large-bore tubing and various intravenous (IV) catheters on rapid fluid resuscitation. This study combines available equipment, which has been demonstrated to increase IV flow rate, into a system. This system is then compared with one commonly used for IV fluid infusion in hypovolemic patients. The new system requires significantly less (P less than 0.0005) time for both drainage of fluid and changing of IV containers. The improvement in flow rate can be attributed to the use of a rapid inflation/deflation pneumatic pressure device instead of the conventional hand-pumped pressure bag and the decrease in resistance through both the large-bore IV tubing and 12-gauge catheter. The rapid manipulation of IV bags is made possible by the rigid structure and the rapid inflation/deflation ability of the external pump. The possibility of faster change of IV bags and an increase in flow rate make this system a practical tool in the treatment of severely hypovolemic patients.     

Pressure area management: a static-led approach

An increase in expenditure and inappropriate use of pressure-relieving equipment, along with high-prevalence figures, initiated the evaluation of current practice of pressure damage prevention in an acute general hospital. This evaluation started with the organization of a project group, who (after a baseline audit) identified and procured the resources required to introduce and manage an effective pressure damage prevention programme. The investment in high-quality static mattresses for all patients including those who are at high risk of developing pressure damage was based on available clinical evidence. This investment meant a reduction in the amount of dynamic systems needed as many of those patients are at high risk of pressure damage, and are now successfully managed on a static system. The development of a local policy including documentation facilitated regular reassessment of risk level based on a risk assessment tool and clinical rationale. This static-led approach has reduced not only costs but also improved the appropriate allocation of pressure-relieving/reducing equipment. The approach has also simplified the choice of equipment for staff who were previously expected to match patients' needs with varying levels of dynamic mattress efficiency. This article highlights the action taken from the initial identification of a specific need through to the effective implementation, management and monitoring of this innovative practice.     

Quantification of pressure relief using interface pressure and tissue perfusion in alternating pressure air mattresses

OBJECTIVE: To examine whether the interface pressure (IP) relief provided by alternating pressure air mattresses (APAMs) is matched with maintenance of tissue perfusion over the points of contact by measuring transcutaneous oxygen and carbon dioxide (tcPO2, tcPCO2). DESIGN: Comparative analysis of 2 APAMs with a 2-parameter continuous time-based method for quantifying pressure relief (PR) and transcutaneous gas measurement for assessing tissue perfusion. SETTING: Rehabilitation research facility in a university hospital. PARTICIPANTS: Eleven able-bodied adult postgraduate student volunteers. MAIN OUTCOME MEASURES: Two full-replacement APAM systems were used. For each mattress the mean maximum and minimum interface pressures; mean peak air pressures in the mattresses; interface pressure durations below 30, 20, and 10 mmHg over a 60-minute period; mean maximum tcPCO2 and minimum tcPO2; and mean area under the tcPO2 and tcPCO2 curves were measured for each subject. RESULTS: IP on the sacrum was held below thresholds of 30, 20, and 10 mmHg longer on a 2-cell, low pressure system than on a 3-cell, high pressure system (p < .001). Integrated over time, tcPO2 levels also indicated that the 2-cell system retained oxygen levels closer to the unloaded baseline than did the 3-cell system (p < .01). tcPCO2 levels did not rise significantly (p > 0.1) compared with the baseline measurement in both mattresses. CONCLUSIONS: PR was sensitive to the design of the APAM, especially its inflation pressure, cycle time, and inflation sequence. If future trials demonstrate that PR values and transcutaneous blood gas measurements correlate significantly with the clinical incidence of pressure sore formation, then this technique may prove useful in assessing the effectiveness of alternating pressure support surfaces.     

Reinterpreting the pressure-volume curve in patients with acute respiratory distress syndrome

New evidence requires a reinterpretation of the inflation pressure-volume curve and suggests that neither the lower nor the upper inflection point provides reliable information to determine safe ventilator settings in the acute respiratory distress syndrome. Recruitment probably continues throughout the inflation pressure-volume curve, and studies of the deflation pressure-volume curve, reinflations after partial deflation, or decremental positive end-expiratory pressure trials after a recruitment maneuver are probably needed to determine open-lung positive end-expiratory pressure.     

Does a solitary lobar collapse give pressure–lung volume relationship similar to that found in acute respiratory distress syndrome? A porcine experimental study

Background: The underlying pathophysiology causing different shapes of static pressure–lung volume (PV) curves is not fully elucidated. In this study the aim was to examine the influence of a solitary lobar collapse on inflation–deflation PV curves. The hypothesis was that a lobar collapse would induce the same changes in the PV‐curve as those found in experimental acute lung injury (ALI) and in acute respiratory distress syndrome (ARDS). Methods: In four mechanically ventilated, anaesthetized pigs, the right lower lobe was collapsed by selective lavage and exsufflation. End‐expiratory lung volume and static inflation–deflation PV curves of the respiratory system, the chest wall and the lung were obtained before formation of the collapse. After creation of the collapse, the same measurements were performed in the non‐collapsed lung and in the whole lung including the lobar collapse. In two animals computed tomography was performed to verify the lobar collapse. Results: The solitary lobar collapse changed the PV curve by inducing a significant hysteresis and a right shift of lower inflexion point (LIP) on the inflation limb, but had minimal influence on the deflation limb. After creation of the lobar collapse, LIP was found at the pressure at which the collapse started to expand. Conclusions: PV curves of lungs with solitary lobar collapse are similar to those found in ALI/ARDS. Inspiratory LIP indicated start of recruitment, and expiratory curves did not indicate the pressure at which collapse occurred.     

Techniques for IABP timing, use--and discontinuance. Counterpulsation can reduce ischemia and improve hemodynamics

By restoring the balance between myocardial oxygen supply and demand, intra-aortic balloon counterpulsation can aid the ischemic or failing left ventricle. Rapid inflation and deflation of the balloon reduces afterload and improves coronary perfusion. Timing of inflation and deflation is adjusted according to the arterial pressure waveform. The balloon should inflate just after the aortic valve closes and deflate just before the aortic valve opens; early or late balloon inflation or deflation blunts the therapeutic effects. Discontinue the IABP support gradually; monitor the patient frequently for any signs of hemodynamic compromise, redevelopment of ischemic chest pain, or electrocardiographic changes.     

Laboratory performance of alternating pressure air mattresses component and sequelae

The performance of three different alternating pressure air mattresses with different geometries of air cell were compared (Nimbus 3, Heritage, Tamora Plus), using simple performance indices based on pressure mapping. The aim of this study was to examine the effect on performance of elevating the backrest and thigh section of the bed into sitting position. Ten healthy volunteers of various sizes were pressure-mapped over the full pressure cycle on three alternating pressure air mattresseses with differing cell geometries. This was then repeated with the beds profiled to a sitting position. Performance of the alternating pressure air mattresses in terms of their ability to redistribute pressure dynamically was assessed in the different positions. The different alternating pressure air mattresses performed similarly with the bed in the lying flat position, but smaller cells appeared to be more effective in the sitting position. A conclusion was made that cell geometry may have an effect on the ability of the mattress to achieve alternating behaviour in the sitting position.     

Effect of Pulmonary Blood Flow upon Lung Mechanics

Airway pressure was continuously recorded in an isolated horizontally mounted canine heart-lung preparation during abrupt, stepwise 100-200 ml inflations to 20-25 cm water pressure, and subsequent deflations. With each change in volume there was a steep rise or fall in pressure, followed by stress relaxation to a static equilibrium airway pressure. Comparison was made between the nonperfused state and during perfusion with whole blood at 100 ml/kg dog wt per min, and left atrial pressure of 10 mm Hg. Pressure tracings were similar during deflation in the perfused and the nonperfused lung. During inflation, in the middle range of lung inflation volumes, the peak inflation and equilibrium airway pressures were greater in the nonperfused state; maximum difference of static pressures in nine preparations averaged 146% of perfused values and the average stress relaxation difference from eight of these was 276%. Lung distensibility was the same with packed red cells or plasma perfusates and was not changed by varying the perfusion rate up to 220 ml/kg per min. During cyclic ventilation, dynamic compliance was similarly greater in the perfused than in the nonperfused state in the middle range of inflation volumes. Static distention of the vascular bed produced similar results with progressive improvement in distensibility in mid-inflation range up to a hydrostatic pressure of 15 cm blood. These data suggest that the distended pulmonary vascular bed provides structural airway support which facilitates entry of gas into the terminal respiratory units at diminished pressure.     

Efficacy and cost-effectiveness of the Thermo contour mattress

Use of air mattresses in reduction of pressure sore incidence is an important part of quality patient care. However, there will never be enough air mattresses to match the demand as increased education and an increase in the general provision of air mattresses can lead to an unrealistic expectation of obtaining air mattresses when required for patients. This raises the demand for air mattresses and increases costs within hospital trusts. This study examined a way to redress the balance through use of an alternative, cost-effective type of mattress (thermoelastic polymer) in the prevention of pressure sores. Prime consideration was given to the comfort of the patient. The claims of the manufacturers, Barrington Healthcare, that 'patients with Waterlow scores of 23 can be nursed safely on this product' were explored as part of the study. A total of 407 patients took part over a 6-month period. Twenty-four Thermo contour mattresses were loaned to one ward for the study and patients were allocated to the mattress through admission to the experimental ward. Patients were then split into two groups on two wards. Group B were allocated to a Thermo contour mattress and group A were nursed on air mattresses and foam mattresses generally used throughout the trust. Results showed that more patients were comfortable on the Thermo contour mattresses than on all other mattresses. Patients with Waterlow scores under 25 did not develop pressure sores on the Thermo contour mattress. However, the sample of patients with a Waterlow score of between 20 and 25 was small and therefore further research is required.     

大型压力蒸汽与臭氧消毒床垫的效果比较

目的比较大型压力蒸汽消毒法和臭氧消毒法对医院床垫的消毒效果。方法选择100张床垫,分为两组,各50张,采用压力蒸汽消毒法和臭氧消毒法,对两组床垫的表面细菌和深部载体定量菌的杀灭率进行对照研究。结果蒸汽消毒法对床垫表面细菌的杀灭率为99.92%,臭氧消毒法为94.86%,二者差异无统计学意义(P>0.05);蒸汽消毒法对床垫深部白色念珠菌的杀灭率为100%,高于臭氧法的98.03%(P<0.01)。结论两种消毒方法消毒效果肯定,但蒸汽消毒法穿透性能好、效率高、环保,值得推广。     

Bed surfaces and pressure sore prevention: an abridged report

PURPOSE: This article summarizes the results of a systematic review of randomized controlled trials testing the effectiveness of special beds, mattresses, and cushions in preventing and treating pressure sores. The review's citation is Cullum, N., Deeks, J., Sheldon, T.A., Song, F., & Fletcher, A.W. (2000). Beds, mattresses and cushions for pressure sore prevention and treatment (Cochrane Review). The Cochrane Library, 4. DESIGN: An integrative research review. SAMPLE: 37 studies were included in the analysis. METHODS: A broad search of databases and unpublished studies was conducted. Data were extracted from those that met the inclusion criteria. Studies were grouped in various ways but mainly by type of product evaluated. FINDINGS: Many special products designed to prevent or treat pressure sores are more effective than standard hospital foam mattresses in preventing and treating pressure sores. CONCLUSIONS: Special pressure-relieving surfaces should be used for patients at risk for skin breakdown. IMPLICATIONS FOR NURSING PRACTICE: Individual practitioners and agencies should have a systematic protocol for assessing patients' risk of skin breakdown and for taking action when patients are determined to be at risk. The findings of this review provide some guidance for choosing particular products, albeit not definitive evidence for matching risk levels to products.     

Prevention is better than cure. A survey of the use of Pegasus Airwave System in pressure sore treatment.

Pressure relieving mattresses play an important role in pressure sore prevention and healing. This study describes the current use and reported effectiveness of the Pegasus Airwave System in pressure sore prevention.     

Do different mattresses affect the quality of cardiopulmonary resuscitation?

Objective To determine the effect of different mattresses on cardiopulmonary resuscitation performance and establish whether emergency deflation of an inflatable mattress improves the quality of resuscitation.Design and setting Randomised controlled cross-over trial performed in a general ICUParticipants Critical care staff from a general ICU.Interventions Cardiopulmonary resuscitation on a manikin on the floor or on a bed with a standard foam mattress and inflated and deflated pressure redistributing mattresses. Maximal compression force was measured at different bed heights.Measurements and results Compression depth, duty cycle and rate and percentage correct expired air ventilation were recorded on a manikin. Compression depth was significantly lower on the foam (35.2 mm), inflated (37.2 mm) and deflated mattress (39.1 mm) than the floor (44.2 mm). There were no clinically important differences in duty cycle or compression rate. The quality of ventilation was poor on all surfaces. Maximal compression force declined as bed height increased.Conclusions Resuscitation performance is adversely affected when performed on a bed (irrespective of mattress type) compared to the floor. There were no differences between the inflated and deflated mattresses, although the deflation process did not adversely affect performance. This study does not support the routine deflation of an inflated mattress during resuscitation and questions the potential benefits from using a backboard. The finding that bed height affects maximal compression forces, challenges the recommendation that cardiopulmonary resuscitation be performed with the bed at middle-thigh level and requires further investigation.Dr. Robert Benny was tragically killed in a road accident during the Summer of 2003. Dr. Benny was at the very start of a promising career in surgery. Our thoughts go to his family and friends.G.D.P. and M.T. have received travelling fellowships from Huntleigh Healthcare, and M.T. has previously delivered lectures for Huntleigh Healthcare.     

Pressure ulcer prevention and treatment. A synopsis of the current evidence from research.

Foam alternatives to the standard hospital foam mattress can reduce the incidence of pressure ulcers in people at risk. The relative merits of AP and CLP devices and of the different AP devices for pressure ulcer prevention are unclear. There is some evidence from one study to suggest that LAL beds may reduce the incidence of pressure ulcers compared with standard intensive care beds. There is insufficient evidence to make conclusions on the value of various CLP devices and sheepskins as pressure ulcer prevention strategies, although Australian Medical Sheepskin was an effective preventive strategy in a recent study. There is evidence from two trials that air-fluidized therapy may improve pressure ulcer healing rates. There is insufficient evidence to make conclusions on the value of other beds and mattresses as pressure ulcer treatments. There is insufficient evidence to recommend any particular wound dressing or debridement technique. Research about pressure ulcer prevention and treatment is generally conducted on a small scale and is of poor quality; few economic evaluations have been undertaken of pressure area care strategies. Only when a clinically relevant research agenda has been developed and appropriate research methods have been used in sufficiently large studies can evidence-based pressure ulcer prevention and treatment be a possibility. Until then, nurses and other health care professionals can only rely on what little research evidence exists together with their professional judgment to make decisions in this field.     

Correlation between alveolar recruitment /derecruitment and inflection points on the pressure-volume curve

Objective To determine whether individual alveolar recruitment/derecruitment (R/D) is correlated with the lower and upper inflections points on the inflation and deflation limb of the whole-lung pressure-volume (P-V) curve. Design and setting Prospective experimental study in an animal research laboratory. Subjects Five anesthetized rats subjected to saline-lavage lung injury. Interventions Subpleural alveoli were filmed continuously using an in vivo microscope during the generation of a whole-lung P-V curve using the super syringe technique. Alveolar R/D was correlated to the calculated inflection points on both limbs of the P-V curve. Measurements and results There was continual alveolar recruitment along the entire inflation limb in all animals. There was some correlation (R ² = 0.898) between the pressure below which microscopic derecruitment was observed and the upper inflection point on the deflation limb. No correlation was observed between this pressure and the lower inflection point on the inflation limb. Conclusions In this physiological experiment in lungs with pure surfactant deactivation we found that individual alveolar recruitment measured by direct visualization was not correlated with the lower inflection point on inflation whereas alveolar derecruitment was correlated with alveolar derecruitment on deflation. These data suggest that inflection points on the P-V curve do not always represent a change in alveolar number. This article is discussed in the editorial available at: .     

Oxygenation index,an indicator of optimal distending pressure during high-frequency oscillatory ventilation?

OBJECTIVE: To test the hypothesis that, during high-frequency oscillatory ventilation (HFOV) of pigs with acute lung injury, the oxygenation index (OI = Paw*FIO(2)*100/PaO(2)) is minimal at the lowest continuous distending pressure (Paw), where the physiological shunt fraction is below 0.1 (Paw(optimal)). DESIGN AND SETTING: Prospective, observational study in a university research laboratory. SUBJECTS: Eight Yorkshire pigs weighing 12.0+/-0.5 kg, with lung injury induced by lung lavage. INTERVENTIONS: After initiation of HFOV, the pigs were subjected to a stepwise increase of Paw to obtain under-inflation, optimal inflation and over-distension of the lungs (inflation) in series, followed by a similar decrease of Paw (deflation). MEASUREMENTS AND RESULTS: At each Paw level, the OI and physiological shunt fraction were determined. The OI reached a minimum of 6.2+/-1.4 at Paw 30+/-4 cmH(2)O during inflation and a minimum of 2.4+/-0.3 at Paw 13+/-2 cmH(2)O during deflation. Paw(optimal) was 32+/-6 cmH(2)O on the inflation limb and 14+/-2 cmH(2)O on the deflation limb. The difference between the Paw at minimal OI and Paw(optimal) was -1.9+/-4.2 cmH(2)O (NS) during inflation and -1.5+/-1.6 cmH(2)O (p<0.05) during deflation. In 15 out of the 16 comparisons, the difference in Paw was within one step (+/-3 cmH(2)O). CONCLUSION: The minimal OI is indicative for the Paw where oxygenation is optimal during HFOV in surfactant-depleted pigs.     

Accurate blood pressure measurement

The most critical requirement for obtaining accurate blood pressure measurements is that the Korotkoff sounds be loud. Loudness can be enhanced by various techniques of cuff inflation and chest piece placement. The type of manometer, cuff size, and cuff placement are also important factors in obtaining accurate blood pressure readings. Correct systolic pressure measurement depends on proper inflation and deflation of the cuff. True diastolic pressure is usually closer to the disappearance point of Korotkoff sounds than to the muffling phase. Blood pressure should be recorded to the nearest 5 mm Hg because measurement to the nearest 2 mm Hg is not meaningful and is too difficult and time-consuming.