Liquid-filled esophageal catheter for measuring pleural pressure in preterm neonates

The precise measurement of esophageal pressure (Pes) as a reflection of pleural pressure (Ppl) is crucial to the measurement of lung mechanics in the newborn. The fidelity of Pes as a measurement of Ppl is determined by the occlusion test in which, during respiratory efforts against an occlusion at the airway opening, changes in pressure (delta Pao) (Pao is assumed to be equal to alveolar pressure) are shown to be equal to changes in Pes (delta Pes). Eight intubated premature infants (640-3,700 g) with chest wall distortion were studied using a water-filled catheter system to measure Pes. During the occlusion test, all patients had a finite region of the esophagus where delta Pes equaled delta Pao, which corresponded to points in the esophagus above the cardia but below the carina. In conclusion, even in the presence of chest wall distortion, a liquid-filled catheter with the tip between the cardia and carina can provide an accurate measurement of Ppl, even in the very small premature infant with chest wall distortion.     

Upper airway pressure-flow relationships in obstructive sleep apnea

We examined the pressure-flow relationships in patients with obstructive sleep apnea utilizing the concepts of a Starling resistor. In six patients with obstructive sleep apnea, we applied incremental levels of positive pressure through a nasal mask during non-rapid-eye-movement sleep. A positive critical opening pressure (Pcrit) of 3.3 +/- 3.3 (SD) cmH2O was demonstrated. As nasal pressure was raised above Pcrit, inspiratory airflow increased in proportion to the level of positive pressure applied until apneas were abolished (P less than 0.01). However, at pressures greater than Pcrit, esophageal pressures either did not correlate or correlated inversely with inspiratory airflow provided that esophageal pressure was less than Pcrit. When pressure was applied to a full face mask, inspiratory airflow did not occur and Pcrit could not be obtained at pressures well above Pcrit demonstrated with the nasal mask. These results are consistent with the view that the upper airway functions as a Starling resistor with a collapsible segment in the oropharynx. These findings offer a unifying construct for the association of sleep apnea, periodic hypopnea, and snoring.     

Measurement of pulmonary mechanics in the newborn lamb: a comparison of three techniques

Although recent interest in neonatal respiratory mechanics has led to the development of a plethora of techniques for measuring lung compliance and resistance, a critical appraisal of the limitations of these techniques in the newborn has not been performed to date. We evaluated three techniques of measuring respiratory mechanics in the newborn lamb, with the reference method (method 1) being the Mead-Whittenberger technique using flow, volume, and esophageal pressure (Pes) by water-filled catheter, and the other two methods entailing the measurement of mouth pressure (Pm) during airway occlusion (method 2 using end-expiratory occlusion; method 3 using end-inspiratory occlusion). Each technique was evaluated during eupnea and tachypnea in intubated and nonintubated newborn lambs. We found that the use of Pes for the measurement of resistance and compliance gave the most reliable results during both eupnea and tachypnea in both the intubated and nonintubated subjects. The airway occlusion techniques that use Pm to derive resistance and compliance (methods 2 and 3) gave more variable results under all conditions of testing. Method 2 was the least precise method of measurement with a variability of greater than 30% compared with a variation of less than 20% for method 1. For all three methods, it was found that the number of breaths needed for reproducible measurements of mechanics was four to six during eupnea and seven to nine during tachypnea.     

Induction of airway collapse with subatmospheric pressure in awake patients with sleep apnea

To determine whether the pharyngeal airway is abnormal in awake patients with obstructive sleep apnea (OSA), we measured the ability of the pharyngeal airway to resist collapse from subatmospheric pressure applied to the nose in awake subjects, 12 with OSA and 12 controls. Subatmospheric pressure was applied to subjects placed in the supine position through a tightly fitting face mask. We measured airflow at the mask as well as mask, pharyngeal, and esophageal pressures. Ten patients developed airway obstruction when subatmospheric pressures between 17 and 40 cmH2O were applied. Obstruction did not occur in two patients with the least OSA. Obstruction did not occur in 10 controls; one obese control subject developed partial airway obstruction when -52 cmH2O was applied as did another with -41 cmH2O. We conclude that patients with significant OSA have an abnormal airway while they are awake and that application of subatmospheric pressure may be a useful screening test to detect OSA.     

Laryngeal response to nasal ventilation in nonsedated newborn lambs.

Although endoscopic studies in adult humans have suggested that laryngeal closure can limit alveolar ventilation during nasal intermittent positive pressure ventilation (nIPPV), there are no available data regarding glottal muscle activity during nIPPV. In addition, laryngeal behavior during nIPPV has not been investigated in neonates. The aim of the present study was to assess laryngeal muscle response to nIPPV in nonsedated newborn lambs. Nine newborn lambs were instrumented for recording states of alertness, electrical activity [electromyograph (EMG)] of glottal constrictor (thyroarytenoid, TA) and dilator (cricothyroid, CT) muscles, EMG of the diaphragm (Dia), and mask and tracheal pressures. nIPPV in pressure support (PS) and volume control (VC) modes was delivered to the lambs via a nasal mask. Results show that increasing nIPPV during wakefulness and quiet sleep led to a progressive disappearance of Dia and CT EMG and to the appearance and subsequent increase in TA EMG during inspiration, together with an increase in trans-upper airway pressure (TUAP). On rare occasions, transmission of nIPPV through the glottis was prevented by complete, active glottal closure, a phenomenon more frequent during active sleep epochs, when irregular bursts of TA EMG were observed. In conclusion, results of the present study suggest that active glottal closure develops with nIPPV in nonsedated lambs, especially in the VC mode. Our observations further suggest that such closure can limit lung ventilation when raising nIPPV in neonates.     

Inhibitory Effect of Nasal Intermittent Positive Pressure Ventilation on Gastroesophageal Reflux

Non-invasive intermittent positive pressure ventilation can lead to esophageal insufflations and in turn to gastric distension. The fact that the latter induces transient relaxation of the lower esophageal sphincter implies that it may increase gastroesophageal refluxes. We previously reported that nasal Pressure Support Ventilation (nPSV), contrary to nasal Neurally-Adjusted Ventilatory Assist (nNAVA), triggers active inspiratory laryngeal closure. This suggests that esophageal insufflations are more frequent in nPSV than in nNAVA. The objectives of the present study were to test the hypotheses that: i) gastroesophageal refluxes are increased during nPSV compared to both control condition and nNAVA; ii) esophageal insufflations occur more frequently during nPSV than nNAVA. Polysomnographic recordings and esophageal multichannel intraluminal impedance pHmetry were performed in nine chronically instrumented newborn lambs to study gastroesophageal refluxes, esophageal insufflations, states of alertness, laryngeal closure and respiration. Recordings were repeated without sedation in control condition, nPSV (15/4 cmH₂O) and nNAVA (~ 15/4 cmH₂O). The number of gastroesophageal refluxes recorded over six hours, expressed as median (interquartile range), decreased during both nPSV (1 (0, 3)) and nNAVA [1 (0, 3)] compared to control condition (5 (3, 10)), (p < 0.05). Meanwhile, the esophageal insufflation index did not differ between nPSV (40 (11, 61) h^-1) and nNAVA (10 (9, 56) h^-1) (p = 0.8). In conclusion, nPSV and nNAVA similarly inhibit gastroesophageal refluxes in healthy newborn lambs at pressures that do not lead to gastric distension. In addition, the occurrence of esophageal insufflations is not significantly different between nPSV and nNAVA. The strong inhibitory effect of nIPPV on gastroesophageal refluxes appears identical to that reported with nasal continuous positive airway pressure.     

Nasal Continuous Positive Airway Pressure Inhibits Gastroesophageal Reflux in Newborn Lambs

BackgroundUsing esophageal pHmetry, nasal CPAP (nCPAP) has been shown to decrease acid gastroesophageal reflux (GER) in adult humans. Although both GER (mainly non-acid) and nCPAP use are very frequent in newborns, the effect of nCPAP on GER in early life is unknown. Having recently shown that the newborn lamb is a unique model for studying neonatal GER, our main objective was to assess the effect of nCPAP on GER in newborn lambs.MethodsEight newborn lambs, aged 2–3 days, were studied. Continuous esophageal pH-Impedance monitoring and polysomnography were performed for six hours during both spontaneous breathing and nCPAP application at 6 cmH₂O (nCPAP6), in a randomized order. Results were compared in the two experimental conditions, as well as without CPAP during the following 6 hours.Resultsi) nCPAP₆ virtually abolished GER [mean ±SD reflux number for 6 h = 9.1±8.6 without nCPAP₆ vs. 0.6±1 with nCPAP₆, P<0.05]; ii) GER number was also reduced during the 6 h-period following nCPAP6 application (18±16 without nCPAP₆ vs. 7±8.1 with nCPAP₆, P<0.05); iii) nCPAP₆ decreased the depth and duration of lower esophageal sphincter relaxation.ConclusionsnCPAP inhibits GER in the newborn lamb. Further clinical studies using different levels of nasal CPAP are needed to confirm this result in human infants.     

Direct measurement of static chest wall compliance in animal and human neonates

The measurement of pulmonary mechanics has been developed extensively for adults, and these techniques have been applied directly to neonates and infants. However, the compliant chest wall of the infant frequently predisposes to chest wall distortion, especially when there is a low dynamic lung compliance (CL,dyn). We describe a technique of directly measuring the static chest wall compliance (Cw,st), developed initially in the newborn lamb and subsequently applied to the premature neonate with chest wall distortion. The mean CL,dyn in seven intubated newborn lambs in normoxia was 2.45 +/- 0.41 ml.cmH2O-1.kg-1, whereas Cw,st was 11.81 +/- 0.25 ml.cmH2O-1.kg-1. These values did not change significantly in seven animals breathing through a tight-fitting face mask or with hypercapnia-induced tachypnea. For the eight premature infants the mean CL,dyn was 1.35 +/- 0.36 ml.cmH2O-1.kg-1, whereas the mean Cw,st was 3.16 +/- 1.01 ml.cmH2O-1.kg-1. This study shows that, under relaxed conditions when measurements of static compliance are performed, the chest wall is more compliant than the lung. The measurement of Cw,st may thus be used to determine the contribution of the respiratory musculature in stabilizing the chest wall.     

Study on effect of diluent osmotic pressure on yeast cell strength and cell size using a novel micromanipulation technique

A new micromanipulation technique which has previously been used to measure the mechanical properties of single animal cells has now been applied to yeast cells. In this study this technique was used to measure yeast cell strength and cell size across a 2l batch fermentation. Alternatively the cell size could also be determined using a Coulter counter while cell measurement was diluted with a conducting fluid (Isoton II). For the cell strength, it was found that the osmotic pressure of diluents did affect cell strength. However, it was also found that there was no significant effect of osmotic pressure of diluents on cell size whether a Coulter counter or micromanipulation was used for measurement. Micromanipulation has been shown to be a powerful technique for measuring the mechanical properties of yeast cells and it will be very useful for studying their behaviour in cell disruption equipment, e.g. high-pressure homogenizers.     

Effect of changes in lung volume on respiratory system compliance in newborn infants

Total respiratory system compliance (Crs) at volumes above the tidal volume (VT) was studied by use of the expiratory volume clamping (EVC) technique in 10 healthy sleeping unsedated newborn infants. Flow was measured with a pneumotachograph attached to a face mask and integrated to yield volume. Volume changes were confirmed by respiratory inductance plethysmography. Crs measured by EVC was compared with Crs during tidal breathing determined by the passive flow-volume (PFV) technique. Volume increases of approximately 75% VT were achieved with three to eight inspiratory efforts during expiratory occlusions. Crs above VT was consistently greater than during tidal breathing (P less than 0.0005). This increase in Crs likely reflects recruitment of lung units that are closed or atelectatic in the VT range. Within the VT range, Crs measured by PFV was compared with that obtained by the multiple-occlusion method (MO). PFV yielded greater values of Crs than MO (P less than 0.01). This may be due to braking of expiratory airflow after the release of an occlusion or nonlinearity of Crs. Thus both volume recruitment and airflow retardation may affect the measurement of Crs in unsedated newborn infants.     

Factors affecting the accuracy of esophageal balloon measurement of pleural pressure in dogs.

Simultaneous measurement of esophageal and tracheal pressures during an occluded inspiratory effort was used to assess the accuracy of the esophageal balloon for measuring pleural pressure in dogs. Esophageal balloons were inserted in five mongrel dogs, and an occlusion test was performed with the balloon tip 5, 10, 15, 20, and 25 cm above the esophageal sphincter; at lung volumes of functional residual capacity (FRC) and FRC + 600 ml; and in supine and right- and left-side lying postures. The protocol was repeated in paralyzed animals. This time the occlusion test was performed by injecting air into a plethysmograph to change the body surface pressure, simulating pressure changes produced by respiratory efforts in spontaneously breathing animals. In 47% of the tests in spontaneously breathing dogs, the slope of esophageal vs. tracheal pressure varied greater than 10% from unity. After paralysis the slope did not vary greater than 5% from unity under any circumstance. These data indicate that the poorer performance of the occlusion test in nonparalyzed dogs is due to active tension in the walls of the esophagus and stress induced in the intrathoracic soft tissues by the descent of the diaphragm during a breathing effort.     

Continuous positive airway pressure and mechanical ventilation by facemask in newborn infants.

During a nine-month period 24 newborn infants were treated with continuous positive airway pressure (CPAP) or mechanical ventilation delivered through a facemask. The mask was held in place in a way that minimised trauma and distortion of the head. The median birth weight of the infants was 1096 g and their median gestational age 29 weeks.The usual reason for treatment was hyaline membrane disease or recurrent apnoea due to inadequate control of breathing. Twenty-one of the infants survived. The technique was simple to apply and complications were minimal. We suggest that it may have advantages over other methods of applying CPAP or mechanical ventilation to infants mildly affected by respiratory illnesses and that it should be useful in avoiding endotracheal intubation or reducing the length of time that infants with more serious illnesses are intubated.     

Determining the filtration efficiency of half-face medical protection mask (N99) against viral aerosol

Hospital-based outbreaks of severe acute respiratory syndrome (SARS) have once again highlighted the vulnerability of healthcare workers (HCWs). Use of personal respiratory protective equipment was the main method used by HCWs to avoid nosocomial transmission. This paper describes the technology used to evaluate the filtration efficiency of the half-face medical protection mask (N99), manufactured by Firmshield Biotechnology, against viral aerosol. Viral aerosol was generated and then sampled simultaneously with and without the test mask. This enables a percentage efficiency value to be calculated against test phage f2 aerosols (surrogates of viral pathogen aerosols). At the same time the mask filtration efficiency against NaCl particle aerosol was determined by use of TSI8130 equipment and face-fit factor was tested by use of TSI8020 equipment. The half-face medical protection mask (N99) evaluated by use of the viral aerosol had a filtration efficiency >99%. The mask filtration efficiency against NaCl particle aerosol was 99.634 ± 0.024% and it had a good face-fit factor. This half-face medical protection mask (N99) can protect the wearer from viral aerosol disease transmission. The test method can be used to assess filtration efficacy against viral aerosol of masks used for respiratory protection.     

Determinants of skin contact pressure formation during non-invasive ventilation

There is no published data about mask features that impact skin contact pressure during mask ventilation.To investigate the physical factors of skin contact pressure formation.We measured masks with original and reduced air cushion size and recorded contact pressure. We determined cushion contact and mask areas by planimetric measurements.Contact pressures necessary to prevent air leakage during inspiration exceed inspiratory pressure by 1.01±0.41 hPa independent of cushion size.Contact area, ventilator pressure and mask area during inspiration and expiration impact contact pressure. Mask contact pressures are higher during expiration. The contact pressure increases with increase in inspiratory pressures independent of the ventilator cycle. During expiration, the contact pressure will increase in proportion to the expiratory pressure reduction of the ventilator. The mask with reduced air cushion size developed higher contact pressures.Contact pressure can be reduced by selecting masks with a small mask area in combination with a large mask cushion.     

How to safely maintain equipment where hazardous materials may lurk

The best protection is preparation. Assess any equipment/device that requires repair or maintenance for potential contamination or source of injury, such as sharp edges. Know where your protective apparel is located and use it. Review decontamination procedures and keep disinfectants available. Know your employee report of injury program and seek medical care whenever you have concerns regarding potential injury or exposure. Know your policies and procedures and where to find them if you need further information. Your infection control staff should be available 24 hours a day. The standard personal protective equipment that your employer is required to make available to you should include gloves, masks, eye protection, and gowns. In addition, if you are expected to enter a negative pressure room while a patient is in Airborne Precautions, you must be properly fit tested with an N95 respirator prior to entering the room. This respirator is very similar to a normal mask, but is able to filter out particles such as the TB bacterium. Infection control boils down to 2 commandments: 1. Wash your hands! 2. Use your head/common sense: If it looks soiled--clean it. If you have concerns--ask for clarification. If you think you have been exposed--seek medical assistance.     

Comparison of arousal responses to tracheal and face mask occlusions in sleeping newborn piglets.

The arousal responses after occlusion of the airway at the mid-trachea were compared with the responses after occlusion of the airway in a face mask in chronically instrumented 3- to 5-day-old piglets. For each site of occlusion arousal latency was significantly longer from active sleep than from quiet sleep. There was a significant increase in the frequency of early arousals after face mask occlusions compared with tracheal occlusions in both sleep states. During quiet sleep the frequency of arousal by 1 s after occlusion was 0.55 with face mask occlusions compared with 0.28 with tracheal occlusion (P less than 0.01). During active sleep the frequency of arousal by 3 s after a face mask occlusion was 0.32 compared with 0.08 after tracheal occlusion (P less than 0.05). Arousal from quiet sleep occurred before changes in arterial oxygen saturation. During active sleep mean saturation at arousal was not different between face mask and tracheal occlusions. Exposure of the upper airway to the pressures generated during airway occlusions results in earlier arousal in both quiet and active sleep, indicating a potential role for upper airway mechanoreceptors in initiating arousal in the newborn piglet.     

The power of feeder-mask respirometry as a method for examining hummingbird energetics

Many birds spend important portions of their time and energy flying. For this reason, quantification of metabolic rates during flight is of crucial importance to understanding avian energy balance. Measurement of organismal gas exchange rates using a mask enclosing the whole head or respiratory orifices has served as an important tool for studying animal energetics because it can free the rest of the body, permitting movement. Application of so-called “mask respirometry” to the study of avian forward flight energetics presents unique challenges because birds must be tethered to gas analysis equipment thus typically necessitating use of a wind tunnel. Resulting potential alterations to a study organism's behaviour, physiology, and aerodynamics have made interpretation of such studies contentious. In contrast, the study of hovering flight energetics in hummingbirds using a specialized form of mask respirometry is comparatively easy and can be done without a wind tunnel. Small size, hovering flight, and a nectarivorous diet are characteristics shared by all hummingbird species that make these birds ideally suited for this approach. Specifically, nectar feeders are modified to function as respirometry masks hummingbirds voluntarily respire into when hover-feeding. Feeder-mask based respirometry has revealed some of the highest vertebrate metabolic rates in hovering hummingbirds. In this review I discuss techniques for the successful measurement of metabolic rate using feeder-mask respirometry. I also emphasize how this technique has been used to address fundamental questions regarding avian flight energetics such as capacities for fuel use and mechanisms by which ecology, behaviour and energy balance are linked.     

Common cavity pressure during gastroesophageal reflux: reassessment using simultaneous pressure, impedance, and ultrasound imaging

An increase in intraesophageal pressure during transient lower esophageal sphincter (LES) relaxation [referred to as common cavity (CC) pressure] is thought to be a marker of gastroesophageal reflux (GER). Multiluminal impedance (MII) measurement is a sensitive marker of reflux entry into the esophagus during GER. We recorded GER using esophageal pressure, pH, impedance, and intraluminal ultrasound (US) images to understand the genesis of the esophageal CC pressure. Nine normal subjects underwent simultaneous MII/pH/pressure and US image recording of the esophagus for 2 h following a standardized meal. MII and pressure transducers were located at 5 and 15 cm above the LES. The US transducer and pH sensors were also placed at 5 cm above the LES. Refluxate entry into the esophagus by MII criteria was determined relative to the onset of CC pressure wave. Esophageal lumen cross-sectional area (CSA) and muscle CSA during GER were determined from the US images. Eighty liquid GER episodes identified using MII criteria, of which 55 were clearly associated with CC pressure waves, were analyzed. The GER reached 15 cm above LES in 49 of 55 (89%) by MII criteria, but the CC pressure wave was observed at 5 and 15 cm during all episodes. The propagation of the CC pressure wave was simultaneous between 5 and 15 cm during 49 of 55 (89%) of the GER episodes, but reflux entry by MII criteria was retrograde during 53 of 55 (96%) of these episodes. During 5 air-reflux episodes, MII showed a simultaneous reflux entry between the 5- and 15-cm site, however, the CC pressure preceded reflux entry during all of these episodes. There was poor correlation between the luminal CSA and the magnitude of CC pressure (R(2) = 0.144). US images revealed a close temporal correlation between CC pressure and the increase in esophageal muscle thickness and muscle CSA (markers of longitudinal muscle contraction). Disassociation between CC pressure and MII-detected reflux suggests that the onset of CC pressure is not due to GER. We speculate that longitudinal muscle contraction plays an important role in the genesis of CC pressure.     

General anesthesia for newborn pigs.

A safe technique for the induction and maintenance of general anesthesia in newborn pigs was accomplished. After premedication with atropine sulphate, anesthesia was induced by giving a mixture of oxygen, nitrous oxide and 2.5% halothane introduced by face mask. An endotracheal tube was inserted, and anesthesia was maintained using 0.5--1% halothane. Recovery was prompt, and there were no postoperative complications related to anesthesia.     

Carbon monoxide as an attenuator of vasoconstriction in piglet cerebral arterioles

Carbon monoxide (CO) is an endogenous dilator in the newborn cerebral circulation. The present study addressed the hypothesis that endogenous CO attenuates pial arteriolar vasoconstriction caused by hypocapnia, platelet activating factor, and elevated blood pressure. Experiments used anesthetized piglets with implanted, closed cranial windows. Topical application of a metal porphyrin inhibitor of heme oxygenase was used to inhibit production of CO. Chromium mesopophyrin increased vasoconstriction in response to hypocapnia. The constrictor response to a topical stimulus, platelet activating factor, was also increased by application of chromium mesoporphyrin. Inhibition of heme oxygenase did not constrict pial arterioles in normotensive newborn pigs (mean arterial pressure of about 70 mmHg), but did constrict pial arterioles of piglets with experimentally induced increases in arterial pressure (mean arterial pressure greater than 90 mmHg). In fact, pial arterioles of normotensive piglets transiently dilated to chromium mesoporphyrin, whereas those of hypertensive piglets progressively constricted during 10 min of chromium mesoporphyrin treatment. Therefore, inhibition of heme oxygenase augments cerebral vasoconstriction in response to several very different constrictor stimuli. These data suggest endogenous CO attenuates vasoconstrictor responses in the newborn cerebral circulation.