Mark S Siobal

Calculation of Physiologic Dead Space: Comparison of Ventilator Volumetric Capnography to Measurements by Metabolic Analyzer and Volumetric CO2 Monitor

BACKGROUND: Calculation of physiologic dead space (dead space divided by tidal volume [VD/VT]) using the Enghoff modification of the Bohr equation requires measurement of the partial pressure of mean expired CO2 (PĒCO2) by exhaled gas collection and analysis, use of a metabolic analyzer, or use of a volumetric CO2 monitor. The Dräger XL ventilator is equipped with integrated volumetric CO2 monitoring and calculates minute CO2 production (V̄CO2). We calculated PĒCO2 and VD/VT from ventilator derived volumetric CO2 measurements of V̄CO2 and compared them to metabolic analyzer and volumetric CO2 monitor measurements. METHODS: A total of 67 measurements in 36 subjects recovering from acute lung injury or ARDS were compared. Thirty-one ventilator derived measurements were compared to measurements using 3 different metabolic analyzers, and 36 ventilator derived measurements were compared to measurements from a volumetric CO2 monitor. RESULTS: There was a strong agreement between ventilator derived measurements and metabolic analyzer or volumetric CO2 monitor measurements of PĒCO2 and VD/VT. The correlations, bias, and precision between the ventilator and metabolic analyzer measurements for PĒCO2 were r = 0.97, r2 = 0.93 (P < .001), bias −1.04 mm Hg, and precision ± 1.47 mm Hg. For VD/VT the correlations were r = 0.95 and r2 = 0.91 (P < .001), and the bias and precision were 0.02 ± 0.04. The correlations between the ventilator and the volumetric CO2 monitor for PĒCO2 were r = 0.96 and r2 = 0.92 (P < .001), and the bias and precision were −0.19 ± 1.58 mm Hg. The correlations between the ventilator and the volumetric CO2 monitor for VD/VT were r = 0.97 and r2 = 0.95 (P < .001), and the bias and precision were 0.01 ± 0.03. CONCLUSIONS: PĒCO2, and therefore VD/VT, can be accurately calculated directly from the Dräger XL ventilator volumetric capnography measurements without use of a metabolic analyzer or volumetric CO2 monitor.

The Rise and Fall of i-STAT Point-of-Care Blood Gas Testing in an Acute Care Hospital

In response to a

Combining Heliox and Inhaled Nitric Oxide as Rescue Treatment for Pulmonary Interstitial EmphysemaThe authors respond

350,000 laboratory budget cut and closure of an intensive care unit-based laboratory and a desire to maintain turnaround times of 10 minutes or less, a multidisciplinary group developed and implemented point-of-care (POC) testing. Only blood gases (pH, PO2, and PCO2) and ionized calcium values were deemed essential stat tests. Three commercially available POC blood gas devices were evaluated; all yielded results comparable to in-house reference methods. The 1 device with a US Food and Drug Administration-approved method for ionized calcium testing and with an existing interface for laboratory information systems was selected. Fiscal analysis predicted annual savings of approximately

A Shout Instead of a Whisper: Let's Get the Graphics Right—Reply

225,000. POC blood gas analysis was implemented in April 1996 coincident with closure of the intensive care unit-based laboratory. Clinical laboratories and POC blood gas test volumes remained constant through August 1998; in contrast, the number of ionized calcium tests decreased dramatically after April 1996. In August 1998, clinically significant (i.e., artificial ventilation parameters would have been altered based on test results) discrepant PCO2 values were observed sporadically and noted only with patient specimens, not with commercial controls or electronic simulators. Because investigation failed to identify the cause, use of the POC device was discontinued in September 1998.

Transnasal Aerosol Delivery to Pediatric Patients: Jet Versus Vibrating Mesh.

In the December 2008 issue of RESPIRATORY CARE, Phatak and associates1 reported a case where they used heliox in combination with inhaled nitric oxide (NO) as rescue treatment for a preterm infant with pulmonary interstitial emphysema. They reported that this novel and innovative intervention was instituted because of the progression of severe cardiorespiratory compromise despite several conventional treatments. My group reported a very similar case in abstract form in 1999.2 The case report from Phatak et al is therefore more likely the second documented case in which the combination of heliox and inhaled NO appeared to improve ventilation and oxygenation in a preterm infant with pulmonary interstitial emphysema and critical gas-exchange defects that were refractory to conventional interventions. A major difference in the way heliox and inhaled NO were simultaneously delivered in those 2 reports should be pointed out. Phatak et al1 used a bleed-in system whereby an 80% helium/20% oxygen mixture was added to the ventilator circuit (Babylog, Dräger, Lübeck, Germany), distal to the injector module of the NO delivery device (INOVent, INO Therapeutics, Clinton, New Jersey). That setup could result in inaccuracies in the gas-concentration measurements and limit the maximum inspired helium concentration that could be delivered before the ventilator would not function.1 My group used an Infrasonics InfantStar ventilator (Tyco-Mallinckrodt, Carlsbad, California) with 80% helium/20% oxygen instead of compressed air, so the INOVent injector module would measure circuit flow comprised of a blended helium-oxygen gas mixture through the ventilator.2 At the initial fractionof inspiredoxygen(FIO2)of0.80, a set dose of 5.4 ppm NO was required to deliver a dose of 10 ppm NO. This effect is caused by the physical properties of helium, which has a higher thermal conductivity than nitrogen, oxygen, or air.3 Heliox more rapidly cools the heated-wire element in the INOVent’s injector module, resulting in a higher calculated flow rate than is actually delivered. This causes the INOVent to inject a larger volume of NO. This effect increases proportional to the concentration of helium in the gas mixture and results in a higher inhaled NO dose delivery at any set dose. To maintain a constant inhaled NO dose delivery, any changes in FIO2, and therefore the helium concentration, require an adjustment in the set inhaled NO dose.2 We studied this effect in a bench-top laboratory experiment with an infant test lung, in which we assessed the functioning of the INOVent and the inhaled NO dose delivery at various heliox concentrations and minute ventilations, by changing the respiratory rate and peak inspiratory pressure. The set dose to deliver a 10-ppm dose of inhaled NO was 9.5–9.4 ppm at a helium concentration of 5%, versus 0.8–0.7 ppm at a helium concentration of 75% (Fig. 1). We found no effects on the formation of nitrogen dioxide, and the INOVent otherwise functioned normally. The 2 cases1,2 have striking similarities. Both involved preterm infants 1,200 g with respiratory distress syndrome followed by the development of pulmonary interstitial emphysema, early use of surfactant, failure of conventional and high-frequency ventilation, use of steroids, and initiation of heliox ventilation followed by combined heliox plus NO. In both cases FIO2 was reduced from approximately 80% to 60% soon after the initiation of heliox plus NO, because of improved oxygenation. This reduction in FIO2 allowed delivery of a higher fraction of inspired helium, which in turn improved CO2 elimination and permitted lower ventilator pressures. This effect on gas exchange, FIO2, and ventilation pressures may have helped to attenuate the cycle of progression of pulmonary interstitial emphysema in these infants. Both infants were subsequently weaned off heliox, then weaned off inhaled NO, and extubated. Heliox reduces gas trapping,4 improves the distribution of ventilation and CO2 clearance,5,6 functions as a carrier gas for better penetration of inhaled NO to distal gas-exchange units,7,8 and reduces lung inflammation,9 all of which may have benefited these critically ill infants with pulmonary interstitial emphysema. In low-birth-weight infants, pulmonary interstitial emphysema is a complication of mechanicalventilationandisassociatedwith chronic respiratory insufficiency and bronchopulmonary dysplasia.10,11 As stated 2 decades ago,10 in regards to the treatment of premature infants with respiratory distress syndrome and the prevention of bronchopulmonary dysplasia, “it would be prudent to use all methods to reduce the concentration and duration of the inspired supplemental oxygen and to reduce peak ventilator pressures and their duration as much as possible.” This holds true today as much as it did then, as pulmonary interstitial emphysema and bronchopulmonary dysplasia continue to be clinically important problems in the neonatal intensive care unit. For that reason the combined use of heliox and inhaled NO may be a means of accomplishing these treatment goals in infants with pulmonary interstitial emphysema who continue to deteriorate despite aggressive conventional interventions. Betit’s editorial12 on the case report from Phatak et al1 identified several important technical issues, questioned the safety of the combined therapy, and raised the ethical dilemma of using unproven therapies. The effect of heliox on the functional and performance characteristics of many mechanical ventilators has been determined.3,13 Several ventilators suitable for infantventilationnow have heliox compatibility, which eliminates some of the technical and safety issues cited by Betit.12 We found no effect on nitrogendioxide formation, and our delivery method allows a more stable titration and control of the heliox mixture and the inhaled NO dose delivered.2 Therefore, given the above solutions for the major technical and safety issues stated by Betit,12 the more important question is whether the combined use of heliox and inhaled NO made a difference in outcome. This can only be determined by further innovation, technical refinement, and clinical study of this potentially beneficial treatment for pulmonary interstitial emphysema.

The Whisper Game—Reply

In reply: After politely conceding to his original objections back in 2013, and thanking him for “his compulsion for accuracy and obsession to detail,”[1][1],[2][2] I am now amused and a bit annoyed by Mr Chatburns letter, which expresses his incessant intellectual banter and expert opinion

Use of Dexmedetomidine to Facilitate Extubation in Surgical Intensive-Care-Unit Patients Who Failed Previous Weaning Attempts Following Prolonged Mechanical Ventilation: A Pilot Study

To the Editor: In the recent paper by El Taoum and associates,[1][1] an in vitro comparison of aerosol dose delivery using jet and vibrating mesh nebulizers with a variety of nasal interface devices in pediatric patient models was presented. The results of this study demonstrated that the jet

Description and evaluation of a delivery system for aerosolized prostacyclin.

In reply: We thank Mr Chatburn for reading our paper with such meticulous and extensive attention to detail as to find and point out what seem to be inconsequential mistakes. These oversights were not unintentional distortions or corrupted information, but perhaps just minor errors overlooked