Calculation Of Dead Space Research Articles

Dead space and weaning from invasive mechanical ventilation in high-altitude residents

Objective: To determine the predictive value of the dead space calculation through the dead space/tidal volume fraction atweaning from invasive mechanical ventilation in critically ill patients at high altitude.Materials and methods: An epidemiological, observational, analytical and prospective study carried out in the Adult IntensiveCare Unit of the Hospital del Norte in the city of El Alto, Bolivia (4,090 m a.s.l.; barometric pressure: 453 mm Hg) from November01, 2016 to March 31, 2017. High-altitude residents under invasive mechanical ventilation were studied. The inclusion criteriawere: a) Altitude residents hospitalized in the Invasive Mechanical Ventilation Therapy Intensive Care Unit. b) Patients with evidence of resolution of the cause that prompted their connection to the invasive mechanical ventilator. c) Patients withpositive weaning criteria and rates. d) Positive spontaneous respiration test. The study variables were the dead space through the Vd/Vt fraction and its relationship with the success or failure of the weaning process from mechanical ventilation. The Vd/Vt fraction was calculated in the study patients and then weaning from invasive mechanical ventilation wasperformed. Patients were divided into two groups according to the need for reintubation and reconnection to themechanical ventilator within 72 hours.Results: Twenty-one (21) patients were included: 7 (33 %) women and 14 men (67 %). The mean age was 41 years with a standard deviation of 22.38 years. Eighteen (18) patients (86 %) succeeded and 3 (14 %) failed in the weaning process from invasive mechanical ventilation. The Vd/Vt values in the success and failure groups were 0.43 and 0.53 (p < 0.011109), respectively, witha sensitivity of 0.61 and specificity of 1; a positive predictive value of 1 and a negative predictive value of 0.3.Conclusions: The calculation of the dead space through the measurement of the dead space/tidal volume fraction predicts the success of weaning of critically ill patients under invasive mechanical ventilation at high altitude.

Estimated dead space fraction and the ventilatory ratio are associated with mortality in early ARDS

BackgroundIndirect indices for measuring impaired ventilation, such as the estimated dead space fraction and the ventilatory ratio, have been shown to be independently associated with an increased risk of mortality. This study aimed to compare various methods for dead space estimation and the ventilatory ratio in patients with acute respiratory distress syndrome (ARDS) and to determine their independent values for predicting death at day 30. The present study is a post hoc analysis of a prospective observational cohort study of ICUs of two tertiary care hospitals in the Netherlands.ResultsIndividual patient data from 940 ARDS patients were analyzed. Estimated dead space fraction and the ventilatory ratio at days 1 and 2 were significantly higher among non-survivors (p < 0.01). Dead space fraction calculation using the estimate from physiological variables [VD/VT phys] and the ventilatory ratio at day 2 showed independent association with mortality at 30 days (odds ratio 1.28 [95% CI 1.02–1.61], p < 0.03 and 1.20 [95% CI, 1.01–1.40], p < 0.03, respectively); whereas, the Harris–Benedict [VD/VT HB] and Penn State [VD/VT PS] estimations were not associated with mortality. The predicted validity of the estimated dead space fraction and the ventilatory ratio improved the baseline model based on PEEP, PaO2/FiO2, driving pressure and compliance of the respiratory system at day 2 (AUROCC 0.72 vs. 0.69, p < 0.05).ConclusionsEstimated methods for dead space calculation and the ventilatory ratio during the early course of ARDS are associated with mortality at day 30 and add statistically significant but limited improvement in the predictive accuracy to indices of oxygenation and respiratory system mechanics at the second day of mechanical ventilation.

Determination of physiological dead space in anaesthetized horses: a method-comparison study

ObjectiveTo compare two methods of Bohr–Enghoff physiological dead space to tidal volume ratio (Vd/VtBohr–Enghoff) determination using a mixing chamber and an E-CAiOVX metabolic monitor. Study designProspective, clinical, method-comparison study. AnimalsTwenty horses anaesthetized for elective orthopaedic procedures. MethodsHorses were anaesthetized with isoflurane in oxygen and the lungs were mechanically ventilated (Vt 15±2 mL kg−1). Arterial blood was sampled to provide arterial partial pressure of carbon dioxide (PaCO2) for dead space calculation using a metabolic monitor. Mixed expired partial pressure of carbon dioxide (PēCO2) obtained from the custom-made mixing chamber was recorded at the time of arterial blood sampling. Dead space fraction was calculated using the Enghoff modification of the Bohr equation. Agreement between the methods was assessed by Bland–Altman test. A clinically acceptable error was defined to be ≤ 10%. ResultsForty-nine simultaneous Vd/VtBohr–Enghoff results were obtained. There was no clinically significant bias between the mixing chamber and E-CAiOVX. The limits of agreement were within a priori defined error (bias±95% limits of agreement: −0.022±0.078). Conclusions and clinical relevanceAcceptable agreement was found between the two methods. The E-CAiOVX metabolic monitor might be a suitable device for measuring Vd/VtBohr–Enghoff in anaesthetized horses.

Evaluation of variables to describe the shape of volumetric capnography curves during bronchoconstriction in dogs

The aim of the study was to investigate changes in volumetric capnography (VC) variables during bronchoconstriction in dogs and compare it with total respiratory resistance (RL) measured with a Fleisch pneumotachograph. Six dogs were challenged with increasing concentrations of carbachol until obvious signs of bronchoconstriction were seen. All VC parameters were obtained before, directly after, 10 and 20min after maximal bronchoconstriction. The slope of phase III (SIII) and airway and alveolar dead space parameters were significantly different from baseline directly after the challenge. The VC curve obtained a typical shape at the time of maximal bronchoconstriction and a trend to return to baseline shape was seen over time. A significant correlation was found for all aforementioned parameters with RL. We conclude that the shape of the VC curve in combination with dead space calculation can be used to verify bronchoconstriction on a breath-to-breath basis.

A comparison of two different methods for dead space measurements in ventilated dogs

The aim of the study was to compare two methods of measuring physiological dead space/tidal volume ratio (Vd/Vt) and alveolar dead space (VdALV). Measurements were obtained by automated single breath CO2 analysis (Ventrak 1550/Capnoguard 1265 (V&C)) and classical calculations were carried out using the Enghoff–Bohr equation in anaesthetized dogs. The V&C consists of a mainstream capnometer, a pneumotachometer, a signal processor, and computer software to determine continuous single-breath CO2 analysis (SBT-CO2). Eleven dogs of mixed breed (five female, six male) mean body mass 35 ± 10 kg, aged 9 months to 8 years were studied. Pre-anaesthetic medication was acepromazine (0.03 mg kg−1) and methadone (0.1 mg kg−1). Anaesthesia was induced with propofol given to effect and maintained with propofol (10 mg kg−1 hour−1) and fentanyl (0.02 mg kg−1 hour−1) by infusion. The dog's trachea were intubated and the carbon dioxide and flow sensor were placed between the tube and the Y-piece of a circle system (FiO2 = 1.0). Controlled ventilation was started (tidal volume 10–15 mL kg−1) and settings were not changed throughout the measurement period. Mixed expired PCO2 (P eCO2) was measured by analyzing expired gas collected in a mixing box in the expiratory limb of the circle system. The dorsal pedal artery was cannulated for arterial blood sampling and analysis. Measurements were done every 15 minutes for 1 hour. The Vd/Vt was automatically calculated and displayed from the SBT-CO2 analysis and also obtained using the Enghoff modification of the Bohr equation (Vd/Vt = (PaCO2 − P eCO2)/PaCO2). Alveolar dead space was determined by calculating the physiological dead space (Vdphys = expired volume × (Vd/Vt)) and subtracting the anatomical dead space measured by SBT-CO2. Values for Vd/Vt and VdALV obtained with both methods were compared using Students t-test. The mean values from the automatic dead space calculation (Vd/Vt: 0.62–0.63; VdALV: 56.1–64.3 mL) did not differ significantly from those calculated arithmetically (Vd/Vt: 0.62–0.63; VdALV: 54.09–66.31 mL). The mean differences and standard deviation in Vd/Vt was 0.63 ± 0.00 and in VdALV 58.98 ± 4.28 mL for the two measurement techniques. Our data indicate that V&C can be used for accurate noninvasive online Vd/Vt and VdALV measurements in anaesthetized ventilated dogs.

Delay time correction of the gas analyzer in the calculation of anatomical dead space of the lung.

By means of a mathematical model, we have studied a way to correct the delay time of the gas analyzer in order to calculate the anatomical dead space using Fowler's graphical method. The mathematical model was constructed of ten tubes of equal diameter but unequal length, so that the amount of dead space varied from tube to tube; the tubes were emptied sequentially. The gas analyzer responds with a time lag from the input of the gas signal to the beginning of the response, followed by an exponential response output. The single breath expired volume-concentration relationship was examined with three types of expired flow patterns of which were constant, exponential and sinusoidal. The results indicate that the time correction by the lag time plus time constant of the exponential response of the gas analyzer gives an accurate estimation of anatomical dead space. Time correction less inclusive than this, e.g. lag time only or lag time plus 50% response time, gives an overestimation, and a correction larger than this results in underestimation. The magnitude of error is dependent on the flow pattern and flow rate. The time correction in this study is only for the calculation of dead space, as the corrected volume-concentration curves does not coincide with the true curve. Such correction of the output of the gas analyzer is extremely important when one needs to compare the dead spaces of different gas species at a rather faster flow rate.

An Approach to Respiratory Monitoring of Anesthetized Patients With VA/Q Mismatch

During the last decade as increasing numbers of patients with severe cardio-pulmonary disorders have become candidates for anesthesia and surgery, it has become apparent that arterial hypoxemia may accompany seemingly adequate pulmonary ventilation with atmospheres containing high inspired concentrations of oxygen even in patients with normal pulmonary function and no impairment of diffusion. A recent review by Markello1 states that ventilationperfusion (VA/Q) mismatch (in the absence of true anatomical shunt) is most often the underlying cause of this hypoxemia. Determinations of A-aDO2 and calculation of dead space to tidal volume ratios (VD/VT), while clearly demonstrating this shunt-like effect, give little information of therapeutic value to help in obtaining a better VA/Q match. The anesthesiologist daily faces the necessity of providing a cyclic flow of alveolar gas distributed as evenly as possible throughout the lung without causing reduction and maldistribution of already impaired perfusion in situations where low cardiac output exists, as is the case in many of the patients undergoing cardiac surgery.

LUNG DIFFUSING CAPACITY: CYCLICALLY AND CONTINUOUSLY VENTILATED CLOSED SYSTEMS.

In previous communications, distribution of ventilation and lung diffusing capacity (Dl) were calculated from changes occurring in N2 and CO concentrations when a subject, whose lungs contained atmospheric air at the beginning of the experiment, respired in a spirometer containing oxygen and a little CO. In the calculations, the system was assumed to be continuously ventilated and without dead space. The present study examines which errors may arise from these simplifications. Equations for cyclic ventilation are derived. Definite values for dead space and for volumes, alveolar ventilation, and Dl of two nonuniformly ventilated lung regions are assumed. Thereupon it is computed how the concentrations of N2 and CO will change, if a subject with the assumed values respires in a spirometer of a given volume. From these changes in concentration, Dl and the distribution of ventilation are calculated using equations for continuous ventilation. The differences between the assumed and the calculated values of Dl are small. mathematics for cyclic ventilation; nonuniform ventilation; calculation of dead space; calculation of lung diffusing capacity Submitted on February 18, 1964

Alveolar air, respiratory dead space, and the "ventilation index.".

SummaryThe ventilatory response of 9 normal persons to muscular exercise can be described in part by a linear relationship between tidal volume and carbon dioxide output per breath. The intercept (Fig. 1) is the effective respiratory dead space and the slope is the reciprocal of the effective alveolar carbon dioxide partial pressure. Thus, during exercise and recovery, involving large changes in oxygen consumption and tidal volume, both effective respiratory dead space and effective alveolar carbon dioxide remain remarkably constant. This circumstance makes possible the calculation of dead space and alveolar partial pressures without recourse to direct sampling methods.