Change in PaCO2 with mechanical dead space during artificial ventilation

Impact of cardiac output and alveolar ventilation in estimating ventilation/perfusion mismatch in ARDS using electrical impedance tomography

INTRODUCTION

Electrical impedance tomography (EIT) can be used to assess ventilation/perfusion (V/Q) mismatch within the lungs. Several methods have been proposed, some of them neglecting the absolute value of alveolar ventilation (V_A) and cardiac output (Q_C). Whether this omission results in acceptable bias is unknown.

METHODS

Pixel-level V/Q maps of 25 ARDS patients were computed once considering (absolute V/Q map) and once neglecting (relative V/Q map) the value of Q_C and V_A. Previously published indices of V/Q mismatch were computed using absolute V/Q maps and relative V/Q maps. Indices computed with relative V/Q maps were compared to their counterparts computed using absolute V/Q maps.

RESULTS

Among 21 patients with ratio of alveolar ventilation to cardiac output (V_A/Q_C) > 1, relative shunt fraction was significantly higher than absolute shunt fraction [37% (24-66) vs 19% (11-46), respectively, p < 0.001], while relative dead space fraction was significantly lower than absolute dead space fraction [40% (22-49) vs 58% (46-84), respectively, p < 0.001]. Relative wasted ventilation was significantly lower than the absolute wasted ventilation [16% (11-27) vs 29% (19-35), respectively, p < 0.001], while relative wasted perfusion was significantly higher than absolute wasted perfusion [18% (11-23) vs 11% (7-19), respectively, p < 0.001]. The opposite findings were retrieved in the four patients with V_A/Q_C < 1.

CONCLUSION

Neglecting cardiac output and alveolar ventilation when assessing V/Q mismatch indices using EIT in ARDS patients results in significant bias, whose direction depends on the V_A/Q_C ratio value.

Determinants of Effect of Extracorporeal CO2 Removal in Hypoxemic Respiratory Failure

BACKGROUND: Dead space and respiratory system elastance (Ers) may influence the clinical benefit of a ventilation strategy combining very low tidal volume (VT) with extracorporeal carbon dioxide removal (ECCO2R) in patients with acute hypoxemic respiratory failure. We sought to evaluate whether the effect of ECCO2R on mortality varies according to ventilatory ratio (VR; a composite variable reflective of dead space and shunt) and Ers. METHODS: Secondary analysis of a trial of a strategy combining very low VT and low-flow ECCO2R planned before the availability of trial results. Bayesian logistic regression was used to estimate the posterior probability of effect moderation by VR, Ers, and severity of hypoxemia (ratio of arterial partial pressure of oxygen to fraction of inspired oxygen [PaO2:FiO2]) on 90-day mortality. Credibility of effect moderation was appraised according to the Instrument for Assessing the Credibility of Effect Modification Analyses criteria. RESULTS: A total of 405 patients were available for analysis. The effect of the intervention on mortality varied substantially with VR (posterior probability of interaction, 94%; high credibility). Benefit was more probable than harm in patients with VR 3 or higher. In patients with VR less than 3, the probability of increased mortality with intervention was high (>90%). The effect of the intervention also varied with PaO2:FiO2 (posterior probability of interaction, >99%; low credibility). Benefit was more probable than harm in patients with PaO2:FiO2 110 mm Hg or higher. The effect of the intervention did not vary substantially with Ers (posterior probability of interaction, 68%; low credibility). CONCLUSIONS: VR has a highly credible influence on the effect of a strategy combining very low VT and low-flow ECCO2R on mortality. This intervention may reduce mortality in patients with high VR. (Funded by an Early Career Investigator Award from the Canadian Institutes of Health Research to Dr. Goligher.)

Refractory Central Neurogenic Hyperventilation: A Novel Approach Utilizing Mechanical Dead Space

This report describes the successful management of a case of central neurogenic hyperventilation (CNH) refractory to high dose sedation by increasing the mechanical dead space. A 46-year-old male presented with a history of multiple neurological symptoms. Following an extensive evaluation, he was diagnosed with primary diffuse CNS lymphoma and started on high dose steroids. After initial symptomatic improvement, the patient developed increasing respiratory distress and tachypnea. He was intubated and transferred to the neurointensive care unit (neuro ICU). While in the ICU the patient remained ventilator dependent with significant tachypnea and respiratory alkalosis resistant to fentanyl and propofol. This prompted an attempt to normalize the PaCO₂ via an increase of the mechanical dead space. This approach successfully increased PaCO₂ and bridged the patient until ongoing therapy for the underlying disease resolved the pervasive breathing pattern typical of CNH. Further investigation is warranted to evaluate this strategy, which upon review of the literature appears underused.

Dead space ventilation in critically ill children with lung injury

STUDY OBJECTIVE

In children with acute lung injury, there is an increase in minute ventilation (E) and inefficient gas exchange due to a high level of physiologic dead space ventilation (VD/VT). Mechanical ventilation with positive end-expiratory pressure, when used in critically ill patients to correct hypoxemia, may contribute to increased VD/VT. The purpose of this study was to measure metabolic parameters and VD/VT in critically ill children.

DESIGN

A cross-sectional study.

SETTING

Pediatric ICU of a university hospital.

PATIENTS

A total of 45 mechanically intubated children (mean age, 5.5 years).

INTERVENTIONS

Indirect calorimetry was used to measure metabolic parameters. VD/VT parameters were calculated using the modified Bohr-Enghoff equation. ARDS was defined based on criteria by The American-European Consensus Conference.

MEASUREMENTS AND RESULTS

The group mean (+/- SD) ventilatory equivalent for oxygen (VeqO(2)) and ventilatory equivalent for carbon dioxide (VeqCO(2)) were 2.9 +/- 1 and 3.3 +/- 1 L per 100 mL, respectively. The group mean VD/VT was 0.48 +/- 0.2. When compared to non-ARDS patients (33 patients), the patients with ARDS (12 patients) had a significantly higher VeqO(2) (3.3 +/- 1 vs 2.8 +/- 1 L per 100 mL, respectively; p < 0.05), a significantly higher VeqCO(2) (3.7 +/- 1 L/100 vs 3.1 +/- 1 L per 100 mL, respectively; p < 0.05), and a significantly higher VD/VT (0.62 +/- 0.14 vs 0.43 +/- 0.15, respectively; p < 0.0005).

CONCLUSIONS

Critically ill children with ARDS have increased VD/VT. Increased VD/VT was the main cause of the excess of E demand in these patients. Increased metabolic demands, as shown by the VeqO(2), VeqCO(2), and ventilatory support, are the major determinants of E requirements in children with ARDS.

Carbon dioxide and oxygen partial pressure in expiratory water condensate are equivalent to mixed expired carbon dioxide and oxygen

This study was to determine whether the PCONCO2 and PCONO2 which collect in the expiratory trap of a ventilator circuit are equivalent to PECO2 and PEO2. Fifty studies were performed in 34 mechanically ventilated male patients. Five milliliters of condensate fluid were collected and PECO2 and PEO2 were measured. Exhaled gases were collected simultaneously with condensate fluid for 5 min in a meteorologic balloon and FECO2 and FEO2 were measured; PECO2 and PEO2 were then calculated. The mean PECO2 was not significantly different from PCONCO2 nor was the PCONO2 significantly different from the condensate PCONO2. There was a high correlation between mixed expired PECO2 and PCONCO2 as well as PEO2 and PCONO2. These data indicate expiratory PCONCO2 and PCONO2 provide a valid reflection of PECO2 and PEO2. The PCONCO2 and PCONO2 measured in a clinical blood gas analyzer are accurate and may be used in calculation of VD/VT and in metabolic assessments.

Lung vascular permeability after reversal of fibronectin deficiency in septic sheep. Correlation with patient studies

Plasma fibronectin deficiency and opsonic dysfunction exist in critically ill septic surgical, trauma, and burn patients with multiple organ failure. Fibronectin deficiency can be reversed by infusion of fresh plasma cryoprecipitate. The influence of therapy with human cryoprecipitate on lung vascular permeability in septic sheep with plasma fibronectin deficiency following surgery was evaluated. Additionally, selected studies on pulmonary function in septic surgical and trauma patients after infusion of plasma cryoprecipitate were completed. In patients, ventilation-perfusion balance appeared to improve as measured by the multiple inert gas elimination technique. With the lung lymph fistula preparation in fibronectin deficient sheep, infusion of human plasma cryoprecipitate (10 units; 250 ml) delayed the onset and minimized the increase in lung vascular permeability during postoperative Pseudomonas sepsis (5 X 10(9) bacteria, I.V.; 5 X 10(10) bacteria, I.P.). For example, in a first group of sheep, the transvascular protein clearance (TPC) at 2 hrs in septic sheep (n = 4) treated with only saline (volume control) was 20.1 +/- 3.1 ml/hr, compared to 11.23 +/- 0.83 ml/hr in the sheep (n =a 4) treated with fibronectin-rich cryoprecipitate (p less than 0.05). In a second group of sheep, cryoprecipitate depleted of fibronectin by affinity chromatography was used as the control solution. It also did not manifest this protective effect with respect to lung vascular permeability. Thus, at 2 hrs the lymph flow (Qlym) was 30.2 ml/hr and the transvascular protein clearance (TPC) was 18.0 ml/hr in septic sheep given fibronectin-deficient cryoprecipitate. In contrast, in the fibronectin-rich cryoprecipitate treated sheep, the Qlym was 14.8 ml/hr and the TPC was 8.12 ml/hr. It is suggested that fibronectin may influence lung vascular integrity during sepsis following surgery and trauma.

Interaction of series and parallel dead space in the lung

The volume of ventilation delivered to unperfused zones of the respiratory system (respiratory dead space) can be divided into the volume occupied by the conducting airways (series dead space) and the volume of unperfused alveolar space (parallel dead space). The effect of the interaction between these two components of dead space on steady-state gas exchange was first evaluated with a mathematical model. The presence of both parallel and series dead space was predicted to underestimate the dead space measured by the inert gas elimination technique (VDIG). This error was largest when the volumes of parallel and series dead space were equal. The size of the parallel dead space in the model could be calculated from measurements of VDIG made before and after adding a series dead space of known volume. In 16 anesthetized dogs series and parallel dead space were quantitated using the multiple inert gas elimination technique with addition of known volumes of series dead space. In five normal dogs, the series and parallel dead space averaged 20% and 13% of the tidal volume, respectively. In eleven dogs with the left pulmonary artery occluded the parallel dead space averaged 26%. This method represents the first means of quantitating these two anatomically separate components of wasted ventilation.

Cardiorespiratory effects of an inspiratory hold and continuous positive pressure ventilation in goats

The cardiorespiratory effects of three different patterns of mechanical ventilation were compared in sixteen anaesthetized goats. Intermittent positive pressure ventilation (IPPV), with an inspiratory: expiratory (I:E) time ratio of 1:3, was compared with an inspiratory hold pattern (IPPVH), with an I:E ratio of 3:1, and with continuous positive pressure ventilation (CPPV) adjusted to produce the same mean airway pressure. In eight animals with normal lungs, IPPVH reduced VD/VT and PaCO2, but produced no changes in oxygenation. CPPV did not significantly alter the efficiency of gas exchange. In a further eight animals, with oleic acid-induced lung damage, both IPPVH and CPPV produced a decrease in both VD/VT and PaCO2. Qs/Qt was significantly reduced by both CPPV and IPPVH, but the effect was more marked with CPPV, and the PaO2 was significantly increased only by CPPV. The increased effectiveness of CPPV in increasing PaO2 in this model may have been due to the greater increase in end-expiratory lung volume produced by this pattern of ventilation.

Opsonic alpha2 surface binding glycoprotein therapy during sepsis

A pronounced depletion of an opsonic protein for hepatic reticuloendothelial (RE) phagocytosis has been demonstrated in critically ill trauma patients. This opsonic alpha(2) surface binding (SB) glycoprotein has immunologic identity and a similar amino acid composition to cold insoluble globulin (CIg). Since CIg can be concentrated in cryoprecipitate, it was utilized as a readily available source of opsonic alpha(2)SB glycoprotein for replacement therapy after injury with documented hypoopsonemia. Six septic patients (2 multiple trauma, 2 thermal burn, and 2 intra-abdominal abscess) were studied to test whether cryoprecipitate infusion would restore this humoral component. Pre- and posttherapy opsonin levels were determined by bioassay and electroimmunoassay. In all patients, severe opsonin depletion was reversed following cryoprecipitate infusion. All patients had a rapid improvement in febrile state, normalization of leukocyte levels, and improvement in pulmonary function as evidenced by decreasing requirements for end expiratory pressure at lowered levels of inspired oxygen. One patient was studied more extensively and demonstrated an increase in cardiac output, limb blood flow, total body and limb oxygen delivery, total body and limb oxygen consumption and a progressive decrease in pulmonary shunt fraction. Thus, opsonic alpha(2)SB glycoprotein deficiency can be reversed by cryoprecipitate infusion in critically ill septic injured patients. Replacement of this humoral factor may be an important therapeutic modality in prevention of multiple organ failure, but it should be administered only after documentation of hypoopsonemia in traumatized patients.

Some adverse physiological effects of hypocarbia and methods of maintaining normocarbia during controlled ventilation--a review

Some of the adverse physiological effects of hypocarbia are described and the methods available to maintain the PaCO2 within or near the normal range during controlled ventilation are reviewed.

Correction for mechanical dead space in the calculation of physiological dead space

When physiological dead space (Vd(p)) is calculated for a patient who has alveolar dead space, e.g., after pulmonary vascular occlusion, less than the full volume of attached mechanical dead space (Vd(m)) appears in the measured dead space (Vd(n)). Under these conditions the traditional subtraction of Vd(m) from Vd(n) leads to underestimation of Vd(p) and can give a falsely small ratio of Vd(p) to tidal volume (Vt) when, in fact, an abnormally large Vd(p)/Vt exists. To make the proper correction for Vd(m), two equations have been derived and validated with seven subjects having Vd(p)/Vt from 0.29 to 0.87, using Vd(m)'s from 120 to 322 ml. With only a small modification, these equations are suitable for routine clinical use and give Vd(p)/Vt within 0.02 of that by the validated equations (32 of 33 comparisons). The fraction of Vd(m) subtracted from Vd(n) is the square of the ratio of effective alveolar to total alveolar ventilation and is never > 1. This fraction is (Pa(CO2)/Pa(CO2))(2), where Pa(CO2) and Pa(CO2) are the mean partial pressures of expired alveolar and of arterial CO(2); in the other equation this fraction is [Pe(CO2)/Pa(CO2) (Vt - Vd(an) - Vd(m))](2) where Pe(CO2) is mixed expired Pco(2) and Vd(an) is anatomical dead space. The second equation requires an estimated Vd(an) and is applicable when Pa(CO2) is not measured or does not plateau (as in exercise).