Correspondence

Permissive Hypoxemia: Is It Time To Change Our Approach?

The impact of blood transfusion on tissue oxygen delivery (DO₂) and tissue oxygen consumption (VO₂) is a subject of current clinical studies. The primary objective of this observational study is to evaluate and measure the parameters involved in determining DO₂ and VO₂, in early phase of septic patients. A secondary objective of this study is to assess the potential benefit of blood transfusion on tissue metabolism by serial measurements of lactic acid (Ac. Lac.).

A group of 29 patients were studied, each patient received between one to three units of fresh packed red blood cells (pRBC). Clinical and paraclinical criteria for sepsis as well as the plasma value of haemoglobin (Hb) below 10 g/dL represented the inclusion criteria in this study. We evaluated Hb, haematocrit (HCT), arterial blood oxigen saturation (SAO₂), central venous oxygen saturation (SCVO₂), parameters which are involved in determination of DO₂ and VO₂, before and after the transfusion of one unit of pRBC. Values of Ac. Lac. were also assessed in order to determine the type of metabolism (aerobic or anaerobic). SCVO₂, SAO₂, Hb, HCT and Ac. Lac. were determined using Epoc blood analyser. The cardiac output (CO) and systemic vascular resistance (SVR) were monitored during blood transfusion, using Vigileo monitor (Edward's Life Science, PreSep catheter kit). SAO₂ was also monitored by pulse-oximetry.

Changes in Hb, HCT and SCVO₂ before and after pRBC transfusion (which further determine VO₂) were statistically significant (P < 0.001). A statistically significant increase (P < 0.001) was obtained in Ac. Lac. values, before and after pRBC transfusion. SAO₂ and CO directly involved in producing DO₂, were clinically monitored during blood transfusion and the results remained constant.

Results obtained in this clinical study show an increase in DO2 in critically ill septic patients and also an increase in oxygen tissue uptake which is similar to VO2, clearly pointing out the benefit of pRBC transfusion. The benefits of pRBC transfusion on tissue metabolism in critically ill septic patients remain elusive because of lactic acid values increase during and after transfusion. Based on our findings we recommend that Hb values used as a single trigger for pRBC transfusion should be further studied and that additional parameters such as SCVO₂ and lactic acid should be considered as possible triggers for transfusion. Values of Hb and HCT should never be neglected.

L’impact de la transfusion sanguine sur la délivrance en oxygène (DO₂) et la consommation tissulaire en oxygène (VO₂) est un sujet d’actualité des études cliniques. Cette étude randomisée vise à évaluer et à mesurer les paramètres qui interviennent dans la détermination du DO₂ et du VO₂, dans le cas de patients atteints de sepsis en phase précoce (quelques heures après l’installation du sepsis). Cette étude évalue également le potentiel bénéfice de la transfusion sanguine sur le métabolisme tissulaire par l’intermédiaire des dosages en série de l’acide lactique.

Cette étude a enrôlé 29 patients. Chaque patient a reçu entre une et trois unités de globules rouges emballés (pRBC). Les critères d’inclusion des patients dans cette étude sont représentés par les critères cliniques et paracliniques de sepsis et le taux de l’hémoglobine plasmatique (Hb) inférieur à 10 g/dL. Nous avons évalué le taux d’Hb, l’hématocrite (HCT), la saturation artérielle en oxygène (SAO₂) et la saturation veineuse centrale en oxygène (SCVO₂), paramètres impliqués dans la détermination du DO₂ et du VO₂, avant et après la transfusion d’une unité de pRBC. Le taux de l’acide lactique a été évalué pour déterminer le type de métabolisme (aérobique ou anaérobique). SCVO₂, SAO₂, le taux d’Hb, d’HCT et de l’acide lactique ont été déterminés à l’aide de l’analyseur du sang Epoc. Le débit cardiaque (DC) et la résistance vasculaire systémique (RVS) ont été monitorisés pendant la transfusion sanguine à l’aide du moniteur Vigileo (PreSep cathéter kit, Edward's Life Science). SAO₂ a été monitorisée cliniquement à l’aide d’un oxymètre de pouls.

Les changements du taux d’Hb, d’HTC et du SCVO₂ avant et après la transfusion du pRBC (qui déterminent ultérieurement le VO₂) ont été statistiquement significatifs (p < 0,001). Une augmentation statistiquement significative (p < 0,001) a été remarqué au niveau des valeurs de l’acide lactique, avant et après la transfusion du pRBC. SAO₂ et DC, directement impliqués dans la production du DO₂, ont été monitorisés cliniquement pendant la transfusion sanguine et leurs valeurs sont restees constantes.

Les résultats obtenus dans cette étude montrent une augmentation du DO₂ chez les patients septiques gravement malades et une augmentation de l’absorption tissulaire d’oxygène, qui est similaire au VO₂. Tous ces résultats montrent clairement le bénéfice de la transfusion du pRBC. L’intérêt de la transfusion du pRBC sur le métabolisme tissulaire chez les patients septiques gravement malades reste insaisissable à cause du fait que les valeurs de l’acide lactique augmentent avant et après la transfusion. À la suite de nos recherches, nous recommandons que les valeurs de l’Hb utilisée comme le seul declencheur pour la transfusion du pRBC soient étudiées de manière plus approfondie et les parametres additionnels, comme par exemple le SCVO₂ et l’acide lactique devraient être considérées comme des possibles declencheurs pour la transfusion. Les valeurs de l’Hb et de l’HTC doivent aussi être prises en compte.

High-frequency oscillatory ventilation (HFOV) is one of lung protective strategies in acute respiratory distress syndrome (ARDS). It is not recommended to be used as initial mode of ventilation. Previous studies showed conflicting results for late use of HFOV (after prolonged period of conventional mechanical ventilation (CMV)). This study investigated the use of HFOV as an early therapy (after 24 h of CMV) in the management of ARDS due to burn.

70 burned ARDS patients were ventilated by CMV during the first 24 h (Day 0). Then, patients were randomly allocated into two equal groups (35 each):

Group 1 (G 1 or CMV): they continued on CMV.

Group 2 (G2 or HFOV): HFOV was instituted for 72 h (Days 1, 2, 3). Then, patients were shifted to CMV on Day 4 to continue on CMV. Ventilator settings, gas exchange parameters, hemodynamics, sedatives, vasoactive and paralytic requirements, barotraumas and hospital mortality were recorded and compared between the two groups.

In Day 0: Demographic data, ventilator settings, gas exchange parameters, and hemodynamics showed no significant difference between both groups. Days 1, 2, 3: there was statistically significant decrease of FiO₂ and OI accompanied by an increase in PaO₂, PaO₂/FiO₂ and PaCO₂ in G2. Day 4: while both groups on CMV, G2 patients showed statistically significant decrease in PEEP and mPaw with same gas exchange findings on Days 1, 2, 3 between two groups. During the study period, Hypotension was observed following HFOV in G2 and was most significant in Day 1. G2 showed statistically significant increase in barotraumas and required more midazolam, atracurium and norepinephrine. There was no statistically significant difference in 30 days mortality between both groups.

Early HFOV therapy is effective in improving oxygenation in burn patients with ARDS, but it failed to reduce hospital mortality.

Acute respiratory distress syndrome (ARDS) is an inflammatory condition of the lungs which can result in refractory and life-threatening hypoxaemic respiratory failure. The risk factors for the development of ARDS are many but include trauma, multiple blood transfusions, burns and major surgery, therefore this condition is not uncommon in the severely injured patient. When ARDS is severe, high-inspired oxygen concentrations are frequently required to minimise hypoxaemia. In these situations clinicians commonly utilise interventions termed ‘hypoxaemic rescue therapies’ in an attempt to improve oxygenation, as without these, conventional mechanical ventilation can be associated with high mortality. However, their lack of efficacy on mortality when used prophylactically in generalised ARDS cohorts has resulted in their use being confined to clinical trials and the subset of ARDS patients with refractory hypoxaemia.

First line hypoxaemic rescue therapies include inhaled nitric oxide, prone positioning, alveolar recruitment manoeuvres and high frequency oscillatory ventilation, which have all been shown to be effective in improving oxygenation. In situations where these first line rescue therapies are inadequate extra-corporeal membrane oxygenation has emerged as a lifesaving second line rescue therapy. Rescue therapies in critically ill patients with traumatic injuries presents specific challenges and requires careful assessment of both the short and longer term benefits, therapeutic limitations, and specific adverse effects before their use.

Approximately 16% of deaths in patients with ARDS results from refractory hypoxemia, which is the inability to achieve adequate arterial oxygenation despite high levels of inspired oxygen or the development of barotrauma. A number of ventilator-focused rescue therapies that can be used when conventional mechanical ventilation does not achieve a specific target level of oxygenation are discussed. A literature search was conducted and narrative review written to summarize the use of high levels of positive end-expiratory pressure, recruitment maneuvers, airway pressure-release ventilation, and high-frequency ventilation. Each therapy reviewed has been reported to improve oxygenation in patients with ARDS. However, none of them have been shown to improve survival when studied in heterogeneous populations of patients with ARDS. Moreover, none of the therapies has been reported to be superior to another for the goal of improving oxygenation. The goal of improving oxygenation must always be balanced against the risk of further lung injury. The optimal time to initiate rescue therapies, if needed, is within 96 h of the onset of ARDS, a time when alveolar recruitment potential is the greatest. A variety of ventilatory approaches are available to improve oxygenation in the setting of refractory hypoxemia and ARDS. Which, if any, of these approaches should be used is often determined by the availability of equipment and clinician bias.