Clinical practice, decision-making, and use of clinical decision support systems in invasive mechanical ventilation: a narrative review.

Invasive mechanical ventilation is a key supportive therapy for patients on intensive care. There is increasing emphasis on personalised ventilation strategies. Clinical decision support systems (CDSS) have been developed to support this. We conducted a narrative review to assess evidence that could inform device implementation. A search was conducted in MEDLINE (Ovid) and EMBASE. Twenty-nine studies met the inclusion criteria. Role allocation is well described, with interprofessional collaboration dependent on culture, nurse:patient ratio, the use of protocols, and perception of responsibility. There were no descriptions of process measures, quality metrics, or clinical workflow. Nurse-led weaning is well-described, with factors grouped by patient, nurse, and system. Physician-led weaning is heterogenous, guided by subjective and objective information, and 'gestalt'. No studies explored decision-making with CDSS. Several explored facilitators and barriers to implementation, grouped by clinician (facilitators: confidence using CDSS, retaining decision-making ownership; barriers: undermining clinician's role, ambiguity moving off protocol), intervention (facilitators: user-friendly interface, ease of workflow integration, minimal training requirement; barriers: increased documentation time), and organisation (facilitators: system-level mandate; barriers: poor communication, inconsistent training, lack of technical support). One study described factors that support CDSS implementation. There are gaps in our understanding of ventilation practice. A coordinated approach grounded in implementation science is required to support CDSS implementation. Future research should describe factors that guide clinical decision-making throughout mechanical ventilation, with and without CDSS, map clinical workflow, and devise implementation toolkits. Novel research design analogous to a learning organisation, that considers the commercial aspects of device design, is required.

The effects of oxygen induced pulmonary vasoconstriction on bedside measurement of pulmonary gas exchange.

In patients with respiratory failure measurements of pulmonary gas exchange are of importance. The bedside automatic lung parameter estimator (ALPE) of pulmonary gas exchange is based on changes in inspired oxygen (FiO2) assuming that these changes do not affect pulmonary circulation. This assumption is investigated in this study. Forty-two out of 65 patients undergoing coronary artery bypass grafting (CABG) had measurements of mean pulmonary arterial pressure (MPAP), cardiac output and pulmonary capillary wedge pressure thus enabling the calculation of pulmonary vascular resistance (PVR) at each FiO2 level. The research version of ALPE was used and FiO2 was step-wise reduced a median of 0.20 and ultimately returned towards baseline values, allowing 6-8 min' steady state period at each of 4-6 levels before recording the oxygen saturation (SpO2). FiO2 reduction led to median decrease in SpO2 from 99 to 92 %, an increase in MPAP of 4 mmHg and an increase in PVR of 36 dyn s cm(-5). Changes were immediately reversed on returning FiO2 towards baseline. In this study changes in MPAP and PVR are small and immediately reversible consistent with small changes in pulmonary gas exchange. This indicates that mild deoxygenation induced pulmonary vasoconstriction does not have significant influences on the ALPE parameters in patients after CABG.

Agreement between mathematically arterialised venous versus arterial blood gas values in patients undergoing non-invasive ventilation: a cohort study.

BACKGROUND: Blood gas analysis is important for assessment of ventilatory function. Traditionally, arterial analysis has been used. A method for mathematically arterialising venous blood gas values has been developed. Our aim was to validate this method in patients undergoing non-invasive ventilation (NIV) in an emergency department (ED). MATERIALS AND METHODS: This post hoc substudy of a prospective cohort study included adult patients undergoing NIV for acute respiratory compromise. When arterial blood gas analysis was required for clinical purposes, a venous sample was also drawn. Mathematically arterialised values were calculated independent of arterial values. Primary outcome of interest was agreement between mathematically arterialised venous and arterial values for pH and pCO2. Bland-Altman agreement plot analysis was used. RESULTS: Eighty sample-pairs (58 patients) were studied. Mean difference for arterial pH (actual-calculated) was 0.01 pH units (95% limits of agreement: -0.04, 0.06). Mean difference for pCO2 (actual-calculated) was -0.06 kPa (95% limits of agreement: -1.34, 1.22). CONCLUSIONS: For patients undergoing NIV in an ED, agreement between mathematically arterialised venous values and arterial values was close for pH but only moderate for pCO2. Depending on clinician tolerance for agreement, this method may be a clinically useful alternative to arterial blood gas analysis in the ED.

Measuring gas exchange with step changes in inspired oxygen: an analysis of the assumption of oxygen steady state in patients suffering from COPD.

Bedside estimation of pulmonary gas exchange efficiency may be possible from step changes in FIO2 and subsequent measurement of arterial oxygenation at steady state conditions. However, a steady state may not be achieved quickly after a change in FIO2, especially in patients with lung disease such as COPD, rendering this approach cumbersome. This paper investigates whether breath by breath measurement of respiratory gas and arterial oxygen levels as FIO2 is changed can be used as a much more rapid alternative to collecting data from steady state conditions for measuring pulmonary gas exchange efficiency. Fourteen patients with COPD were studied using 4-5 step changes in FIO2 in the range of 0.15-0.35. Values of expired respiratory gas and arterial oxygenation were used to calculate and compare the parameters of a mathematical model of pulmonary gas exchange in two cases: from data at steady state; and from breath by breath data prior to achievement of a steady state. For each patient, the breath by breath data were corrected for the delay in arterial oxygen saturation changes following each change in FIO2. Calculated model parameters were shown to be similar for the two data sets, with Bland-Altman bias and limits of agreement of -0.4 and -3.0 to 2.2 % for calculation of pulmonary shunt and 0.17 and -0.47 to 0.81 kPa for alveolar to end-capillary PO2, a measure of oxygen abnormality due to shunting plus regions of low [Formula: see text] A/[Formula: see text] ratio. This study shows that steady state oxygen levels may not be necessary when estimating pulmonary gas exchange using changes in FIO2. As such this technique may be applicable in patients with lung disease such as COPD.

A Prospective, Longitudinal Study Evaluating the Efficacy of an Automated Secretion Removal Technology.

BACKGROUND: Endotracheal suctioning causes discomfort, is associated with adverse effects, and is resource-demanding. An artificial secretion removal method, known as an automated cough, has been developed, which applies rapid, automated deflation, and inflation of the endotracheal tube cuff during the inspiratory phase of mechanical ventilation. This method has been evaluated in the hands of researchers but not when used by attending nurses. The aim of this study was to explore the efficacy of the method over the course of patient management as part of routine care. METHODS: This prospective, longitudinal, interventional study recruited 28 subjects who were intubated and mechanically ventilated. For a maximum of 7 d and on clinical need for endotracheal suctioning, the automatic cough procedure was applied. The subjects were placed in a pressure-regulated ventilation mode with elevated inspiratory pressure, and automated cuff deflation and inflation were performed 3 times, with this repeated if deemed necessary. Success was determined by resolution of the clinical need for suctioning as determined by the attending nurse. Adverse effects were recorded. RESULTS: A total of 84 procedures were performed. In 54% of the subjects, the artificial cough procedure was successful on > 70% of occasions, with 56% of all procedures considered successful. Ninety percent of all the procedures were performed in subjects who were spontaneously breathing and on pressure-support ventilation with peak inspiratory pressures of 20 cm H2O. Rates of adverse events were similar to those seen in the application of endotracheal suctioning. CONCLUSIONS: This study solely evaluated the efficacy of an automated artificial cough procedure, which illustrated the potential for reducing the need for endotracheal suctioning when applied by attending nurses in routine care.

Time to steady state after changes in FIO(2) in patients with COPD.

BACKGROUND: International guidelines recommend that when changing FIO2 in patients with COPD receiving Long-Term Oxygen Therapy (LTOT), 30 minutes should be waited for steady state before measurement of arterial blood gasses. This study evaluates whether 30 minutes is really necessary, as a smaller duration might improve the logistics of care, potentially reducing the time spent by patients at the out-patient clinic. METHODS: 12 patients with severe to very severe COPD according to the GOLD guidelines were included. Patients had a median FEV1% of 23% of the predicted value (range 15-64%), median FEV1/FVC 0.43 (range 0.26-0.63), and chronic respiratory failure necessitating LTOT, 1-4 liters/minute, minimum 16 hours/day. Following a FIO2 reduction (wash out), arterial blood gases were measured at 0, 1, 2, 4, 8, 12, 17, 22, 32 and 34 minutes. FIO2 was then increased to baseline levels (wash in) and blood gasses measured at 0, 1, 2, 4, 8, 12, 17, 22, 32, and 34 minutes. Data were analyzed to examine the dynamics of arterial PO2 and saturation (SO2) wash out and wash in by calculating the time constants, tau (ô), and to evaluate the time required to reach values which might be considered clinically stable, defined as PO2 within 0.5 kPa and SO2 within 1% of equilibrium values. RESULTS: For arterial PO2 values of time constants were about 3 minutes and similar for both wash out and wash in. A median of 5 minutes was required to reach clinically stable values of PO2 in both wash out and wash in, with 7-8 minutes sufficient in 75% of patients, and in the worst case 14 minutes. For SO2, values of the time constant were 4.5 and 1.4 minutes for wash out and wash in, respectively. The time required to reach clinically stable values was different in the two phases. For wash out the median time was 7.4 minutes, and in the worst case 15.6 minutes. For wash in the median time was 2.6 minutes and in worst case 6.8 minutes. No significant changes in PCO2 or pH were seen during FIO2 changes. DISCUSSION/CONCLUSION: This study shows that oxygen equilibration relevant for clinical interpretation requires only 10 minutes following an increase and 16 minutes following a decrease in FIO2. over the range studied.

Clinical refinement of the automatic lung parameter estimator (ALPE).

The automatic lung parameter estimator (ALPE) method was developed in 2002 for bedside estimation of pulmonary gas exchange using step changes in inspired oxygen fraction (FIO2). Since then a number of studies have been conducted indicating the potential for clinical application and necessitating systems evolution to match clinical application. This paper describes and evaluates the evolution of the ALPE method from a research implementation (ALPE1) to two commercial implementations (ALPE2 and ALPE3). A need for dedicated implementations of the ALPE method was identified: one for spontaneously breathing (non-mechanically ventilated) patients (ALPE2) and one for mechanically ventilated patients (ALPE3). For these two implementations, design issues relating to usability and automation are described including the mixing of gasses to achieve FIO2 levels, and the automatic selection of FIO2. For ALPE2, these improvements are evaluated against patients studied using the system. The major result is the evolution of the ALPE method into two dedicated implementations, namely ALPE2 and ALPE3. For ALPE2, the usability and automation of FIO2 selection has been evaluated in spontaneously breathing patients showing that variability of gas delivery is 0.3 % (standard deviation) in 1,332 breaths from 20 patients. Also for ALPE2, the automated FIO2 selection method was successfully applied in 287 patient cases, taking 7.2 ± 2.4 min and was shown to be safe with only one patient having SpO2 < 86 % when the clinician disabled the alarms. The ALPE method has evolved into two practical, usable systems targeted at clinical application, namely ALPE2 for spontaneously breathing patients and ALPE3 for mechanically ventilated patients. These systems may promote the exploration of the use of more detailed descriptions of pulmonary gas exchange in clinical practice.

Efficacy of an Automated Secretion Removal Technology at Different Inspiratory Pressures.

BACKGROUND: Endotracheal suctioning is resource demanding, causes patient discomfort, and is associated with adverse effects. A new artificial cough method has been developed for automated secretion removal by using rapid deflation and inflation of the endotracheal tube cuff during the inspiratory phase of mechanical ventilation. This method has been evaluated in a bench model and in animals but not in human subjects. The aim of this study was to investigate whether this method can remove the need for endotracheal suctioning in subjects and whether this is dependent on ventilator settings. METHODS: This prospective, non-controlled study recruited 20 subjects on invasive mechanical ventilation. On the clinical need for endotracheal suctioning, the automatic cough procedure was applied 3 times over 30 s, with this repeated at higher ventilatory pressure and lower respiratory frequency if considered unsuccessful. Success was determined by removal of the clinical need for suctioning. Subject safety and comfort was measured by using the Critical-Care Pain Observation Tool before and after the procedure, and negative effects were recorded. To assess intra-subject variability, the procedure was performed on 3 different occasions for each subject. RESULTS: The procedure was successful in 18 of 20 subjects (90%), with mean subject success rates of 53% at low settings (peak inspiratory pressure 21.8 ± 3.8 cm H2O) and 83% at high settings (peak inspiratory pressure 25.6 ± 3.6 cm H2O). The Critical-Care Pain Observation Tool category remained unchanged in 30 procedures (77%), improved in 7 (18%), and deteriorated in 2 (5%). CONCLUSIONS: This study illustrated the potential for significant reduction in the clinical need for endotracheal suctioning after the use of an automated artificial cough procedure at both low and high peak inspiratory pressures, and that was well tolerated.

Inspiratory pressure waveform influences time to failure, respiratory muscle fatigue, and metabolism during resistive breathing.

Increased ventilatory work beyond working capacity of the respiratory muscles can induce fatigue, resulting in limited respiratory muscle endurance (Tlim ). Previous resistive breathing investigations all applied square wave inspiratory pressure as fatigue-inducing pattern. Spontaneous breathing pressure pattern more closely approximate a triangle waveform. This study aimed at comparing Tlim , maximal inspiratory pressure (PImax ), and metabolism between square and triangle wave breathing. Eight healthy subjects (Wei = 76 ± 10 kg, H = 181 ± 7.9 cm, age = 33.5 ± 4.8 years, sex [F/M] = 1/7) completed the study, comprising two randomized matched load resistive breathing trials with square and triangle wave inspiratory pressure waveform. Tlim decreased with a mean difference of 8 ± 7.2 min (p = 0.01) between square and triangle wave breathing. PImax was reduced following square wave (p = 0.04) but not for triangle wave breathing (p = 0.88). Higher VO2 was observed in the beginning and end for the triangle wave breathing compared with the square wave breathing (p = 0.036 and p = 0.048). Despite higher metabolism, Tlim was significantly longer in triangle wave breathing compared with square wave breathing, showing that the pressure waveform has an impact on the function and endurance of the respiratory muscles.

Effect of expiratory flow limitation on ventilation/perfusion mismatch and perioperative lung function during pneumoperitoneum and Trendelenburg position.

BACKGROUND: Laparoscopic surgery and Trendelenburg position may affect the respiratory function and alter the gas exchange. Further the reduction of the lung volumes may contribute to the development of expiratory flow limitation (EFL). The latter is associated with an increased risk of postoperative pulmonary complications. Our aim was to investigate the incidence of EFL and to evaluate its effect on pulmonary function and intraoperative V/Q mismatch. METHODS: This is a prospective study on patients undergoing elective laparoscopic gynecological surgery. We evaluated respiratory mechanics, V/Q mismatch and presence of EFL after anesthesia induction, during pneumoperitoneum and Trendelenburg position and at the end of surgery. Intraoperative gas exchange and hemodynamic were also recorded. Clinical data were collected until seven days after surgery to evaluate the onset of pulmonary postoperative complications (PPCs). RESULTS: Among the 66 patients enrolled, 25/66 (38%) exhibited EFL during surgery, of whom 10/66 (15%) after anesthesia induction, and the remaining 15 patients after pneumoperitoneum and Trendelenburg position. Median PEEP able to reverse flow limitation was 7 [7-10] cmH2O after anesthesia induction and 9 [8-15] cmH2O after pneumoperitoneum and Trendelenburg position. Patients with EFL had significantly higher shunt (17 [2-25] vs. 9 [1-19]; P=0.05), low V̇/Q̇ (27 [20-70] vs. 15 [10-22]; P=0.05) and high V̇/Q̇ (10 [7-14] vs. 6 [4-7]; P=0.024). At the end of surgery, only high V/Q was significantly higher in EFL patients. Further, they exhibited higher incidence of postoperative pulmonary complication (48% (12/25) vs. 15% (6/41), P=0.005), hypoxemia and hypercapnia (80% [20/25] vs. 32% [13/41]; P<0.001). CONCLUSIONS: Expiratory flow limitation is a common phenomenon during gynecological laparoscopic surgery associated with worsen gas exchange, increased V/Q mismatch and altered lung mechanics. Our study showed that patients experiencing EFL during surgery showed a higher risk for PPCs.

Calculating acid-base and oxygenation status during COPD exacerbation using mathematically arterialised venous blood.

BACKGROUND: Repeated arterial puncture is painful. A mathematical method exists for transforming peripheral venous pH, PCO 2 and PO 2 to arterial eliminating the need for arterial sampling. This study evaluates this method to monitor acid base and oxygenation during admission for exacerbation of chronic obstructive pulmonary disease (COPD). METHODS: Simultaneous arterial and peripheral venous blood was analysed. Venous values were used to calculate arterial pH, PCO 2 and PO 2, with these compared to measured values using Bland-Altman analysis and scatter plots. Calculated values of PO 2 were assessed with previously defi ned rules.Differences between maximal changes of calculated and measured values were compared using a t-test, with trends analysed by inspection of plots. RESULTS: Fifty-four patients, median age 67 years (range 62 ­ 75), were studied on average 3 days. Mean values of pH,PCO 2 and PO 2 were 7.432 } 0.047, 6.8 } 1.7 kPa and 9.2 } 1.5kPa, respectively. Calculated and measured arterial pH and PCO 2 agreed well, differences having small bias and SD(0.000 } 0.022 pH, ­ 0.06 } 0.50 kPa PCO 2 ), significantly better than venous blood alone. Calculated PO 2 obeyed the clinical rules. Calculated values could track patients, with no significant differences in maximal changes in measured and calculated values (pH p = 0.96, PCO 2 p = 0.62, PO 2 p = 0.33), and time-course plots matching quantity and pattern of change in measurements. CONCLUSIONS: This study shows that arterial pH, PCO 2 and PO 2 can be calculated from peripheral venous values so as to characterise changes seen during exacerbation. Application of the method has potential to reduce arterial sampling, decrease discomfort and enable venous sampling as routine practice.

Decision support system to evaluate ventilation in the acute respiratory distress syndrome (DeVENT study)-trial protocol.

BACKGROUND: The acute respiratory distress syndrome (ARDS) occurs in response to a variety of insults, and mechanical ventilation is life-saving in this setting, but ventilator-induced lung injury can also contribute to the morbidity and mortality in the condition. The Beacon Caresystem is a model-based bedside decision support system using mathematical models tuned to the individual patient's physiology to advise on appropriate ventilator settings. Personalised approaches using individual patient description may be particularly advantageous in complex patients, including those who are difficult to mechanically ventilate and wean, in particular ARDS. METHODS: We will conduct a multi-centre international randomised, controlled, allocation concealed, open, pragmatic clinical trial to compare mechanical ventilation in ARDS patients following application of the Beacon Caresystem to that of standard routine care to investigate whether use of the system results in a reduction in driving pressure across all severities and phases of ARDS. DISCUSSION: Despite 20 years of clinical trial data showing significant improvements in ARDS mortality through mitigation of ventilator-induced lung injury, there remains a gap in its personalised application at the bedside. Importantly, the protective effects of higher positive end-expiratory pressure (PEEP) were noted only when there were associated decreases in driving pressure. Hence, the pressures set on the ventilator should be determined by the diseased lungs' pressure-volume relationship which is often unknown or difficult to determine. Knowledge of extent of recruitable lung could improve the ventilator driving pressure. Hence, personalised management demands the application of mechanical ventilation according to the physiological state of the diseased lung at that time. Hence, there is significant rationale for the development of point-of-care clinical decision support systems which help personalise ventilatory strategy according to the current physiology. Furthermore, the potential for the application of the Beacon Caresystem to facilitate local and remote management of large numbers of ventilated patients (as seen during this COVID-19 pandemic) could change the outcome of mechanically ventilated patients during the course of this and future pandemics. TRIAL REGISTRATION: ClinicalTrials.gov identifier NCT04115709. Registered on 4 October 2019, version 4.0.

Is venous blood a more reliable description of acid-base state following simulated hypo- and hyperventilation?

BACKGROUND: ABGs are performed in acute conditions as the reference method for assessing the acid-base status of blood. Hyperventilation and breath-holding are common ventilatory changes that occur around the time of sampling, rapidly altering the 'true' status of the blood. This is particularly relevant in emergency medicine patients without permanent arterial catheters, where the pain and anxiety of arterial punctures can cause ventilatory changes. This study aimed to determine whether peripheral venous values could be a more reliable measure of blood gases following acute changes in ventilation. METHODS: To allow for characterisation of ventilatory changes typical of acutely ill patients, but without the confounding influence of perfusion or metabolic disturbances, 30 patients scheduled for elective surgery were studied in a prospective observational study. Following anaesthesia, and before the start of the surgery, ventilator settings were altered to achieve a + 100% or - 60% change in alveolar ventilation ('hyper-' or 'hypoventilation'), changes consistent with the anticipation of a painful arterial puncture commonly encountered in the emergency room. Blood samples were drawn simultaneously from indwelling arterial and peripheral venous catheters at baseline, and at 15, 30, 45, 60, 90 and 120 s following the ventilatory change. Comparisons between the timed arterial (or venous) samples were done using repeated-measures ANOVA, with post-hoc analysis using Bonferroni's correction. RESULTS: Arterial blood pH and PCO2 changed rapidly within the first 15-30s after both hyper- and hypoventilation, plateauing at around 60s (∆pH = ±0.036 and ∆PCO2 = ±0.64 kPa (4.7 mmHg), respectively), with peripheral venous values remaining relatively constant until 60s, and changing minimally thereafter. Mean arterial changes were significantly different at 30s (P < 0.001) when compared to baseline, in response to both hyper- and hypoventilation. CONCLUSION: This study has shown that substantial differences in arterial and peripheral venous acid-base status can be due to acute changes in ventilation, commonly seen in the ER over the 30s necessary to sample arterial blood. If changes are transient, peripheral venous blood may provide a more reliable description of acid-base status.

Enabling a learning healthcare system with automated computer protocols that produce replicable and personalized clinician actions.

Clinical decision-making is based on knowledge, expertise, and authority, with clinicians approving almost every intervention-the starting point for delivery of "All the right care, but only the right care," an unachieved healthcare quality improvement goal. Unaided clinicians suffer from human cognitive limitations and biases when decisions are based only on their training, expertise, and experience. Electronic health records (EHRs) could improve healthcare with robust decision-support tools that reduce unwarranted variation of clinician decisions and actions. Current EHRs, focused on results review, documentation, and accounting, are awkward, time-consuming, and contribute to clinician stress and burnout. Decision-support tools could reduce clinician burden and enable replicable clinician decisions and actions that personalize patient care. Most current clinical decision-support tools or aids lack detail and neither reduce burden nor enable replicable actions. Clinicians must provide subjective interpretation and missing logic, thus introducing personal biases and mindless, unwarranted, variation from evidence-based practice. Replicability occurs when different clinicians, with the same patient information and context, come to the same decision and action. We propose a feasible subset of therapeutic decision-support tools based on credible clinical outcome evidence: computer protocols leading to replicable clinician actions (eActions). eActions enable different clinicians to make consistent decisions and actions when faced with the same patient input data. eActions embrace good everyday decision-making informed by evidence, experience, EHR data, and individual patient status. eActions can reduce unwarranted variation, increase quality of clinical care and research, reduce EHR noise, and could enable a learning healthcare system.

A Pilot Bench Study of Decision Support for Proportional Assist Ventilation.

The purpose was to develop a bench setup for testing a decision support system (DSS) for proportional assist ventilation (PAV). The test setup was based on a patient simulator connected to a mechanical ventilator with the DSS measurement sensors connected to the respiratory circuit. A test case was developed with parameters of lung mechanics reflecting a patient with mild acute respiratory distress syndrome. Five experiments were performed starting at different levels of percentage support (%Supp) and continuing until the DSS advised to remain at current settings. Final advice ranged from %Supp of 50-70%, indicating some dependence of baseline level, but with resulting patient effort estimates indicating that this may not be clinically important. Further studies are required of test cases reflecting different patient types and in patients.

Correlation between acid-base parameters measured in arterial blood and venous blood sampled peripherally, from vena cavae superior, and from the pulmonary artery.

OBJECTIVE: In intensive care units arterial blood sampling is routine for analysing acid-base and oxygenation status. In nonintensive departments arterial blood sampling is seldom performed. Venous blood sampling is routine but not usually analysed for acid-base and oxygenation status. This study describes the correlation between arterial and peripheral, central and mixed venous pH, PCO2 and PO2 in a wide range of adult patients. METHODS: Arterial and venous blood samples were taken anaerobically and simultaneously. The values of pH, PCO2 and PO2 were compared using Bland-Altman plots. RESULTS: A total of 103 patients were included. The arteriovenous difference (bias+/-SD) for pH was 0.026+/-0.023 and for PCO2 -0.60+/-0.57 kPa (peripheral venous blood), 0.036+/-0.014 and -0.79+/-0.26 kPa (central venous blood) and 0.026+/-0.010 and -0.67+/-0.22 kPa (mixed venous blood). The arteriovenous difference for PO2 for peripheral, central and mixed venous blood was 6.27+/-4.36, 8.33+/-3.94 and 11.00+/-4.87 kPa, respectively. CONCLUSION: The venous values of pH, corrected for bias, can give arterial values which are within reasonable laboratory and clinical acceptance criteria. For PCO2 this is also true, except for peripheral blood, where the standard deviation is outside laboratory acceptance criteria but within clinical acceptance criteria. For PO2 the arteriovenous differences are not randomly distributed and even for PO2

A decision support system for suggesting ventilator settings: retrospective evaluation in cardiac surgery patients ventilated in the ICU.

Selecting appropriate ventilator settings decreases the risk of ventilator-induced lung injury. A decision support system (DSS) has been developed based on physiological models, which can advise on setting of tidal volume (Vt), respiratory frequency (f) and fraction of inspired oxygen (FiO2). The aim of this study is to assess the feasibility of the DSS by comparing its advice with the values used in clinical practice. Data from 20 patients following uncomplicated coronary artery bypass grafting (CABG) with cardiopulmonary bypass was used to test the DSS. Ventilator settings suggested by the DSS were compared to the settings selected by the clinician. When compared to the clinician the DSS suggested: lowering FiO2 (by median 7%, range 2-17%) at high SpO2 and increasing FiO2 (by median 2%, range 1-5%) at low SpO2; lowering ventilation volume (by median 0.57 l min(-1), range 0.2-1.1 l min(-1)) at high pHa and increasing ventilation volume (by median 0.4 l min(-1), range 0.1-0.9 l min(-1)) at low pHa. Suggested changes in ventilation volume were such that simulated values of PIP were < or = 22.9 cmH2O and respiratory frequency < or = 18 breaths min(-1). In all cases, computer suggested values of FiO2, Vt or f were consistent with maintaining sufficient oxygenation, normalising pH and obtaining low values of PIP.

Variation in the PaO2/FiO2 ratio with FiO2: mathematical and experimental description, and clinical relevance.

INTRODUCTION: Previous studies have shown through theoretical analyses that the ratio of the partial pressure of oxygen in arterial blood (PaO2) to the inspired oxygen fraction (FiO2) varies with the FiO2 level. The aim of the present study was to evaluate the relevance of this variation both theoretically and experimentally using mathematical model simulations, comparing these ratio simulations with PaO2/FiO2 ratios measured in a range of different patients. METHODS: The study was designed as a retrospective study using data from 36 mechanically ventilated patients and 57 spontaneously breathing patients studied on one or more occasions. Patients were classified into four disease groups (normal, mild hypoxemia, acute lung injury and acute respiratory distress syndrome) according to their PaO2/FiO2 ratio. On each occasion the patients were studied using four to eight different FiO2 values, achieving arterial oxygen saturations in the range 85-100%. At each FiO2 level, measurements were taken of ventilation, of arterial acid-base and of oxygenation status. Two mathematical models were fitted to the data: a one-parameter 'effective shunt' model, and a two-parameter shunt and ventilation/perfusion model. These models and patient data were used to investigate the variation in the PaO2/FiO2 ratio with FiO2, and to quantify how many patients changed disease classification due to variation in the PaO2/FiO2 ratio. An F test was used to assess the statistical difference between the two models' fit to the data. A confusion matrix was used to quantify the number of patients changing disease classification. RESULTS: The two-parameter model gave a statistically better fit to patient data (P < 0.005). When using this model to simulate variation in the PaO2/FiO2 ratio, disease classification changed in 30% of the patients when changing the FiO2 level. CONCLUSION: The PaO2/FiO2 ratio depends on both the FiO2 level and the arterial oxygen saturation level. As a minimum, the FiO2 level at which the PaO2/FiO2 ratio is measured should be defined when quantifying the effects of therapeutic interventions or when specifying diagnostic criteria for acute lung injury and acute respiratory distress syndrome. Alternatively, oxygenation problems could be described using parameters describing shunt and ventilation/perfusion mismatch.