Pathophysiology and clinical consequences of arterial blood gases and pH after cardiac arrest

Xylazine Exacerbates Fentanyl-Induced Respiratory Depression and Bradycardia

Fatal opioid overdoses in the United States have nearly tripled during the past decade, with greater than 92% involving a synthetic opioid like fentanyl. Fentanyl potently activates the μ-opioid receptor to induce both analgesia and respiratory depression. The danger of illicit fentanyl has recently been exacerbated by adulteration with xylazine, an α2-adrenergic receptor agonist typically used as a veterinary anesthetic. In 2023, over a 1,000% increase in xylazine-positive overdoses was reported in some regions of the U.S. Xylazine has been shown to potentiate the lethality of fentanyl in mice, yet a mechanistic underpinning for this effect has not been defined. Herein, we evaluate fentanyl, xylazine, and their combination in whole-body plethysmography (to measure respiration) and pulse oximetry (to measure blood oxygen saturation and heart rate) in male and female CD-1 mice. We show that xylazine decreases breathing rate more than fentanyl by increasing the expiration time. In contrast, fentanyl primarily reduces breathing by inhibiting inspiration, and xylazine exacerbates these effects. Fentanyl but not xylazine decreased blood oxygen saturation, and when combined, xylazine did not change the maximum level of fentanyl-induced hypoxia. Xylazine also reduced heart rate more than fentanyl. Finally, loss in blood oxygen saturation correlated with the frequency of fentanyl-induced apneas, but not breathing rate. Together, these findings provide insight into how the addition of xylazine to illicit fentanyl may increase the risk of overdose.

Neurological outcomes and mortality of hyperoxaemia in patients with acute brain injury: protocol for a systematic review and meta-analysis

INTRODUCTION

Oxygen is frequently prescribed in neurocritical care units. Avoiding hypoxaemia is a key objective in patients with acute brain injury (ABI). However, several studies suggest that hyperoxaemia may also be related to higher mortality and poor neurological outcomes in these patients. The evidence in this direction is still controversial due to the limited number of prospective studies, the lack of a common definition for hyperoxaemia, the heterogeneity in experimental designs and the different causes of ABI. To explore the correlation between hyperoxaemia and poor neurological outcomes and mortality in hospitalised adult patients with ABI, we will conduct a systematic review and meta-analysis of observational studies and RCTs.

METHODS AND ANALYSIS

The systematic review methods have been defined according to Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines and follow the PRISMA-Protocols structure. Studies published until June 2024 will be identified in the electronic databases MEDLINE, Embase, Scopus, Web of Science, The Cochrane Library, Cumulative Index to Nursing and Allied Health Literature and ClinicalTrials.gov. Retrieved records will be independently screened by four authors working in pairs, and the selected variables will be extracted from studies reporting data on the effect of 'hyperoxaemia' versus 'no hyperoxaemia on neurological outcomes and mortality in hospitalised patients with ABI. We will use covariate-adjusted ORs as outcome measures when reported since they account for potential cofounders and provide a more accurate estimate of the association between hyperoxaemia and outcomes; when not available, we will use univariate ORs. If the study presents the results as relative risks, it will be considered equivalent to the OR as long as the prevalence of the condition is close to 10%. Pooled estimates of both outcomes will be calculated applying random-effects meta-analysis. Interstudy heterogeneity will be assessed using the I2 statistic; risk of bias will be assessed through Risk Of Bias In Non-Randomised Studies of Interventions, Newcastle-Ottawa or RoB2 tools. Depending on data availability, we plan to conduct subgroup analyses by ABI type (traumatic brain injury, postcardiac arrest, subarachnoid haemorrhage, intracerebral haemorrhage and ischaemic stroke), arterial partial pressure of oxygen values, study quality, study time, neurological scores and other selected clinical variables of interest.

ETHICS AND DISSEMINATION

Specific ethics approval consent is not required as this is a review of previously published anonymised data. Results of the study will be shared with the scientific community via publication in a peer-reviewed journal and presentation at relevant conferences and workshops. It will also be shared key stakeholders, such as national or international health authorities, healthcare professionals and the general population, via scientific outreach journals and research institutes' newsletters.

The early change in pH values after out-of-hospital cardiac arrest is not associated with neurological outcome at hospital discharge

Background

The association between pH values and outcome for patients after out-of-hospital cardiac arrest (OHCA) was not fully elucidated; besides, the relationship of change in pH values and neurological outcome was unknown. The aim was to explore the association of pH values as well as change in pH values and neurological outcome for OHCA cardiac patients.

Methods

The adult patients with non-traumatic out-of-hospital cardiac arrest, shock-refractory ventricular fibrillation or pulseless ventricular tachycardia, and at least two arterial blood gases analysis recorded after admission were included. The change in pH values is calculated as the difference between the second and first pH value, and divided by time interval got the rate of change in pH values. The primary outcome was modified Rankin Score (mRS), dichotomized to good (mRS 0-3) and poor (mRS 4-6) outcomes at hospital discharge. The independent relationship of the first pH value, second pH value, and changes in pH values with neurological outcome was investigated with multivariable logistic regression models, respectively.

Results

A total of 1388 adult patients were included for analysis, of which 514 (37%) had good neurological outcome. The median first pH value and second pH value after admission were 7.21 (interquartile range [IQR] 7.09-7.29) and 7.28 (IQR 7.20-7.36), respectively. The median absolute, relative change, and rate of changes in pH values were 0.08 (IQR 0.01-0.16), 1.10% (IQR 0.11-2.22%), and 0.02 (IQR 0-0.06) per hour, respectively. After adjusting for confounders, the higher first pH value (odds ratio [OR] 3.81, confidence interval [CI] 1.60-9.24, P = 0.003) and higher second pH value (OR 9.54, CI 3.45-26.87, P < 0.001) after admission were associated with good neurological outcome, respectively. The absolute (OR 1.58, CI 0.58-4.30, P = 0.368) and relative (OR 1.03, CI 0.96-1.11, P = 0.399) change as well as the rate of change (OR 0.98, CI 0.33-2.71, P = 974) in pH values were not associated with neurological outcome.

Conclusions

For OHCA patients, abnormality in pH values was very common, with a more acidic pH value indicating poor neurological outcome. However, the change in pH values was not associated with outcomes.

Individualized Thresholds of Hypoxemia and Hyperoxemia and their Effect on Outcome in Acute Brain Injured Patients: A Secondary Analysis of the ENIO Study

BACKGROUND

In acute brain injury (ABI), the effects of hypoxemia as a potential cause of secondary brain damage and poor outcome are well documented, whereas the impact of hyperoxemia is unclear. The primary aim of this study was to assess the episodes of hypoxemia and hyperoxemia in patients with ABI during the intensive care unit (ICU) stay and to determine their association with in-hospital mortality. The secondary aim was to identify the optimal thresholds of arterial partial pressure of oxygen (PaO2) predicting in-hospital mortality.

METHODS

We conducted a secondary analysis of a prospective multicenter observational cohort study. Adult patients with ABI (traumatic brain injury, subarachnoid aneurysmal hemorrhage, intracranial hemorrhage, ischemic stroke) with available data on PaO2 during the ICU stay were included. Hypoxemia was defined as PaO2 < 80 mm Hg, normoxemia was defined as PaO2 between 80 and 120 mm Hg, mild/moderate hyperoxemia was defined as PaO2 between 121 and 299 mm Hg, and severe hyperoxemia was defined as PaO2 levels ≥ 300 mm Hg.

RESULTS

A total of 1,407 patients were included in this study. The mean age was 52 (±18) years, and 929 (66%) were male. Over the ICU stay, the fractions of patients in the study cohort who had at least one episode of hypoxemia, mild/moderate hyperoxemia, and severe hyperoxemia were 31.3%, 53.0%, and 1.7%, respectively. PaO2 values below 92 mm Hg and above 156 mm Hg were associated with an increased probability of in-hospital mortality. Differences were observed among subgroups of patients with ABI, with consistent effects only seen in patients without traumatic brain injury.

CONCLUSIONS

In patients with ABI, hypoxemia and mild/moderate hyperoxemia were relatively frequent. Hypoxemia and hyperoxemia during ICU stay may influence in-hospital mortality. However, the small number of oxygen values collected represents a major limitation of the study.

Monitoring of Brain Tissue Oxygen Tension in Cardiac Arrest: a Translational Systematic Review from Experimental to Clinical Evidence

BACKGROUND

Cardiac arrest (CA) is a sudden event that is often characterized by hypoxic-ischemic brain injury (HIBI), leading to significant mortality and long-term disability. Brain tissue oxygenation (PbtO2) is an invasive tool for monitoring brain oxygen tension, but it is not routinely used in patients with CA because of the invasiveness and the absence of high-quality data on its effect on outcome. We conducted a systematic review of experimental and clinical evidence to understand the role of PbtO2 in monitoring brain oxygenation in HIBI after CA and the effect of targeted PbtO2 therapy on outcomes.

METHODS

The search was conducted using four search engines (PubMed, Scopus, Embase, and Cochrane), using the Boolean operator to combine mesh terms such as PbtO2, CA, and HIBI.

RESULTS

Among 1,077 records, 22 studies were included (16 experimental studies and six clinical studies). In experimental studies, PbtO2 was mainly adopted to assess the impact of gas exchanges, drugs, or systemic maneuvers on brain oxygenation. In human studies, PbtO2 was rarely used to monitor the brain oxygen tension in patients with CA and HIBI. PbtO2 values had no clear association with patients' outcomes, but in the experimental studies, brain tissue hypoxia was associated with increased inflammation and neuronal damage.

CONCLUSIONS

Further studies are needed to validate the effect and the threshold of PbtO2 associated with outcome in patients with CA, as well as to understand the physiological mechanisms influencing PbtO2 induced by gas exchanges, drug administration, and changes in body positioning after CA.

Prehospital Advanced Airway Management and Ventilation for Out-of-Hospital Cardiac Arrest with Prehospital Return of Spontaneous Circulation: A Prospective Observational Cohort Study in Japan

BACKGROUND

The relationship among advanced airway management (AAM), ventilation, and oxygenation in patients with out-of-hospital cardiac arrest (OHCA) who achieve prehospital return of spontaneous circulation (ROSC) has not been validated. This study was designed to evaluate ventilation and oxygenation for each AAM technique (supraglottic devices [SGA] or endotracheal intubation [ETI]) using arterial blood gas (ABG) results immediately after hospital arrival.

METHODS

This observational cohort study, using data from the Japanese Association for Acute Medicine OHCA Registry, included patients with OHCA with prehospital and hospital arrival ROSC between July 1, 2014, and December 31, 2019. The primary outcomes were the partial pressure of carbon dioxide in the arterial blood (PaCO2) and partial pressure of oxygen in the arterial blood (PaO2) in the initial ABG at the hospital for each AAM technique (SGA or ETI) performed by paramedics. The secondary outcome was favorable neurological outcome (cerebral performance category [CPC] 1 or 2) for specific PaCO2 levels, which were defined as good ventilation (PaCO2 ≤45 mmHg) and insufficient ventilation (PaCO2 >45 mmHg).

RESULTS

This study included 1,527 patients. Regarding AAM, 1,114 and 413 patients were ventilated using SGA and ETI, respectively. The median PaCO2 and PaO2 levels were 74.50 mmHg and 151.35 mmHg in the SGA group, while 66.30 mmHg and 173.50 mmHg in the ETI group. PaCO2 was significantly lower in the ETI group than in the SGA group (12.55 mmHg; 95% CI 15.27 to 8.20, P-value < 0.001), while no significant difference was found in PaO2 by multivariate linear regression analysis. After stabilizing inverse probability of weighting (IPW), the adjusted odds ratio for favorable neurological outcome at 1 month was significant in the good ventilation group compared to the insufficient ventilation cohort (adjusted odds ratio = 2.12, 95%CI: 1.40 to 3.19, P value < 0.001).

CONCLUSION

The study showed that in OHCA patients with prehospital ROSC, the PaCO2 levels in the initial ABG were lower in the group with AAM by ETI than in the SGA group. Furthermore, patients with prehospital ROSC and PaCO2 ≤45 mmHg on arrival had an increased odds of favorable neurological outcome after stabilized IPW adjustment.

Carbon dioxide regulates cholesterol levels through SREBP2

In mammals, O2 and CO2 levels are tightly regulated and are altered under various pathological conditions. While the molecular mechanisms that participate in O2 sensing are well characterized, little is known regarding the signaling pathways that participate in CO2 signaling and adaptation. Here, we show that CO2 levels control a distinct cellular transcriptional response that differs from mere pH changes. Unexpectedly, we discovered that CO2 regulates the expression of cholesterogenic genes in a SREBP2-dependent manner and modulates cellular cholesterol accumulation. Molecular dissection of the underlying mechanism suggests that CO2 triggers SREBP2 activation through changes in endoplasmic reticulum (ER) membrane cholesterol levels. Collectively, we propose that SREBP2 participates in CO2 signaling and that cellular cholesterol levels can be modulated by CO2 through SREBP2.

Temperature Control After Cardiac Arrest: A Narrative Review

Cardiac arrest (CA) is a critical public health issue affecting more than half a million Americans annually. The main determinant of outcome post-CA is hypoxic-ischemic brain injury (HIBI), and temperature control is currently the only evidence-based, guideline-recommended intervention targeting secondary brain injury. Temperature control is a key component of a post-CA care bundle; however, conflicting evidence challenges its wide implementation across the vastly heterogeneous population of CA survivors. Here, we critically appraise the available literature on temperature control in HIBI, detail how the evidence has been integrated into clinical practice, and highlight the complications associated with its use and the timing of neuroprognostication after CA. Future clinical trials evaluating different temperature targets, rates of rewarming, duration of cooling, and identifying which patient phenotype benefits from different temperature control methods are needed to address these prevailing knowledge gaps.

Mechanical ventilation during cardiopulmonary resuscitation: influence of positive end-expiratory pressure and head-torso elevation

BACKGROUND

Efficient ventilation is important during cardiopulmonary resuscitation (CPR). Nevertheless, there is insufficient knowledge on how the patient's position affects ventilatory parameters during mechanically assisted CPR. We studied ventilatory parameters at different positive end-expiratory pressure (PEEP) levels and when using an inspiratory impedance valve (ITD) during horizontal and head-up CPR (HUP-CPR).

METHODS

In this human cadaver experimental study, we measured tidal volume (V_T) and pressure during CPR at different randomized PEEP levels (0, 5 or 10 cmH₂O) or with an ITD. CPR was performed, in the following order: horizontal (FLAT), at 18° and then at 35° head-thorax elevation. During the inspiratory phase we measured the net tidal volume (V_T) adjusted to predicted body weight (V_TPBW), reversed airflow (RAF), and maximum and minimum airway pressure (P_max and P_min).

RESULTS

Using ten thawed fresh-frozen cadavers we analyzed the inspiratory phase of 1843 respiratory cycles, 229 without CPR and 1614 with CPR. In a mixed linear model, thoracic position and PEEP significantly impacted V_TPBW (p < 0.001 for each), and the insufflation time, thoracic position and PEEP significantly affected the RAF (p < 0.001 for each) and P_max (p < 0.001). For P_min, only PEEP was significant (p < 0.001). In subgroup analysis, at 35° V_TPBW and P_max were significantly reduced compared with the flat or 18° position.

CONCLUSION

When using mechanical ventilation during CPR, it seems that the PEEP level and patient position are important determinants of respiratory parameters. Moreover, tidal volume seems to be lower when the thorax is positioned at 35°.

Ten rules for optimizing ventilatory settings and targets in post-cardiac arrest patients

Cardiac arrest (CA) is a major cause of morbidity and mortality frequently associated with neurological and systemic involvement. Supportive therapeutic strategies such as mechanical ventilation, hemodynamic settings, and temperature management have been implemented in the last decade in post-CA patients, aiming at protecting both the brain and the lungs and preventing systemic complications. A lung-protective ventilator strategy is currently the standard of care among critically ill patients since it demonstrated beneficial effects on mortality, ventilator-free days, and other clinical outcomes. The role of protective and personalized mechanical ventilation setting in patients without acute respiratory distress syndrome and after CA is becoming more evident. The individual effect of different parameters of lung-protective ventilation, including mechanical power as well as the optimal oxygen and carbon dioxide targets, on clinical outcomes is a matter of debate in post-CA patients. The management of hemodynamics and temperature in post-CA patients represents critical steps for obtaining clinical improvement. The aim of this review is to summarize and discuss current evidence on how to optimize mechanical ventilation in post-CA patients. We will provide ten tips and key insights to apply a lung-protective ventilator strategy in post-CA patients, considering the interplay between the lungs and other systems and organs, including the brain.

Oxygen targets and 6-month outcome after out of hospital cardiac arrest: a pre-planned sub-analysis of the targeted hypothermia versus targeted normothermia after Out-of-Hospital Cardiac Arrest (TTM2) trial

BACKGROUND

Optimal oxygen targets in patients resuscitated after cardiac arrest are uncertain. The primary aim of this study was to describe the values of partial pressure of oxygen values (PaO2) and the episodes of hypoxemia and hyperoxemia occurring within the first 72 h of mechanical ventilation in out of hospital cardiac arrest (OHCA) patients. The secondary aim was to evaluate the association of PaO2 with patients' outcome.

METHODS

Preplanned secondary analysis of the targeted hypothermia versus targeted normothermia after OHCA (TTM2) trial. Arterial blood gases values were collected from randomization every 4 h for the first 32 h, and then, every 8 h until day 3. Hypoxemia was defined as PaO2 < 60 mmHg and severe hyperoxemia as PaO2 > 300 mmHg. Mortality and poor neurological outcome (defined according to modified Rankin scale) were collected at 6 months.

RESULTS

1418 patients were included in the analysis. The mean age was 64 ± 14 years, and 292 patients (20.6%) were female. 24.9% of patients had at least one episode of hypoxemia, and 7.6% of patients had at least one episode of severe hyperoxemia. Both hypoxemia and hyperoxemia were independently associated with 6-month mortality, but not with poor neurological outcome. The best cutoff point associated with 6-month mortality for hypoxemia was 69 mmHg (Risk Ratio, RR = 1.009, 95% CI 0.93-1.09), and for hyperoxemia was 195 mmHg (RR = 1.006, 95% CI 0.95-1.06). The time exposure, i.e., the area under the curve (PaO2-AUC), for hyperoxemia was significantly associated with mortality (p = 0.003).

CONCLUSIONS

In OHCA patients, both hypoxemia and hyperoxemia are associated with 6-months mortality, with an effect mediated by the timing exposure to high values of oxygen. Precise titration of oxygen levels should be considered in this group of patients.

TRIAL REGISTRATION

clinicaltrials.gov NCT02908308 , Registered September 20, 2016.

The difference between arterial pCO2 and etCO2 after cardiac arrest - Outcome predictor or marker of unfavorable resuscitation circumstances?

INTRODUCTION

In former studies, the arterio-alveolar carbon dioxide gradient (ΔCO₂) predicted in-hospital mortality after initially survived cardiac arrest. As early outcome predictors are urgently needed, we evaluated ΔCO₂ as predictor for good neurological outcome in our cohort.

METHODS

We retrospectively analyzed all patients ≥18 years of age after non-traumatic in- and out of hospital cardiac arrest in the year 2018 from our resuscitation database. Patients without advanced airway management, incomplete datasets or without return of spontaneous circulation were excluded. The first arterial pCO₂ after admission and the etCO₂ in mmHg at the time of blood sampling were recorded from patient's charts. We then calculated ΔCO₂ (pCO₂ - etCO₂). For baseline analyses, ΔCO₂ was dichotomized into a low and high group with separation at the median. Good neurological outcome on day 30, expressed as Cerebral Performance Category 1-2, defined our primary endpoint. Survival to 30 days was used as secondary endpoint.

RESULTS

Out of 302 screened patients, 128 remained eligible for analyses. ΔCO₂ was lower in 30-day survivors with good neurological outcome (12.2 mmHg vs. 18.8 mmHg, p = 0.009) and in 30-day survivors (12.5 mmHg vs. 20.0 mmHg, p = 0.001). In patients with high ΔCO₂, a cardiac etiology of arrest was found less often. They had a higher body mass index, longer duration of resuscitation, higher amounts of epinephrine, lower pO₂ levels but both higher pCO2 and blood lactate levels, resulting in lower blood pH and HCO₃^- levels at admission. In a crude binary logistic regression analysis, ΔCO₂ was associated with 30-day neurological outcome (OR = 1.041 per mmHg of ΔCO₂, 95% CI 1.008-1.074, p = 0.014). This association persisted after the adjustment for age, sex, witnessed arrest and shockable first rhythm. However, after addition of the duration of resuscitation or the cumulative epinephrine dosage to the model, ΔCO₂ lost its association.

CONCLUSION

ΔCO₂ at admission after a successfully resuscitated cardiac arrest is associated with 30 days survival with good neurological outcome. However, a higher ΔCO₂ may rather be a surrogate for unfavorable resuscitation circumstances than an independent outcome predictor.

Ventilatory settings in the initial 72 h and their association with outcome in out-of-hospital cardiac arrest patients: a preplanned secondary analysis of the targeted hypothermia versus targeted normothermia after out-of-hospital cardiac arrest (TTM2) trial

PURPOSE

The optimal ventilatory settings in patients after cardiac arrest and their association with outcome remain unclear. The aim of this study was to describe the ventilatory settings applied in the first 72 h of mechanical ventilation in patients after out-of-hospital cardiac arrest and their association with 6-month outcomes.

METHODS

Preplanned sub-analysis of the Target Temperature Management-2 trial. Clinical outcomes were mortality and functional status (assessed by the Modified Rankin Scale) 6 months after randomization.

RESULTS

A total of 1848 patients were included (mean age 64 [Standard Deviation, SD = 14] years). At 6 months, 950 (51%) patients were alive and 898 (49%) were dead. Median tidal volume (V_T) was 7 (Interquartile range, IQR = 6.2-8.5) mL per Predicted Body Weight (PBW), positive end expiratory pressure (PEEP) was 7 (IQR = 5-9) cmH₂0, plateau pressure was 20 cmH₂0 (IQR = 17-23), driving pressure was 12 cmH₂0 (IQR = 10-15), mechanical power 16.2 J/min (IQR = 12.1-21.8), ventilatory ratio was 1.27 (IQR = 1.04-1.6), and respiratory rate was 17 breaths/minute (IQR = 14-20). Median partial pressure of oxygen was 87 mmHg (IQR = 75-105), and partial pressure of carbon dioxide was 40.5 mmHg (IQR = 36-45.7). Respiratory rate, driving pressure, and mechanical power were independently associated with 6-month mortality (omnibus p-values for their non-linear trajectories: p < 0.0001, p = 0.026, and p = 0.029, respectively). Respiratory rate and driving pressure were also independently associated with poor neurological outcome (odds ratio, OR = 1.035, 95% confidence interval, CI = 1.003-1.068, p = 0.030, and OR = 1.005, 95% CI = 1.001-1.036, p = 0.048). A composite formula calculated as [(4*driving pressure) + respiratory rate] was independently associated with mortality and poor neurological outcome.

CONCLUSIONS

Protective ventilation strategies are commonly applied in patients after cardiac arrest. Ventilator settings in the first 72 h after hospital admission, in particular driving pressure and respiratory rate, may influence 6-month outcomes.

Dynamic changes in arterial blood gas during cardiopulmonary resuscitation in out-of-hospital cardiac arrest

We aimed to investigate the prognostic value of dynamic changes in arterial blood gas analysis (ABGA) measured after the start of cardiopulmonary resuscitation (CPR) for return of spontaneous circulation (ROSC) in patients with out-of-hospital cardiac arrest (OHCA). This prospective observational study was conducted at the emergency department of a university hospital from February 2018 to February 2020. All blood samples for gas analysis were collected from a radial or femoral arterial line, which was inserted during CPR. Changes in ABGA parameters were expressed as delta (Δ), defined as the values of the second ABGA minus the values of the initial ABGA. The primary outcome was sustained ROSC. Out of the 80 patients included in the analysis, 13 achieved sustained ROSC after in-hospital resuscitation. Multivariable logistic analysis revealed that ΔpaO₂ (odds ratio [OR] = 1.023; 95% confidence interval [CI] = 1.004–1.043, p = 0.020) along with prehospital shockable rhythm (OR = 84.680; 95% CI = 2.561–2799.939, p = 0.013) and total resuscitation duration (OR = 0.881; 95% CI = 0.805–0.964, p = 0.006) were significant predictors for sustained ROSC. Our study suggests a possible association between ΔpaO₂ in ABGA during CPR and an increased rate of sustained ROSC in the late phase of OHCA.