Oxygen therapy in critical illness: precise control of arterial oxygenation and permissive hypoxemia

Extreme, expedition, and wilderness medicine

Extreme, expedition, and wilderness medicine are modern and rapidly evolving specialties that address the spirit of adventure and exploration. The relevance of and interest in these specialties are changing rapidly to match the underlying activities, which include global exploration, adventure travel, and military deployments. Extreme, expedition, and wilderness medicine share themes of providing best available medical care in the outdoors, especially in austere or remote settings. Early clinical and logistics decision making can often have important effects on subsequent outcomes. There are lessons to be learned from out-of-hospital care, military medicine, humanitarian medicine, and disaster medicine that can inform in-hospital medicine, and vice-versa. The future of extreme, expedition, and wilderness medicine will be defined by both recipients and practitioners, and empirical observations will be transformed by evidence-based practice.

Re-tooling critical care to become a better intensivist: something old and something new

Developments in recent years have placed powerful new tools of diagnosis, therapy, and communication at the disposal of medicine in general, and of critical care in particular. The art of healing requires not only technical proficiency, but also personal connection, multidisciplinary teamwork, and commitment to the venerable traditions of our profession. The latter often seem to be under assault by today's high-pressure, high-efficiency, and increasingly business-driven hospital environments. Re-tooling critical care for the future generations of caregivers requires something old--empathetic connection--as well as the exciting newer technologies of our science and practice.

Improved in vivo performance of amperometric oxygen (PO2) sensing catheters via electrochemical nitric oxide generation/release

A novel electrochemically controlled release method for nitric oxide (NO) (based on electrochemical reduction of nitrite ions) is combined with an amperometric oxygen sensor within a dual lumen catheter configuration for the continuous in vivo sensing of the partial pressure of oxygen (PO2) in blood. The on-demand electrochemical NO generation/release method is shown to be fully compatible with amperometric PO2 sensing. The performance of the sensors is evaluated in rabbit veins and pig arteries for 7 and 21 h, respectively. Overall, the NO releasing sensors measure both venous and arterial PO2 values more accurately with an average deviation of -2 ± 11% and good correlation (R(2) = 0.97) with in vitro blood measurements, whereas the corresponding control sensors without NO release show an average deviation of -31 ± 28% and poor correlation (R(2) = 0.43) at time points >4 h after implantation in veins and >6 h in arteries. The NO releasing sensors induce less thrombus formation on the catheter surface in both veins and arteries (p < 0.05). This electrochemical NO generation/release method could offer a new and attractive means to improve the biocompatibility and performance of implantable chemical sensors.

Targeting normoxemia in acute respiratory distress syndrome may cause worse short-term outcomes because of oxygen toxicity

It was suggested that targeting normoxemia (PaO2 85-110 mm Hg) in patients with acute respiratory distress syndrome (ARDS) might prevent neurocognitive dysfunction in survivors. However, targeting normoxemia may cause detrimental effects to the lungs from oxygen toxicity. Some have suggested that oxygen is not harmful to the lungs at FiO2 (fraction of inspired oxygen) levels less than 0.6-0.7, but contrasting evidence in normal humans suggests that there can be untoward effects of moderate FiO2 levels. Furthermore, in experimental models of the acute respiratory distress syndrome, coexisting lung inflammation increases susceptibility to oxygen toxicity. Coexisting lung inflammation may lower the threshold for oxygen toxicity in patients with ARDS or in other acute illnesses in the lung. Moreover, physicians frequently prescribe higher FiO2 levels than are necessary to achieve their arterial oxygenation goal, further increasing the risk of oxygen toxicity. Targeting normoxemia in patients with ARDS may prevent some long-term neurocognitive deficits in survivors, but it may increase lung inflammation and cause worse short-term clinical outcomes. We advocate for a clinical trial in patients with ARDS to determine more appropriate goals for arterial oxygenation.

Design and conduct of Xtreme Everest 2: An observational cohort study of Sherpa and lowlander responses to graduated hypobaric hypoxia

Objective: Oxygen availability falls with ascent to altitude and also as a consequence of critical illness. Because cellular sequelae and adaptive processes may be shared in both circumstances, high altitude exposure (‘physiological hypoxia’) assists in the exploration of the response to pathological hypoxia. We therefore studied the response of healthy participants to progressive hypobaric hypoxia at altitude. The primary objective of the study was to identify differences between high altitude inhabitants (Sherpas) and lowland comparators.

Methods: We performed an observational cohort study of human responses to progressive hypobaric hypoxia (during ascent) and subsequent normoxia (following descent) comparing Sherpas with lowlanders. Studies were conducted in London (35m), Kathmandu (1300m), Namche Bazaar (3500m) and Everest Base Camp (5300m). Of 180 healthy volunteers departing from Kathmandu, 64 were Sherpas and 116 were lowlanders. Physiological, biochemical, genetic and epigenetic data were collected. Core studies focused on nitric oxide metabolism, microcirculatory blood flow and exercise performance. Additional studies performed in nested subgroups examined mitochondrial and metabolic function, and ventilatory and cardiac variables. Of the 180 healthy participants who left Kathmandu, 178 (99%) completed the planned trek. Overall, more than 90% of planned testing was completed. Forty-four study protocols were successfully completed at altitudes up to and including 5300m. A subgroup of identical twins (all lowlanders) was also studied in detail.

Conclusion: This programme of study (Xtreme Everest 2) will provide a rich dataset relating to human adaptation to hypoxia, and the responses seen on re-exposure to normoxia. It is the largest comprehensive high altitude study of Sherpas yet performed. Translational data generated from this study will be of relevance to diseases in which oxygenation is a major factor.

Double-blind, placebo-controlled pilot randomized trial of methylprednisolone infusion in pediatric acute respiratory distress syndrome

OBJECTIVE

Low-dose methylprednisolone therapy in adults with early acute respiratory distress syndrome reduces systemic inflammation, duration of mechanical ventilation, and ICU length of stay. We report a pilot randomized trial of glucocorticoid treatment in early pediatric acute respiratory distress syndrome.

DESIGN

Double-blind, placebo-controlled randomized clinical trial.

SETTING

Le Bonheur Children's Hospital, Memphis, TN.

PATIENTS

Children (0-18 yr) with acute respiratory distress syndrome undergoing mechanical ventilation.

INTERVENTIONS

Patients were randomly assigned to steroid or placebo groups within 72 hours of intubation. IV methylprednisolone administered as loading dose (2 mg/kg) and continuous infusions (1 mg/kg/d) on days 1-7 and then tapered over days 8-14. Both groups were ventilated according to the Acute Respiratory Distress Syndrome Network protocol modified for children. Daily surveillance was performed for adverse effects.

MEASUREMENTS AND MAIN RESULTS

Thirty-five patients were randomized to the steroid (n = 17, no death) and placebo groups (n = 18, two deaths). No differences occurred in length of mechanical ventilation, ICU stay, hospital stay, or mortality between the two groups. At baseline, higher plateau pressures (p = 0.006) and lower Pediatric Logistic Organ Dysfunction scores (p = 0.04) occurred in the steroid group; other characteristics were similar. Despite higher plateau pressures on days 1 (p = 0.006) and 2 (p = 0.025) due to poorer lung compliance in the steroid group, they had lower PaCO2 values on days 2 (p = 0.009) and 3 (p = 0.014), higher pH values on day 2 (p = 0.018), and higher PaO2/FIO2 ratios on days 8 (p = 0.047) and 9 (p = 0.002) compared with the placebo group. Fewer patients in the steroid group required treatment for postextubation stridor (p = 0.04) or supplemental oxygen at ICU transfer (p = 0.012). Steroid therapy was not associated with detectable adverse effects.

CONCLUSION

This study demonstrates the feasibility of administering low-dose glucocorticoid therapy and measuring clinically relevant outcomes in pediatric acute respiratory distress syndrome. Changes in oxygenation and/or ventilation are consistent with early acute respiratory distress syndrome pathophysiology and results of similar clinical trials in adults. We propose and design a larger randomized trial to define the role of glucocorticoid therapy in pediatric acute respiratory distress syndrome.

High cumulative oxygen levels are associated with improved survival of children treated with mild therapeutic hypothermia after cardiac arrest

AIM

The aim of this study was to analyze the relationship between the partial pressure of arterial oxygen (PaO2) and in-hospital (IH) mortality in children after cardiac arrest (CA) using the conventional cutoff analysis, which was compared with the cumulative analysis, a new method in PaO2 analysis. Additionally, we analyzed this relationship for children with and without mild therapeutic hypothermia (MTH; 32-34 °C).

METHODS

This observational cohort study included all children (aged >28 days) with CA and return of spontaneous circulation (ROSC) between 2002 and 2011. The first research question was the association between PaO2 and IH mortality after ROSC. This was analyzed for three hyperoxia cutoff values, and for three time intervals using the cumulative PaO2 determined with the area under the curve (AUC). For the second research question, these analyses were repeated for children with and without MTH.

RESULTS

Of the 200 patients included (median age 2.6 years), 84 (42%) survived to hospital discharge. Fifty-eight children (29%) were treated with MTH. With the cutoff analysis and the AUC analysis we found no relationship between PaO2 and IH mortality. However, analysis of the MTH-group showed a lower IH mortality in children with high cumulative PaO2 levels on two of the three time intervals. Multivariable analysis showed significantly higher odds of survival (0.643 (95% confidence interval (CI) 0.424-0.976), 0.554 (95% CI 0.335-0.916)).

CONCLUSIONS

Cumulative PaO2 analysis showed that the IH mortality is significantly lower in MTH-treated children with high PaO2 levels. The effects of cumulative PaO2 on the outcome need to be studied further, and this will help us to achieve individualized goal-directed therapy.

Extracorporeal life support in critically ill adults

Extracorporeal life support (ECLS) has become increasingly popular as a salvage strategy for critically ill adults. Major advances in technology and the severe acute respiratory distress syndrome that characterized the 2009 influenza A(H1N1) pandemic have stimulated renewed interest in the use of venovenous extracorporeal membrane oxygenation (ECMO) and extracorporeal carbon dioxide removal to support the respiratory system. Theoretical advantages of ECLS for respiratory failure include the ability to rest the lungs by avoiding injurious mechanical ventilator settings and the potential to facilitate early mobilization, which may be advantageous for bridging to recovery or to lung transplantation. The use of venoarterial ECMO has been expanded and applied to critically ill adults with hemodynamic compromise from a variety of etiologies, beyond postcardiotomy failure. Although technology and general care of the ECLS patient have evolved, ECLS is not without potentially serious complications and remains unproven as a treatment modality. The therapy is now being tested in clinical trials, although numerous questions remain about the application of ECLS and its impact on outcomes in critically ill adults.

Self-reported attitudes versus actual practice of oxygen therapy by ICU physicians and nurses

BACKGROUND

High inspiratory oxygen concentrations are frequently administered in ventilated patients in the intensive care unit (ICU) but may induce lung injury and systemic toxicity. We compared beliefs and actual clinical practice regarding oxygen therapy in critically ill patients.

METHODS

In three large teaching hospitals in the Netherlands, ICU physicians and nurses were invited to complete a questionnaire about oxygen therapy. Furthermore, arterial blood gas (ABG) analysis data and ventilator settings were retrieved to assess actual oxygen practice in the same hospitals 1 year prior to the survey.

RESULTS

In total, 59% of the 215 respondents believed that oxygen-induced lung injury is a concern. The majority of physicians and nurses stated that minimal acceptable oxygen saturation and partial arterial oxygen pressure (PaO2) ranges were 85% to 95% and 7 to 10 kPa (52.5 to 75 mmHg), respectively. Analysis of 107,888 ABG results with concurrent ventilator settings, derived from 5,565 patient admissions, showed a median (interquartile range (IQR)) PaO2 of 11.7 kPa (9.9 to 14.3) [87.8 mmHg], median fractions of inspired oxygen (FiO2) of 0.4 (0.4 to 0.5), and median positive end-expiratory pressure (PEEP) of 5 (5 to 8) cm H2O. Of all PaO2 values, 73% were higher than the upper limit of the commonly self-reported acceptable range, and in 58% of these cases, neither FiO2 nor PEEP levels were lowered until the next ABG sample was taken.

CONCLUSIONS

Most ICU clinicians acknowledge the potential adverse effects of prolonged exposure to hyperoxia and report a low tolerance for high oxygen levels. However, in actual clinical practice, a large proportion of their ICU patients was exposed to higher arterial oxygen levels than self-reported target ranges.

Pulse oximetry: fundamentals and technology update

Oxygen saturation in the arterial blood (SaO₂) provides information on the adequacy of respiratory function. SaO₂ can be assessed noninvasively by pulse oximetry, which is based on photoplethysmographic pulses in two wavelengths, generally in the red and infrared regions. The calibration of the measured photoplethysmographic signals is performed empirically for each type of commercial pulse-oximeter sensor, utilizing in vitro measurement of SaO₂ in extracted arterial blood by means of co-oximetry. Due to the discrepancy between the measurement of SaO₂ by pulse oximetry and the invasive technique, the former is denoted as SpO₂. Manufacturers of pulse oximeters generally claim an accuracy of 2%, evaluated by the standard deviation (SD) of the differences between SpO₂ and SaO₂, measured simultaneously in healthy subjects. However, an SD of 2% reflects an expected error of 4% (two SDs) or more in 5% of the examinations, which is in accordance with an error of 3%–4%, reported in clinical studies. This level of accuracy is sufficient for the detection of a significant decline in respiratory function in patients, and pulse oximetry has been accepted as a reliable technique for that purpose. The accuracy of SpO₂ measurement is insufficient in several situations, such as critically ill patients receiving supplemental oxygen, and can be hazardous if it leads to elevated values of oxygen partial pressure in blood. In particular, preterm newborns are vulnerable to retinopathy of prematurity induced by high oxygen concentration in the blood. The low accuracy of SpO₂ measurement in critically ill patients and newborns can be attributed to the empirical calibration process, which is performed on healthy volunteers. Other limitations of pulse oximetry include the presence of dyshemoglobins, which has been addressed by multiwavelength pulse oximetry, as well as low perfusion and motion artifacts that are partially rectified by sophisticated algorithms and also by reflection pulse oximetry.

Oxygen Therapy in Critical Illness: Friend or Foe? A Review of Oxygen Therapy in Selected Acute Illnesses

In recent years there has been a gradual shift away from using uncontrolled high concentrations of inspired oxygen in some acute illnesses. Oxygen is perhaps the most frequently used drug in medicine, and understanding the balance of benefits and harms is essential knowledge for all anaesthetists and intensivists. While current teaching and practice emphasise avoiding hypoxaemia over concerns about hyperoxaemia, it may transpire that oxygen excess is more harmful than previously thought. As with many interventions in intensive care medicine, striving to achieve physiological normality may sometimes do more harm than good, and tolerance of abnormal values may on occasion be in patients' best interests. Incorporating Single Best Answers (see page 197: answers on page 237).

Can we optimize long-term outcomes in acute respiratory distress syndrome by targeting normoxemia?

Since its original description in 1967, acute respiratory distress syndrome (ARDS) has been recognized as a devastating condition associated with significant morbidity and mortality. Advances in critical care medicine and ARDS management have led to a substantial increase in the number of ARDS survivors. Long-term cognitive impairment after critical illness is a significant public health concern. ARDS survivors frequently experience long-term cognitive impairment, as well as physical impairment, psychiatric morbidity, and reduced health-related quality of life. At present, no intensive care unit-based intervention has been proven to reduce the risk of long-term cognitive impairment after ARDS. To address the urgent need to identify strategies to preserve long-term health, investigators have advocated the measurement of short- and long-term outcomes in clinical trials. Maintaining adequate oxygen delivery to preserve organ function is of vital importance in ARDS management. The optimal target range for arterial oxygenation in ARDS remains unknown, due in part to the challenge to maintain adequate tissue oxygenation and to minimize harm, such as oxygen toxicity. An approach targeted to subnormal oxygenation values (partial pressure of arterial oxygen, 55-80 mm Hg) has emerged as a means to accomplish these aims. In this perspective, we critically evaluate this strategy from short- and long-term perspectives, with a focus on the potential long-term cognitive effects of the strategy. We conclude with a proposal to consider resetting the target range for arterial oxygenation higher (85-110 mm Hg) as a potential strategy to improve the long-term outcomes of ARDS survivors.

Postoperative nosocomial infections among children with congenital heart disease

OBJECTIVE

To study the pathogen distribution, antimicrobial susceptibility and risk factors of postoperative nosocomial infections among children with congenital heart disease.

METHODS

Three hundreds children with congenital heart disease admitted to our hospital to receive surgeries from February 2010 to February 2013 were selected.

RESULTS

A total of 120 children were tested as positive by sputum culture, with the infection rate of 40.0%. The top five most common pathogenic microorganisms included Staphylococcus epidermidis, Staphylococcus aureus, Enterococcus, Pseudomonas aeruginosa, and Candida albicans. S. epidermidis, S. aureus and Enterococcus were highly resistant to penicillin, azithromycin and erythromycin, moderately susceptible to levofloxacin and cefazolin, and completely susceptible to vancomycin. Multivariate Logistic regression analysis showed that hospitalization stay length, combined use of antibiotics, systemic use of hormones, mechanical ventilation and catheter indwelling were the independent risk factors of postoperative nosocomial infections (P<0.05).

CONCLUSION

Nosocomial infection, which was the most frequent postoperative complication of pediatric congenital heart disease, was predominantly induced by Gram-positive bacteria that were highly susceptible to cephalosporins and vancomycin. Particular attention should be paid to decrease relevant risk factors to improve the prognosis.

Blood content analysis for evaluating cardiopulmonary function

PURPOSE OF REVIEW

To review the role of blood analysis in cardiac and pulmonary evaluation.

RECENT FINDINGS

A comprehensive understanding of arterial blood gas analysis - often aided by venous blood gas analysis, to help determine the ideal levels of blood oxygenation and hemoglobin (Hb), is reviewed in light of the recent findings. Serum lactate plays an important role in assessing cardiopulmonary function; yet, with the exception of prognosis, as to exactly what an elevated lactate implies remains unclear. Despite considerable research into the pulmonary-specific biomarkers, none are currently sufficiently robust to use in daily practice. Of the cardiac biomarkers, the natriuretic peptides are yet to be established in critical care practice apart from a role in assisting weaning from a ventilator, and cardiac troponin (cTn) continues to be an essential tool.

SUMMARY

The ongoing research on blood contents has not resulted in any new outstanding markers to the critical care physician to use in evaluating cardiopulmonary function, with the exception of high-sensitive cTn. Yet, many recent studies assist the clinician by providing a better understanding of how to use routine tools, like arterial and venous blood gas analysis, more effectively and offer guidance in optimizing Hb and lactate blood levels.

Intensive care clinicians' opinion of conservative oxygen therapy (SpO₂ 90-92%) for mechanically ventilated patients

BACKGROUND

In the ICU, SpO2≥96% are regularly targeted implying that more oxygen may be given than desirable. To reduce exposure to hyperoxia a conservative oxygen therapy protocol (targeted SpO2 90-92% using lowest FiO2) for mechanically ventilated patients was introduced in a single tertiary ICU in September 2012.

OBJECTIVES

To describe intensive care clinicians' opinion of conservative oxygen therapy for mechanically ventilated adult patients.

METHODS

A structured multi-choice questionnaire of intensive care clinicians was conducted between February and March 2013.

RESULTS

Responses were received from 90 staff members: 81 intensive care nurses and 9 medical doctors. A majority of respondents (60.7%) considered oxygen related lung injury as 'a major concern'. Most respondents (81/89; 91.1%) felt conservative oxygen therapy was easy to perform and few respondents (6/88; 8%) considered performing conservative oxygen therapy to be stressful. Most respondents (58%) reported not performing more arterial blood gases to monitor PaO2 during conservative oxygen therapy and 90% (81/90) of respondents indicated a desire to continue conservative oxygen therapy. Free text comments indicated adoption of this protocol was a paradigm shift yet more education and research to elucidate the benefits/harm of lower oxygen saturation targeting is needed.

CONCLUSIONS

Intensive care clinicians readily accepted the introduction of a conservative oxygen therapy protocol into their practice. Most respondents found conservative oxygen therapy easy and not stressful to perform. Further evaluation the administration of oxygen therapy, its management by intensive care clinicians and possible impact on outcome for mechanically ventilated patients appears well accepted by clinical staff.

Ventilator settings and monitoring parameter targets for initiation of continuous mandatory ventilation: a questionnaire study

PURPOSE

To inform development of educational tools, we sought to identify initial ventilator settings and monitoring targets for 3 scenarios.

METHOD

A survey was e-mailed to Canadian Society of Respiratory Therapists members with 2 reminders in March/April 2011.

RESULTS

Total evaluable surveys were 363. More participants selected pressure as opposed to volume ventilation for acute respiratory distress syndrome (ARDS; 77%) than for chronic obstructive pulmonary disease (COPD; 50%) and postoperative ventilation (32%; P < .001). Mean tidal volume was lower for ARDS than for COPD and postoperative ventilation (5.7, 6.9, and 7.2 mL/kg, respectively; P < .001). Maximum acceptable plateau pressures were highest for ARDS (30 cm H2O vs 29 cm H2O [COPD] and 27 cm H2O [postoperative], P < .001). Initial positive expiratory end pressure (12 cm H2O vs 7 cm H2O vs 5 cm H2O) and fraction of inspired oxygen (Fio2; 1.0 vs 0.5 vs 0.3) were also higher for ARDS (both P < .001); however, only 8% selected a positive expiratory end pressure/Fio2 combination as recommended by ARDSnet. Values of oxygen saturation as measured by pulse oximetry of 97% (ARDS) and 94% (COPD and postoperative) were considered appropriate for Fio2 reduction. The lowest pH was 7.28 vs 7.23 vs 7.26; the highest pH was 7.46 vs 7.44 vs 7.46 (P < .001). Partial pressure of carbon dioxide (arterial) of 51 mm Hg (postoperative) to 65 mm Hg (ARDS) was considered acceptable.

CONCLUSION

Lung protective ventilation was favored, yet distinct differences in ventilator settings were evident. Monitoring targets suggested relatively conservative practices for Fio2 reduction but an understanding of permissive hypercapnia.

Xtreme Everest 2: unlocking the secrets of the Sherpa phenotype?

Xtreme Everest 2 (XE2) was part of an ongoing programme of field, laboratory and clinical research focused on human responses to hypoxaemia that was conducted by the Caudwell Xtreme Everest Hypoxia Research Consortium. The aim of XE2 was to characterise acclimatisation to environmental hypoxia during a standardised ascent to high altitude in order to identify biomarkers of adaptation and maladaptation. Ultimately, this may lead to novel diagnostic and treatment strategies for the pathophysiological hypoxaemia and cellular hypoxia observed in critically ill patients. XE2 was unique in comparing participants drawn from two distinct populations: native ancestral high-altitude dwellers (Sherpas) and native lowlanders. Experiments to study the microcirculation, mitochondrial function and the effect that nitric oxide metabolism may exert upon them were focal to the scientific profile. In addition, the genetic and epigenetic (methylation and histone modification) basis of observed differences in phenotype was explored. The biological samples and phenotypic metadata already collected during XE2 will be analysed as an independent study. Data generated will also contribute to (and be compared with) the bioresource obtained from our previous observational high-altitude study, Caudwell Xtreme Everest (2007).

Ventilator management in the cardiac intensive care unit

Patients admitted to the Cardiac Intensive Care Unit (CICU) are of increasing complexity and often require ventilatory support. A deep understanding of respiratory physiology and the interactions between the cardiovascular and respiratory systems is essential. Ventilatory support should be tailored to the specific patient condition, ensuring effective minute ventilation, reducing work of breathing and minimizing adverse hemodynamic effects. The weaning process can stress the cardiovascular system and cardiac failure is a common cause of failure to wean. Identification of patients likely to fail and prompt pre-emptive intervention is crucial for successful weaning and avoiding complications related to prolonged mechanical ventilation.

Cardiac responses to 24 hrs hyperoxia in Bmp2 and Bmp4 heterozygous mice

BACKGROUND

Hyperoxia or clinical oxygen (O2) therapy is known to result in increased oxidative burden. Therefore, understanding susceptibility to hyperoxia exposure is clinically important. Bone morphogenetic proteins (BMPs) 2 and 4 are involved in cardiac development and may influence responses to hyperoxia.

METHODS

Bmp2(+/)(-). Bmp4(+/)(-) and wild-type mice were exposed to hyperoxia (100% O2) for 24 hrs. Electrocardiograms (ECG) were recorded before and during exposure by radio-telemetry.

RESULTS

At baseline, a significantly higher low frequency (LF) and total power (TP) heart rate variability (HRV) were found in Bmp2(+/)(-) mice only (p < 0.05). Twenty-four hours hyperoxia-induced strain-independent reductions in heart rate, QTcB and ST-interval and increases in QRS, LF HRV and standard deviation of RR-intervals were observed. In Bmp4(+/)(-) mice only, increased PR-interval (PR-I) (24 hrs), P-wave duration (P-d; 18 and 21-24 hrs), PR-I minus P-d (PR - Pd; 24 hrs) and root of the mean squared differences of successive RR-intervals (24 hrs) were found during hyperoxia (p < 0.05).

DISCUSSION

Elevated baseline LF and TP HRV in Bmp2(+/)(-) mice suggests an altered autonomic nervous system regulation of cardiac function in these mice. However, this was not related to strain specific differences in responses to 24 hrs hyperoxia. During hyperoxia, Bmp4(+/-) mice were the most susceptible in terms of atrioventricular conduction changes and risk of atrial fibrillation, which may have important implications for patients treated with O2 who also harbor Bmp4 mutations. This study demonstrates significant ECG and HRV responses to 24 hrs hyperoxia in mice, which highlights the need to further work on the genetic mechanisms associated with cardiac susceptibility to hyperoxia.