A Piglet Perinatal Asphyxia Model to Study Cardiac Injury and Hemodynamics after Cardiac Arrest, Resuscitation, and the Return of Spontaneous Circulation.

Neonatal piglets have been extensively used as translational models for perinatal asphyxia. In 2007, we adapted a well-established piglet asphyxia model by introducing cardiac arrest. This enabled us to study the impact of severe asphyxia on key outcomes, including the time taken for the return of spontaneous circulation (ROSC), as well as the effect of chest compressions according to alternative protocols for cardiopulmonary resuscitation. Due to the anatomical and physiological similarities between piglets and human neonates, piglets serve as good models in studies of cardiopulmonary resuscitation and hemodynamic monitoring. In fact, this cardiac arrest model has provided evidence for guideline development through research on resuscitation protocols, pathophysiology, biomarkers, and novel methods for hemodynamic monitoring. Notably, the incidental finding that a substantial fraction of piglets have pulseless electrical activity (PEA) during cardiac arrest may increase the applicability of the model (i.e., it may be used to study pathophysiology extending beyond the perinatal period). However, the model generation is technically challenging and requires various skill sets, dedicated personnel, and a fine balance of the measures, including the surgical protocols and the use of sedatives/analgesics, to ensure a reasonable rate of survival. In this paper, the protocol is described in detail, as well as experiences with adaptations to the protocol over the years.

Reliability of pulse oximetry in hypoxic newborn pigs.

OBJECTIVE: The reliability of pulse oximetry in the lower SaO2-ranges has not been fully investigated. We wanted to investigate pulse oximeter performance in a pig model of perinatal asphyxia. METHODS: Asphyxia was induced in 22 newborn pigs. Pulse oximetry-values for SpO2, as well as SaO2 from blood gas analyses were recorded multiple times before, during and after asphyxiation. The relationship between SpO2 and SaO2 in normoxia and hypoxia in this model was quantified. Calculations were made for 5% increments of SaO2 increasing successively from ≥95% to ≥20%. RESULTS: The pigs became severely hypoxic, acidotic and hypotensive. The bias in the total data was +13.7 (p < 0.001), i.e. SpO2 was on average higher than SaO2 for the entire interval of SaO2. However, bias was not significantly different from zero in most SpO2-ranges. Accuracy and precision were considerably higher than stated by the manufacturer in all SpO2-ranges, with both accuracy and precision increasing when SpO2-values down to 60% and lower were included. CONCLUSION: Pulse oximetry was less reliable at SaO2-values below 60%. Tissue acidosis and reduced peripheral perfusion may have contributed to this. It is unknown whether the difference between SpO2 and SaO2 is clinically relevant.

Brain inflammation induced by severe asphyxia in newborn pigs and the impact of alternative resuscitation strategies on the newborn central nervous system.

BACKGROUND: We compared the current guidelines for neonatal resuscitation with alternative measures and aimed to find out whether this modulated brain inflammation. METHODS: Progressive asphyxia was induced in 94 newborn pigs until asystole. With the reference being resuscitation guidelines, 30 s of initial positive-pressure ventilation before compression (C) and ventilation (V) (C:V; 3:1) in 21% oxygen, pigs were randomized to (i) ventilation for 30, 60, or 90 s before chest compressions; (ii) C:V ratios of 3:1, 9:3, or 15:2; or (iii) 21% or 100% oxygen. Concentrations of inflammatory markers in the cerebrospinal fluid (CSF) and gene expression in the hippocampus and frontal cortex were measured for different interventions. RESULTS: In CSF, S100 was higher with 90 s than with 30 or 60 s of initial positive-pressure ventilation, whereas concentrations of interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α) were higher with 30 than with 60 s. Matrix metalloproteinase-2 (MMP-2) and intracellular adhesion molecule 1 (ICAM-1) were higher with 30 than with 60 s. No other comparison between ratios and oxygen concentrations used yielded significant results. CONCLUSION: With respect to signs of brain inflammation, newly born pigs at asystole should be ventilated for longer than 30 s before chest compressions start. C:V ratios of 9:3 and 15:2 as compared with 3:1, or air instead of pure oxygen, did not modulate inflammatory markers.

Lung Injury in Asphyxiated Newborn Pigs Resuscitated from Cardiac Arrest - The Impact of Supplementary Oxygen, Longer Ventilation Intervals and Chest Compressions at Different Compression-to-Ventilation Ratios.

INTRODUCTION: Non-specific lung inflammatory events caused by severe asphyxia may be intensified by the way we resuscitate the newly born. Assessing lung injury is potentially important because if alternative resuscitation approaches induces similar inflammatory responses or less lung injury. then we may choose the resuscitation approach that is most gentle, and easiest to perform and learn. We investigated the levels of lung inflammatory markers by comparing different ventilation, chest compression and inhaled oxygen fraction strategies in resuscitation of newly born pigs at cardiac arrest. MATERIALS AND METHODOLOGY: Progressive asphyxia in newborn pigs was induced until asystole occurred. With current resuscitation guidelines as a reference group, pigs were randomized to receive initial ventilation before chest compressions for 30s, 60s or 90s, or to compression-to-ventilation ratios 3:1or 9:3, or to resuscitation using pure oxygen or air. We analysed inflammatory markers in bronchoalveolar lavage fluid (BAL), IL8 and TNFα, and lung tissue qPCR for genes matrix metalloproteinases (MMP)2, MMP9, TNFα and ICAM-1. RESULTS: BAL-levels of TNFα and IL8 tended to be higher in the 30s group compared to 60s group (p = 0.028 and p = 0.023, respectively) as was gene expression in lung tissue of ICAM-1 and MMP2 (p=0.012 and p=0.043, respectively). MMP2 expression was slightly higher in the 30s group compared to 90s group (p = 0.020). No differences were found between pigs resuscitated with C:V ratio 9:3 and 3:1 or pure oxygen versus air. CONCLUSION: Compared to current guidelines, with respect to lung injury, resuscitation with longer initial ventilation should be considered. Longer series of chest compressions did not change the lung inflammatory response, neither did the use of air instead of pure oxygen in severely asphyxiated pigs resuscitated from asystole.

Antioxidant protects against increases in low molecular weight hyaluronan and inflammation in asphyxiated newborn pigs resuscitated with 100% oxygen.

BACKGROUND: Newborn resuscitation with 100% oxygen is associated with oxidative-nitrative stresses and inflammation. The mechanisms are unclear. Hyaluronan (HA) is fragmented to low molecular weight (LMW) by oxidative-nitrative stresses and can promote inflammation. We examined the effects of 100% oxygen resuscitation and treatment with the antioxidant, N-acetylcysteine (NAC), on lung 3-nitrotyrosine (3-NT), LMW HA, inflammation, TNFα and IL1ß in a newborn pig model of resuscitation. METHODS & PRINCIPAL FINDINGS: Newborn pigs (n = 40) were subjected to severe asphyxia, followed by 30 min ventilation with either 21% or 100% oxygen, and were observed for the subsequent 150 minutes in 21% oxygen. One 100% oxygen group was treated with NAC. Serum, bronchoalveolar lavage (BAL), lung sections, and lung tissue were obtained. Asphyxia resulted in profound hypoxia, hypercarbia and metabolic acidosis. In controls, HA staining was in airway subepithelial matrix and no 3-NT staining was seen. At the end of asphyxia, lavage HA decreased, whereas serum HA increased. At 150 minutes after resuscitation, exposure to 100% oxygen was associated with significantly higher BAL HA, increased 3NT staining, and increased fragmentation of lung HA. Lung neutrophil and macrophage contents, and serum TNFα and IL1ß were higher in animals with LMW than those with HMW HA in the lung. Treatment of 100% oxygen animals with NAC blocked nitrative stress, preserved HMW HA, and decreased inflammation. In vitro, peroxynitrite was able to fragment HA, and macrophages stimulated with LMW HA increased TNFα and IL1ß expression. CONCLUSIONS & SIGNIFICANCE: Compared to 21%, resuscitation with 100% oxygen resulted in increased peroxynitrite, fragmentation of HA, inflammation, as well as TNFα and IL1ß expression. Antioxidant treatment prevented the expression of peroxynitrite, the degradation of HA, and also blocked increases in inflammation and inflammatory cytokines. These findings provide insight into potential mechanisms by which exposure to hyperoxia results in systemic inflammation.

Return of spontaneous circulation with a compression:ventilation ratio of 15:2 versus 3:1 in newborn pigs with cardiac arrest due to asphyxia.

OBJECTIVE: International guidelines recommend a compression to ventilation (C:V) ratio of 3:1 in neonates, and 15:2 for other paediatric age groups. The authors aimed to compare these two C:V ratios in a neonatal swine model of cardiac arrest following asphyxia. DESIGN: Experimental animal study. SETTING: Facility for animal research. SUBJECTS: 22 newborn pigs (age 12-36 h, weight 2.0-2.7 kg). INTERVENTIONS: Progressive asphyxia until asystole. Animals were randomised to receive C:V 3:1 (n=11) or 15:2 (n=11). MAIN OUTCOME MEASURES: Return of spontaneous circulation (ROSC) was defined as a heart rate ≥ 100 bpm. Also of interest were haemodynamic parameters, cerebral and systemic oxygen saturation and the proinflammatory cytokine interleukin-1ß (IL-1ß). RESULTS: Two animals in each group did not achieve ROSC. Mean (SD) increase in diastolic blood pressure (DBP; mm Hg) during compression cycles was significantly higher at a C:V ratio of 15:2 than 3:1 (7.1 (2.8) vs 4.8 (2.6)). Median time (IQR) to ROSC for the 3:1 group was 150 (140-180) s, and 195 (145-358) s for the 15:2 group. There were no significant differences in the temporal changes in haemodynamic parameters or oxygen saturation indices between the groups. IL-1ß levels in cerebrospinal and bronchoalveolar lavage fluid was comparable between the groups. CONCLUSION: In neonatal pigs with asphyxia-induced cardiac arrest, the response to a C:V ratio of 15:2 is not better than the response to a C:V ratio of 3:1 despite better generation of DBP during resuscitation.

Delayed onset of cardiac compressions in cardiopulmonary resuscitation of newborn pigs with asphyctic cardiac arrest.

BACKGROUND: When 30 s of initial positive pressure ventilation fails to stabilize the heart rate (HR) of newborns in the delivery room, the International Liaison Committee on Resuscitation guidelines recommend initiation of cardiac compressions. However, it may take longer than 30 s to establish effective pulmonary gas exchange. Whether a longer period of initial ventilation to reverse asphyxia would result in less need for cardiac compressions is unknown. OBJECTIVES: Our purpose was to investigate the effect of three different initial ventilation intervals prior to initiation of cardiac compressions on hemodynamic parameters, arterial blood gases, oxygen saturations and markers of inflammation and hypoxic damage in a piglet model of asystole due to asphyxia. METHODS: Noroc piglets were anesthetized and mechanically ventilated. Progressive asphyxia was induced until asystole occurred. Randomization was made to ventilation with 21% O(2) for (1) 30 s (n = 16), (2) 1 min (n = 16), or (3) 1.5 min (n = 8) before initiation of cardiac compressions. Return of spontaneous circulation (ROSC) was defined as HR ≥100 min(-1). RESULTS: Piglets initially ventilated for 30 s and 1 and 1.5 min achieved ROSC in a median of 150 (interquartile range 115-180),163 (124-177) and 282 (199-364) s, respectively. p value for group 1 versus group 2 was 0.51 and <0.001 for group 1 versus group 3. There were no differences in temporal changes in oxygen saturations, mean arterial blood pressure, HR, pH, pCO(2), interleukin-1ß or lactate/pyruvate ratios between groups. CONCLUSION: Although an additional 30 s to ensure effective ventilation does not impair the speed or success in achieving ROSC, delaying circulatory support for as long as 1.5 min of initial ventilation may be harmful.

Extended series of cardiac compressions during CPR in a swine model of perinatal asphyxia.

BACKGROUND: The rationale for a compression to ventilation ratio of 3:1 in neonates with primary hypoxic, hypercapnic cardiac arrest is to emphasize the importance of ventilation; however, there are no published studies testing this approach against alternative methods. An extended series of cardiac compressions offers the theoretical advantage of improving coronary perfusion pressures and hence, we aimed to explore the impact of compression cycles of two different durations. MATERIALS AND METHODS: Newborn swine (n = 32, age 12-36 h, weight 2.0-2.7 kg) were progressively asphyxiated until asystole occurred. Animals were randomized to receive compressions:ventilations 3:1 (n=16) or 9:3 (n=16). Return of spontaneous circulation (ROSC) was defined as a heart rate ≥ 100 beats min⁻¹. RESULTS: All animals except one in the 9:3 group achieved ROSC. One animal in the 3:1 group suffered bradycardia at baseline, and was excluded, leaving us with 15 animals in each group surviving to completion of protocol. Time to ROSC (median and interquartile range) was 150 s (115-180) vs. 148 s (116-195) for 3:1 and 9:3, respectively (P = 0.74). There were no differences in diastolic blood pressure during compression cycles or in markers of hypoxia and inflammation. The temporal changes in mean arterial blood pressure, heart rate, arterial blood gas parameters, and systemic and regional oxygen saturation were comparable between groups. CONCLUSION: Neonatal pigs with asphyxia-induced cardiac arrest did not respond to a compression:ventilation ratio of 9:3 better than to 3:1. Future research should address if alternative compression:ventilation ratios offer advantages over the current gold standard of 3:1.

Resuscitation of severely asphyctic newborn pigs with cardiac arrest by using 21% or 100% oxygen.

BACKGROUND: In spite of evidence suggesting that resuscitation with 100% O(2) is detrimental, international guidelines still recommend its use. Clinical studies comparing 21% and 100% O(2) included many infants with only mild and moderate asphyxia. OBJECTIVES: We aimed to investigate the effect of these oxygen fractions on haemodynamic parameters, arterial blood gases, oxygen saturation indices and markers of inflammation and hypoxic damage when resuscitating asystolic newborn pigs following asphyxia. METHODS: Newborn swine (n = 32, age 12-36 h, weight 2.0-2.7 kg) were progressively asphyxiated until asystole occurred. Cardiopulmonary resuscitation was initiated with ventilation with either 21% (n = 16) or 100% O(2) (n = 16). Return of spontaneous circulation (ROSC) was defined as a heart rate >or= 100 min(-1). RESULTS: Mean time of hypoxia, pH, base excess and pCO(2) at asystole were comparable between the groups. All animals except 2 in the 100% group achieved ROSC. One animal in the 21% group suffered bradycardia at baseline and was excluded. For the remaining 15 animals resuscitated with 21% O(2), median time to ROSC (interquartile range) was 150 s (115-180), whereas animals in the 100% group achieved ROSC after 135 s (113-168); p = 0.80. There were no differences in the temporal changes in mean arterial blood pressure, heart rate, pH, pCO(2), interleukin-1beta or lactate/pyruvate ratios. However, systemic and regional cerebral oxygen saturations were higher in the animals resuscitated with 100% oxygen. CONCLUSION: In this animal model of severe perinatal asphyxia, resuscitation with room air seemed to be as safe and effective as the use of 100% oxygen.

Blood pressure in the neonate: three non-invasive oscillometric pressure monitors compared with invasively measured blood pressure.

AIM: To compare blood pressure in neonates obtained from three different oscillometer blood pressure monitors (Dinamap, Criticare, Hewlett-Packard) with arterial blood pressure (Hewlett-Packard' invasive unit). STUDY DESIGN: A total of 32 randomized, non-invasive blood pressure series, each consisting of three measurements from each monitor, were obtained from 20 neonates (birthweight 531-4660 g). Each measurement was compared with the invasive pressure. RESULTS: Two factors appear to have a systematic effect on the difference between oscillometric and invasive pressure (the measurement deviance): the size of the infant, e.g., the arm circumference, and the monitor system. For small infants, the non-invasively measured value tends to be too high. The deviance is partly reversed for larger infants (dependency on size significant for mean and diastolic pressure, p < 0.001). The difference between monitor systems is clearly significant (p < 0.001). Hewlett-Packard gives the lowest pressure values for all pressures. Thus, Criticare and Dinamap tended to show values too high in the smallest infants, while Hewlett-Packard tended to give values too low in larger infants. Birthweight, present weight and arm circumference affected measurement deviance approximately equally strongly, while factors such as the infant's sex, need of breath support and umbilical or radial arterial line were non-significant. CONCLUSIONS: Blood pressure should preferably be measured invasively in severely ill neonates and preterm infants, being aware of pitfalls with measurements using different oscillometer monitors and the size/arm circumference of the infant.