Keep your eye on the carbon dioxide level in your patient with HIE.  It may matter a lot!

Keep your eye on the carbon dioxide level in your patient with HIE. It may matter a lot!

Hypoxic ischemic encephalopathy is a very scary condition for both families and health care providers.  In my career as a Neonatologist one of the greatest accomplishments has been the recognition that 72 hours of moderate hypothermia can make a big difference to the outcomes of such children.  In days gone by our best estimates of outcome relied on Sarnat staging of HIE.  

Since the cooling approach was adopted widely however I have relied more on a wait and see approach when advising families on what to expect.  On some occasions, is spite of cooling babies go on to develop significant cerebral palsy but in other cases babies who one would have predicted would have dismal outcomes have done quite well.  Our best estimates at the moment are that cooling for HIE reduces the risk of death or moderate to severe disability by about 25% with a confidence interval of 17 – 32% around that estimate.

Why would pCO2 matter?

Carbon dioxide has a role to play in outcome and has been the subject of several papers.  The theoretical point is that very low carbon dioxide levels lead to vasoconstriction of blood vessels.  When it comes to HIE one would be quite worried about vasoconstriction of blood vessels such as the carotids carrying oxygenated blood to an injured brain.  Once injured the brain is not going to tolerate further oxygen deprivation and in particular those areas that are teetering on the edge of survival could be tipped the wrong way if further hypoxia is experienced. 

Another reason why CO2 matters is due to something called the Bohr effect.  For those of you who are scratching your heads and recollecting this term from your training it has to do with the influence of pCO2 on hemoglobin oxygen saturation.   The relationship is represented by the following figure. 

In the presence of declining pCO2 there is a shift of the oxygen dissociation curve to the left.  This means for that as PCO2 declines more of the circulating oxygen will be bound to hemoglobin.  In most cases you want your hemoglobin to be great at carrying oxygen but when your tissues are starved of oxygen and injured that is not what you want.  You want a selfless hemoglobin molecule that is more than happy to release its oxygen to the tissue.  That is not what you get as pCO2 drops.

Why would pCO2 be low at all?

There are a few reasons for this.  The first is that many infants born after an asphyxial event have a metabolic acidosis.  Our bodies naturally like to maintain a normal pH.  In order to do so if your HCO3 in the blood is low you need to blow off CO2 to compensate.  The hypocarbia in this case is compensatory but the body in so doing could make matters worse for the brain.

The second reason has to do with both injured tissue and that which is cooled.  As metabolic rate decreases the amount of CO2 generated will drop.  If you remember the Krebs cycle (shudder) there is a fair bit of CO2 generated from aerobic (oxygen rich) metabolism. If this is reduced so to will the production of CO2.  As cooling serves to reduce metabolic rate so the CO2 production would be expected to decrease.

So does it really matter?

The reason for all this preamble is that a “mini-systematic review” has found the CO2 matters to outcome.  The review is entitled Hypocarbia is associated with adverse outcomes in hypoxic ischaemic encephalopathy (HIE) and included 9 studies on influence of pCO2 on outcome.  Before we look at the results it is important to acknowledge that all of the included studies were retrospective so methodology in each study is not standardized.  How one even defines severe hypocarbia varied from <20 mmHg to anything under 35 mm Hg.  The other issue is that each study looked at a different period of exposure from the effect of a couple hours to the effect over the first three days of life.  The included infants were all cooled so it gives us at least an idea of the effect in a modern cohort of cooled infants.

The summary of the results was that CO2 mattered.  As little as a couple hours of very low CO2 levels were found to be associated with adverse outcomes.

The problem of course is the chicken and the egg argument.  The most severe hypocarbia might be seen in those with the worst metabolic acidosis. As mentioned above the response to metabolic acidosis is to blow off CO2.  Therefore, the worse the metabolic acidosis the greater that respiratory drive.

Strategies to control the pCO2 of course exist.  In the presence of a critically low pCO2 one can intubate and control ventilation through sedation and paralysis.  This can lead to other issues though as if you normalize the pCO2 in the presence of a significant metabolic acidosis the pH is likely to take a nosedive.  The myocardium as it turns out doesn’t like low pH and in fact cardiac output in animal models begins to decrease the closer you get to a pH of 7 and becomes significantly worse as you go beyond that point.

At best then I think one can aim for converting severe hypocarbia to moderate until the HCO3 begins to recover.  Based on theoretical issues of oxygen delivery to tissues and cerebral vasoconstriction, notwithstanding the retrospective nature of this review it does make sense to me that there would be  a link between severe hypocarbia and outcome.  We will likely never see an RCT targeting normalization of pCO2 vs tolerance of hypocarbia in this population so for the purists out there that don’t like this type of retrospective analysis I suspect outside of an animal model this is as good as its going to get. 

Maybe avoiding anything with the word severe attached to it though is sensible when it comes to this population.

Newborn Brains After Hypoxia Don’t Like Hyponatremia

Newborn Brains After Hypoxia Don’t Like Hyponatremia

The newborn brain could be described as finicky at best. One of the most difficult things to treat are those things that we can’t see. When a baby is delivered and goes on to develop neurological manifestations, it remains a difficult puzzle to sort out as to what the cause is. Of course, we use all manner of technology to sort this out. The use of EEG, amplitude integrated EEG are helpful in this regard to give us a window into brain excitability but we use all manner of technology to sort this out. When it comes down to it, though we often rely on clinical signs to give us a best guess into whether or not hypoxic injury is at the root of the problem. This, of course, is not always easy as although we have criteria such as those written out by the Canadian paediatric Society to rely on, not all babies fit nicely into the box that provides an easy diagnosis.

For reference, these are the criteria that are recommended by the Canadian paediatric Society for determining who should receive therapeutic hypothermia.

In spite of these criteria, sometimes when babies have Apgar scores or cord blood gases that don’t meet criteria for therapeutic hypothermia, they may still go on to have a seizure. In some of these babies, it is likely that they still experienced a hypoxic injury at some point in time that they have recovered from. In these cases, having a super imposed, metabolic, derangement can tip the scales and cause an already excitable brain to manifest neurological manifestations.

The Brain Does Not Like Low Sodiums

One such abnormality that can tip the scales is a low serum sodium. Babies can develop such derangements from a hypoxic insult that leads to an acute kidney injury. The resultant damage leads to water retention from a poorly functioning, kidney and a dilutional effect on the bloodstream. This usually occurs over time and is not commonly present in the first few hours after birth. When this is seen though with sodium levels below 125 in the first few hours after birth, the likely cause is not renal injury. What is interesting about this phenomenon is that the etiology is most likely related to factors that occurred during labour.

Pregnancy its self has a tendency towards, maternal impairment of water excretion. There is a higher volume status in the pregnant woman and some degree of impairment of excretion of a water load. Maternal hyponatraemia has been described in situations of maternal water, intoxication or provision of excessive dilute fluids to the labouring mother. Add to this, that there is cross-reactivity between oxytocin and ADH receptors in the kidney, and you create a potential problem that a mother can become hyponatraemic simply from frequent administration of oxytocin or Syntocinon. It is possible therefore to have a mother in labour who receives an excessive amount of fluid whether by oral intake or IV and with oxytocin administration develop hyponatremia herself. What follows in terms of the fetus who is an innocent bystander is the eventual development of hyponatremia in utero. As the maternal sodium concentration declines this leads to a difference in maternal and fetal sodium levels. Water flows by osmosis to the fetus and begins to dilute out their blood and bring the sodium levels in line with maternal levels. What comes next can be troublesome to the fetus.

Resultant Seizures

Blake O et al published the paper Therapeutic hypothermia and outcome in hyponatraemic encephalopathy secondary to maternal water intoxication which describes this exact scenario in the setting of maternal water intoxication. The K-series describes three babies all whom developed seizures and had mild The case series describes three babies all whom developed seizures and had mild perinatal asphyxia yet went on to develop seizures. The laboratory results are shown below.

What is most remarkable from the table is the level of serum sodium in the newborns at 1 hour of age. Generally levels of sodium below 125 and certainly 120 can lead to neurological manifestations including seizures and these infants were certainly affected. Much like I explained at the outset of this piece children could be afflicted with a mild form of encephalopathy from hypoxia, and in these cases, each infant by 10 minutes of age had excellent Apgar scores. What I propose, though is that the brain after even a more mild degree of Perinatal asphyxia is more prone to neonatal seizures. I have to say over the years. I have often checked electrolytes after a baby presents with seizures and rarely are they sufficiently abnormal to explain the finding. What I am presenting to you. Here is a special circumstance, in which babies who might not otherwise have seizures, such as those with mild asphyxia go on to have significant convulsions due to the superimposed insult.

The goal of this post was to increase awareness of this phenomenon. Next time you are looking into the events leading up to seizures in a newborn, don’t forget to ask about what fluids a mother received during labour and specifically what her oral intake was like. Don’t forget to have a careful look as well at the amount of oxytocin she received during labour as the combination may be just enough to tip the scales and lied to Neonatal seizures in a baby, who otherwise would not have developed any of those manifestations! While you are at it, take the time to check a maternal sodium and if mother and baby match or at least are both hyponatremic to a similar level you likely have your answer as to what the ethology is.

A bigger question and one that we don’t have the answer to is whether in the presence of hyponatremia and mild asphyxia therapeutic hypothermia offers much benefit. Unfortunately this answer is going to be a tough one to come by as you can’t create an RCT since the numbers are so small but I suspect that most when in doubt will choose to get that temperature down!

Newborn Brains After Hypoxia Don’t Like Hyponatremia

How much does rate matter with NIPPV?

When it comes to non-invasive ventilation the field has become a little more crowded in recent years at least in our institution. In the recent past if one decided to extubate an ELGAN the biggest decision was what CPAP pressure to use. These days we have the option of high frequency nasal ventilation (nHFOV) or non-invasive positive pressure ventilation (NIPPV) to choose from as additional options. Both of these modalities have their uses and I have written about nHFOV before as in Nasal High Frequency Oscillatory Ventilation For Preventing Intubation. On this post though I want to look at NIPPV which has actually been around longer as a modality. The gist of this mode is that one chooses a delta P, peep, Ti and rate much like you would on a conventional ventilator. When ventilating through a nasal interface the device provides ventilation although it is questionable I suppose how much of that is alveolar ventilation. The study we are going to talk about here caught my eye as the information gleaned from it gives me at least an idea of how this mode may work to help prevent reintubations.

The Effect of Rate in NIPPV

Authors from Haifa, Israel performed a fairly elegant study entitled The effect of changing respiratory rate settings on CO2 levels during nasal intermittent positive pressure ventilation (NIPPV) in premature infants.

In this study each patient served as their own control and alternated between either a start of a rate of 10 BPM or a rate of 30 BPM as shown in the following diagram. The infants were all between 24 +0 and 32 +6 weeks gestation to be included in the study. Delivery of NIPPV was through the Leoni Ventilator using RAM cannulae and importantly the mode was non-synchronized. Each infant needed to be stable on their settings for at least 6 hours before being included. The authors hypothesis was that rate matters to clear carbon dioxide. To monitor CO2 levels they used transcutaneous CO2 measurements to allow for continuous measurement over each hour of the study. Given this belief, there was safety built into the protocol such that patients were excluded if on the set rate of 10 bpm the tcCO2-related pCO2 was <40 mmHg, or on NIPPV if the set rate of 30 bpm had a tcCO2-related-pCO2 is 60 mmHg, In other words, if rate matters and your tcCO2 was already less than 40 on a low rate then it would not be safe to blow off more CO2 and vice versa with high CO2 and low rates. To ensure that only rate affected the results “during the 3 h of the study no changes in PIP, PEEP or FiO2 were allowed
with the following exceptions: if spO2 was <90% or >95% for more than
20 s, an increase or decrease in FiO2 were allowed to keep spO2 90–94%,
and were documented”.

So does rate matter?

It turns out the authors found no difference in CO2 levels based on rate changes alone.

This of course is contrary to what the authors expected to find. The question is why this might be. What follows now is just speculation on my part but given the finding of no difference I can offer a few thoughts. The first is that NIPPV does not involve a distal delivery of gas like the situation of an endotracheal tube near the carina. With an endotracheal tube in place the delta P or pressure above the set peep is delivered to the gas exchanging areas of the lung. With NIPPV you are delivering the pressure at the nose and therefore there is a fair amount of dead space in between the exit of the gas into the baby and the lung. Might you just be really ventilating dead space for the most part?

Secondly, depending on the fit of the mask or the degree that the mouth was open how do we know how much of the non-invasive ventilation reached the infant? Lastly, in our own centre we have not been impressed with the RAM cannulae as we have found that whether the prongs are in or out of the nose the pressure being detected as being delivered seems to stay the same at least as the ventilator sees it. If the prongs were not in the nose properly and the atmosphere was being ventilated would one really know that the pressures weren’t really getting into the nose?

Lastly, the Leoni ventilator is not capable of delivering synchronized NIPPV. Now that there is the availability of synchronization on ventilators such as on the Puritan Bennett 980 ventilator it would be interesting to see the same study done again. If you are delivering non-synchronized breaths which are not in sync with the patient should we expect a change in CO2? What if half the breaths for example by chance are delivered on exhalation? Not much effect on CO2 I would think.

I am not saying that rate doesn’t matter at all but I suppose I am saying within the context of this study it doesn’t matter to CO2. My best guess as to how NIPPV works to prevent reintubation may be secondary to two things. The first would be by irritating the baby with the puffs of delta P. Think of it like intermittent stimulation. The second possibility is that the same puffs of air help keep the pharynx open and minimizes the obstructive portion of apnea of prematurity. Whatever the reason NIPPV appears to work to prevent reintubation in some infants!

I have no doubt the group here will look at the effect of delta P on CO2 soon enough and I wonder if we will see much difference there either. It also will be important to look at the effect of rate in a synchronized fashion! Time will tell.

Skull fractures after birth.  Don’t be so quick to blame.

Skull fractures after birth. Don’t be so quick to blame.

Anyone who has watch the delivery of a baby knows that in some cases things go very smoothly and in others every care provider in the room would likely have tachycardia themselves. In some cases where labour is quite prolonged and some degree of cephalopelvic disproportion exists, the fetal head can become quite wedged in the pelvis. When this occurs it is not uncommon to hear of an ob/gyn having to dislodge the entrapped head from below and then perform a c-section to get the baby out safely.

In some of these cases though on the newborn exam a depression of the skull is found such as with the figure on the right. As our brains like to link things together we may jump to the conclusion that the pressure exerted on the head from below led to a fracture. This fracture in turn may lead to injury to the underlying brain. At least that is what our brains want us to think but what if the reason for the fracture has nothing to do with the maneuver as described?

Spontaneous Skull Fractures

This exact situation has been described in two cases and with a review of the literature in a paper entitled Spontaneous Intrauterine Depressed Skull Fractures: Report of 2 Cases Requiring Neurosurgical Intervention and Literature Review. In this report they describe two cases, the first of which was a term infant born via SVD without instrumentation and was described as atraumatic. The figure above was from this infant and thankfully the underlying brain was free of hemorrhage. The second case was also term and again there was no need for forceps or vacuum. In this case there was significant parietal fracture with a small amount of subdural blood collection. This infant unlike the other one due to significant depression required neurosurgical intervention to correct the skull deformity and lift the bone off the brain. As the authors go on to describe there have been 39 other such patients described in the literature with the features as shown in the table from the paper. While there are 4 more in the paper they had vacuum extractions so I wouldn’t count them.

Why Does This Happen?

The short answer in most cases is a tight fit! In the 1960s this was postulated that in the right occiput posterior and and right occiput transverse positions the fetal head becomes compressed between the sacral promontory and pubic bones. Other implicating factors have been maternal fibroids leading to chronic pressure on the developing skull along with oligohydramnios that may lead to fetal compression as well.

When you look at the above table though what stands out is failure to progress as an ethology. One can imagine the contracting uterus attempting to propel the fetus forward and if impacted in the pelvis the pressure on the skull may well lead to fracture.

The other thing of note is the overwhelming involvement of the parietal bone in these cases. A presentation in another bone might lead one to think of a different etiology.

As far as treatment, many of these as you can see are simply observed but in the presence of significant bleeding neurosurgical intervention is needed. At the outset it is sensible to consult neurosurgery as one never knows which ones need intervention and which ones do not.

As you can see, the presence of a fracture and a history of forceful pushing from below MAY be related to a fracture but on the other hand these may occur simply with protracted labours themselves. In these situations while it may be tempting to blame the ob/gyn we also need to ask ourselves what the alternative they had was. Should they have let the mother continue to push with the potential risk of asphyxia or potentially even uterine rupture? At some point the delivering physician needs to get the baby out and if that is what needs to be done to extract the baby then that is what they will need to do. At the end of the day one thing is for sure that we don’t know for sure what caused the fracture and as tempting as it may be to blame the ob/gyn or GP delivering a baby it just might have been spontaneous!

Newborn Brains After Hypoxia Don’t Like Hyponatremia

Do antenatal steroids really benefit 22 and 23 weekers?

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It’s been a while since my last post. Like many centers across North America and worldwide the resuscitation of premature infants as young as 22 weeks is becoming more commonplace. Our own center is in the process of working towards coming up with evidence-based approaches to the care of these fragile infants. One of the questions that has long been asked is whether antenatal steroids really make a difference at these earliest gestational ages. The argument against effectiveness would be that the cards are just so stacked up against these preemies that even steroids may not help. Making matters worse is that the number of babies at this early gestational age included in antenatal steroid trials are extremely small making any conclusions difficult.

A Study To Help Us?

You can imagine my delight and then when I saw the following study published this past week, Association of Antenatal Steroid Exposure at 21 to 22 Weeks of Gestation With Neonatal Survival and Survival Without Morbidities.

In short, the goal of the study was to look at survival and survival without major morbidities for infant born between 22 and 0 days to 23 weeks and 6 days gestational age who either received no antenatal steroids, 1 dose or 2 doses 24 hours apart. Only those mothers who received betamethasone were included and the doses were provided at either 21 or 22 weeks of gestation prior to delivery at 22 and 23 weeks of gestation.
The study was retrospective and looked at NICHD neonatal research network data from January 1, 2016 to December 31, 2019. In comparison to all the previous prospective studies in existence which recruited less than 50 preterm infants this young this study managed to recruit 431 infants. In the groups analyzed, there were 25.5% infants who received no antenatal steroids, 18.6% infants receiving a partial course and 55.9% infants receiving complete antenatal steroids.

What did they find?

The authors found evidence that I believe will be reassuring to practitioners deciding whether to provide a course of steroids at these gestational ages. There are questions though that will be raised when looking at this data as well.

The data in table to show a number of interesting findings. Most notably a primary outcome of survival at hospital discharge was improved with a complete course of steroids but not with partial or none. Similarly there were reductions in severe intracranial hemorrhage and survival at 36 weeks postmenstrual age without major medical morbidities.

Figure 2 shows survival to hospital discharge and survival without major neonatal morbidities graphically. What one can more clearly see is that if you are going to give steroids the outcome is best if the mother receives both doses.

Challenges

On the one hand you might say that this is a slam dunk finding and we should be giving antenatal steroids to all women presenting at 21 and 22 weeks gestational age. I mentioned there would be questions and one of them will have to do with the avoidance of a repeat course of antenatal steroids. There is some literature that suggests repeat dosing of antenatal steroids later in pregnancy is associated with adverse developmental outcomes and also structural changes to the developing brain. This then leads the practitioner and a bit of a quagmire. If the woman presents at 21 or 22 weeks with threatened preterm labor do give her the steroids knowing that only a full course will help her versus waiting to see if she is truly in labor as you are considering whether you should save dosing for a later time in pregnancy. I have no doubt there will be some providers that we will hesitate to give the 1 course if that is their institution practice at this gestational age. This will not be an easy selection to make.

The other question that we will come up as we start to see a single dose antenatal steroid trials coming out is whether such infants will be included in prospective trials. The upcoming SNACS trial which we are participating in is one such trial that will include infants as young as these. It will be interesting to see if prospectively collected clinical trials with adequate numbers of such small infants will demonstrate similar findings that 2 doses really are required to make a meaningful reduction in adverse outcomes. As we have seen with many retrospective studies though such as this one the outcomes may in fact be different when you randomize patients in a prospective fashion.

For now I think the evidence as good as it is we will favor giving steroids to mother’s presenting at these gestational ages. Curious what you think?