It has to be one of the most common questions you will hear uttered in the NICU. What were the cord gases? You have a sick infant in front of you and because we are human and like everything to fit into a nicely packaged box we feel a sense of relief when we are told the cord gases are indeed poor. The congruence fits with our expectation and that makes us feel as if we understand how this baby in front of us looks the way they do.
Take the following case though and think about how you feel after reading it. A term infant is born after fetal distress (late deceleration to as low as 50 BPM) is noted on the fetal monitor. The infant is born flat with no heart rate and after five minutes one is detected. By this point the infant has received chest compressions and epinephrine twice via the endotracheal tube. The cord gases are run as the baby is heading off to the NICU for admission and low and behold you get the following results back; pH 7.21, pCO2 61, HCO3 23, lactate 3.5. You find yourself looking at the infant and scratching your head wondering how the baby in front of you that has left you moist with perspiration looks as bad as they do when the tried and true cord gas seems to be betraying you. To make matters worse at one hour of age you get the following result back; pH 6.99, pCO2 55, HCO3 5, lactate 15. Which do you believe? Is there something wrong with the blood gas analyzer?
How Common Is This Situation
You seem to have an asphyxiated infant but the cord gas isn’t following what you expect as shouldn’t it be low due to the fetal distress that was clearly present? It turns out, a normal or mildly abnormal cord gas may be found in asphyxiated infants just as commonly as what you might expect. In 2012 Yeh P et al looked at this issue in their paper The relationship between umbilical cord arterial pH and serious adverse neonatal outcome: analysis of 51,519 consecutive validated samples. The authors sampled a very large number of babies over a near 20 year period to come up with a sample of 51519 babies and sought to pair the results with what they knew of the outcome for each baby. This is where things get interesting. When looking at the outcome of encephalopathy with seizures and/or death you will note that only 21.71% of the babies with this outcome had a gas under 7.00. If you include those under 7.10 as still being significantly distressed then this percentage rises to 34.21%. In other words almost 66% of babies who have HIE with seizures and/or death have a arterial cord pH above 7.1! The authors did not look at encephalopathy without seizures but these are the worst infants and almost 2/3 have a cord gas that you wouldn’t much as glance at and say “looks fine”
How do we reconcile this?
The answer lies in the fetal circulation. When an fetus is severely stressed, anaerobic metabolism takes over and produces lactic acid and the metabolic acidosis that we come to expect. For the metabolites to get to the umbilcal artery they must leave the fetal tissues and enter the circulation. If the flow of blood through these tissues is quite poor in the setting of compromised myocardial contractility the acids sit in the tissues. The blood that is therefore sitting in the cord at the time of sampling actually represents blood that was sent to the placenta “when times were good”. When the baby is delivered and we do our job of resuscitating the circulation that is restored then drives the lactic acid into the blood stream and consumes the buffering HCO3 leading to the more typical gases we are accustomed to seeing and reestablishing the congruence our brains so desire. This in fact forms the basis for most HIE protocols which includes a requirement of a cord gas OR arterial blood gas in the first hour of life with a pH < 7.00.
Acidosis May Be Good For the Fetus
To bend your mind just a little further, animal evidence suggests that those fetuses who develop acidosis may benefit from the same and be at an advantage over those infants who don’t get acidemia. Laptook AR et al published Effects of lactic acidinfusions and pH on cerebral blood flow and metabolism. In this study of piglets, infusion of lactic acid improved cerebral blood flow. I would suggest improvement in cerebral blood flow of the stressed fetus would be a good thing. Additionally we know that lactate may be used by the fetus as additional metabolic fuel for the brain which under stress would be another benefit. Finally the acidemic fetus is able to offload O2 to the tissues via the Bohr effect. In case you have forgotten this phenomenon, it is the tendency for oxygen to more readily sever its tie to hemoglobin and move into the tissues.
I hope you have found this as interesting as I have in writing it. The next time you see a good cord gas in a depressed infant, pause for a few seconds and ask yourself is this really a good or a bad thing?
One of the first things a student of any discipline caring for newborns is how to calculate the apgar score at birth. Over 60 years ago Virginia Apgar created this score as a means of giving care providers a consistent snapshot of what an infant was like in the first minute then fifth and if needed 10, 15 and so on if resuscitation was ongoing. For sure it has served a useful purpose as an apgar score of 0 and 0 gives one cause for real worry. What about a baby with an apgar of 3 and 7 or 4 and 8? There are certainly infants who have done very well who initially had low apgar scores and conversely those who had higher apgar scores who have had very significant deleterious outcomes including death. I don’t mean to suggest that the apgar scores don’t provide any useful predictive value as they are used as part of the criteria to determine if a baby merits whole body cooling or not. The question is though after 60+ years, has another score been created to provide similar information but enhance the predictive value derived from a score?
The Neonatal Resuscitation and Adaptation Score (NRAS)
Back in 2015 Jurdi et al published Evaluation of a Comprehensive Delivery Room Neonatal Resuscitation and Adaptation Score (NRAS) Compared to the Apgar Score. This new score added into a ten point score resuscitative actions taken at the 1 and 5 minute time points to create a more functional score that included interventions. The other thing this new score addressed was more recent data that indicated a blue baby at birth is normal (which is why we have eliminated asking the question “is the baby pink?” in NRP. Knowing that, the colour of the baby in the apgar score may not really be that relevant. Take for example a baby with an apgar score of 3 at one minute who could have a HR over 100 and be limp, blue and with shallow breathing. Such a baby might get a few positive pressure breaths and then within 10 seconds be breathing quite well and crying. Conversely, they might be getting ongoing PPV for several minutes and need oxygen. Were they also getting chest compressions? If I only told you the apgar score you wouldn’t have much to go on. Now look at the NRAS and compare the information gathered using two cardiovascular (C1&2), one neurological test (N1) and two respiratory assessments (R1&2).
The authors in this study performed a pilot study on only on 17 patients really as a proof of concept that the score could be taught and implemented. Providers reported both scores and found “superior interrater reliability (P < .001) and respiratory component reliability (P < .001) for all gestational ages compared to the Apgar score.”
A Bigger Study Was Needed
The same group in 2018 this time led by Witcher published Neonatal Resuscitation and Adaptation Score vs Apgar: newborn assessment and predictive ability. The primary outcome was the ability of a low score to predict mortality with a study design that was a non-inferiority trial. All attended deliveries were meant to have both scores done but due to limited numbers of trained personnel who could appropriately administer both scores just under 90% of the total deliveries were assigned scores for comparison. The authors sought to recruit 450 infants to show that a low NRAS score (0–3) would not be inferior to a similar Apgar at predicting death. Interestingly an interim analysis found the NRAS to be superior to Apgar when 75.5% of the 450 were enrolled, so the study was stopped. What led the apgar score to perform poorly in predicting mortality (there were only 12 deaths though in the cohort) was the fact that 49 patients with a 1 minute apgar score of 0-3 survived compared to only 7 infants with a low NRAS score.
The other interesting finding was the ability of the NRAS to predict the need for respiratory support at 48 hours with a one minute apgar score of 0-3 being found in 39% of those on support compared to 100% of those with a low NRAS. Also at 5 minutes a score of 4-6 for the apgar was found in 48% of those with respiratory support at 48 hours vs 87% of those with a similar range NRAS. These findings were statistically significant while a host of other conditions such as sepsis, hypoglycemia, hypothermia and others were no different in terms of predictive ability of the scores.
An Even Bigger Study is Needed
To be sure, this study is still small and missed just over 90% of all deliveries so it is possible there is some bias that is not being detected here. I do think there is something here though which a bigger study that has an army of people equipped to provide the scoring will add to this ongoing story. Every practitioner who resuscitates an infant is asked at some point in those first minutes to hour “will my baby be ok?”. The truth is that the apgar score has never lived up to the hope that it would help us provide an accurate clairvoyant picture of what lies ahead for an infant. Where this score gives me hope is that a score which would at the very least help me predict whether an infant would likely still be needing respiratory support in 48 hours provides the basic answer to the most common question we get in the unit once admitted; “when can I take my baby home”. Using this score I could respond with some greater confidence in saying “I think your infant will be on support for at least 48 hours”. The bigger question though which thankfully we don’t have to address too often for the sickest babies at birth is “will my baby survive?”. If a larger study demonstrates this score to provide a greater degree of accuracy then the “Tipping Point” might just be that to switching over to the NRAS and leaving the apgar score behind. That will never happen overnight but medicine is always evolving and with time you the reader may find yourself becoming very familiar with this score!
It is hard to believe but it has been almost 3 years since I wrote a piece entitled A 200 year old invention that remains king of all tech in newborn resuscitation. In the post I shared a recent story of a situation in which the EKG leads told a different story that what our ears and fingers would want us to believe. The concept of the piece was that in the setting of pulseless electrical activity (where there is electrical conductance in the myocardium but lack of contraction leaves no blood flow to the body) one could pick up a signal from the EKG leads when there is in fact no pulse or perfusion to vital organs. This single experience led me to postulate that this situation may be more common than we think and the application of EKG leads routinely could lead to errors in decision making during resuscitation of the newborn. It is easy to see how that could occur when you think about the racing pulses of our own in such situations and once chest compressions start one might watch the monitor and forget when they see a heart rate of 70 BPM to check for a corresponding pulse or listen with the stethoscope. I could see for example someone stopping chest compressions and continuing to provide BVM ventilation despite no palpable pulse when they see the QRS complex clearly on the monitor. I didn’t really have much evidence to support this concern but perhaps there is a little more to present now.
A Crafty Animal Study Provides The Evidence
I haven’t presented many animal studies but this one is fairly simple and serves to illustrate the concern in a research model. For those of you who haven’t done animal research, my apologies in advance as you read what happened to this group of piglets. Although it may sound awful, the study has demonstrated that the concern I and others have has is real.
For this study 54 newborn piglets (equivalent to 36-38 weeks GA in humans) were anesthetized and had a flow sensor surgically placed around the carotid artery. ECG leads were placed as well and then after achieving stabilization, hypoxia was induced with an FiO2 of 0.1 and then asphyxia by disconnecting the ventilator and clamping the ETT. By having a flow probe around the carotid artery the researchers were able to determine the point of no cardiac output and simultaneously monitor for electrical activity via the EKG leads. Auscultation for heart sounds was performed as well.
The results essentially confirm why I have been concerned with an over reliance on EKG leads.
Of the 57 piglets, 14 had asystole and no carotid flow but in 23 there was still a heart rate present on the EKG with no detectable carotid flow. This yields a sensitivity of only 37%. Moreover, the overall accuracy of the ECG was only 56%.
Meanwhile the stethoscope which I have referred to previously as the “king” in these situations had 100% sensitivity so remains deserving of that title.
What do we do with such information?
I think the results give us reason to pause and remember that faster isn’t always better. Previous research has shown that signal acquisition with EKG leads is faster than with oximetry. While a low heart rate detected quickly is helpful to know what the state of the infant is and begin the NRP pathway, we simply can’t rely on the EKG to tell us the whole story. We work in interdisciplinary teams and need to support one another in resuscitations and provide the team with the necessary information to perform well. The next time you are in such a situation remember that the EKG is only one part of the story and that auscultation for heart sounds and palpation of the umbilical cord for pulsation are necessary steps to demonstrate conclusively that you don’t just have a rhythm but a perfusing one.
I would like to thank the Edmonton group for continuing to put out such important work in the field of resuscitation!
The lungs of a preterm infant are so fragile that over time pressure limited time cycled ventilation has given way to volume guaranteed (VG) or at least measured breaths. It really hasn’t been that long that this has been in vogue. As a fellow I moved from one program that only used VG modes to another program where VG may as well have been a four letter word. With time and some good research it has become evident that minimizing excessive tidal volumes by controlling the volume provided with each breath is the way to go in the NICU and was the subject of a Cochrane review entitled Volume-targeted versuspressure-limited ventilation in neonates. In case you missed it, the highlights are that neonates ventilated with volume instead of pressure limits had reduced rates of:
death or BPD
severe cranial ultrasound pathologies
duration of ventilation
These are all outcomes that matter greatly but the question is would starting this approach earlier make an even bigger difference?
Volume Ventilation In The Delivery Room
I was taught a long time ago that overdistending the lungs of an ELBW in the first few breaths can make the difference between a baby who extubates quickly and one who goes onto have terribly scarred lungs and a reliance on ventilation for a protracted period of time. How do we ventilate the newborn though? Some use a self inflating bag, others an anaesthesia bag and still others a t-piece resuscitator. In each case one either attempts to deliver a PIP using the sensitivity of their hand or sets a pressure as with a t-piece resuscitator and hopes that the delivered volume gets into the lungs. The question though is how much are we giving when we do that?
High or Low – Does it make a difference to rates of IVH?
One of my favourite groups in Edmonton recently published the following paper; Impact of delivered tidal volume on the occurrence of intraventricular haemorrhage in preterm infants during positive pressure ventilation in the delivery room. This prospective study used a t-piece resuscitator with a flow sensor attached that was able to calculate the volume of each breath delivered over 120 seconds to babies born at < 29 weeks who required support for that duration. In each case the pressure was set at 24 for PIP and +6 for PEEP. The question on the authors’ minds was that all other things being equal (baseline characteristics of the two groups were the same) would 41 infants given a mean volume < 6 ml/kg have less IVH compared to the larger group of 124 with a mean Vt of > 6 ml/kg. Before getting into the results, the median numbers for each group were 5.3 and 8.7 mL/kg respectively for the low and high groups. The higher group having a median quite different from the mean suggests the distribution of values was skewed to the left meaning a greater number of babies were ventilated with lower values but that some ones with higher values dragged the median up.
< 6 mL/kg
> 6 ml/kg
Grade 3 or 4
Let’s be fair though and acknowledge that much can happen from the time a patient leaves the delivery room until the time of their head ultrasounds. The authors did a reasonable job though of accounting for these things by looking at such variables as NIRS cerebral oxygenation readings, blood pressures, rates of prophylactic indomethacin use all of which might be expected to influence rates of IVH and none were different. The message regardless from this study is that excessive tidal volume delivered after delivery is likely harmful. The problem now is what to do about it?
Unless I am mistaken, there isn’t a volume regulated bag-mask device that we can turn to for control of delivered tidal volume. Given that all the babies were treated the same with the same pressures I have to believe that the babies with stiffer lungs responded less in terms of lung expansion so in essence the worse the baby, the better they did in the long run at least from the IVH standpoint. The babies with the more compliant lungs may have suffered from being “too good”. Getting a good seal and providing good breathes with a BVM takes a lot of skill and practice. This is why the t-piece resuscitator grew in popularity so quickly. If you can turn a couple of dials and place it over the mouth and nose of a baby you can ventilate a newborn. The challenge though is that there is no feedback. How much volume are you giving when you start with the same settings for everyone? What may seem easy is actually quite complicated in terms of knowing what we are truly delivering to the patient. I would put to you that someone far smarter than I needs to develop a commercially available BVM device with real-time feedback on delivered volume rather than pressure. Being able to adjust our pressure settings whether they be manual or set on a device is needed and fast!
Perhaps someone reading this might whisper in the ear of an engineer somewhere and figure out how to do this in a device that is low enough cost for everyday use.