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An innovation in patient monitoring and respiratory care: Respiratory Function Monitoring during neonatal resuscitation

Georg Marcus Schmölzer MD, PhD
24 July, 2015  

Conflict of Interest: None

Funding sources
GMS is a recipient of the Heart and Stroke Foundation/University of Alberta Professorship of Neonatal Resuscitation and a Heart and Stroke Foundation Canada Research Scholarship.

No relation with industry exists.

Introduction
Establishing breathing and improving oxygenation after birth is vital for survival and long-term health of preterm infants. The majority of preterm infants successfully make the transition from foetal to neonatal life without any help,1 however an estimated 10% of preterm infants require assistance during neonatal transition.1 When infants fail to breathe at birth, positive pressure ventilation (PPV) along with a baseline positive end-expiratory pressure (PEEP) should be provided.1 The goals of PPV are to establish lung aeration, deliver adequate tidal volume (VT) to facilitate gas exchange and stimulate breathing while minimising lung injury.2

During PPV a peak inflation pressure (PIP) is chosen with the assumption this will deliver an adequate VT.1,2 However, the delivered VT is rarely measured and therefore PIP is not adjusted to optimise VT delivery.3 The appropriate VT to be delivered during various phases of resuscitation is unknown. A VT that is too high can damage the lungs by over inflation and a VT that is too small will result in inadequate gas exchange and atelectasis.2 Several delivery room (DR) studies reported a large variation of VT delivery during PPV, which has been shown to cause both volutrauma and atelectasis.4 In the neonatal intensive care unit (NICU), arterial blood gases, transcutaneous partial O2 saturation, end-tidal carbon dioxide, and respiratory functions are routinely used to guide effectiveness of respiratory support. However, these methods are not commonly applied in the DR. In the DR, PPV is usually guided by changes in heart rate, however, if heart rate does not increase, chest rise should be assessed to gauge PPV.1 Observational studies in the DR have shown that assessment of chest rise is imprecise and should not be used to adjust mask ventilation.3,5 Recently, oxygen saturation and heart rate have been considered important indirect indicators of adequate transition of newborn infants in the DR.6,7 More recently, observational and randomised controlled studies described how gas flow and VT waves can be used to guide PPV during neonatal simulation,8 neonatal transport9 and in the DR.4,10 However, parameters to directly assess the efficiency of ventilation are lacking. Hooper et al. recently demonstrated that monitoring exhaled carbon dioxide (ECO2) during PPV identifies lung aeration.11 In addition, monitoring changes of ECO2 over time has the potential to guide mask PPV in the DR.11

Respiratory function monitor (RFM)
Several different RFMs are available to guide mask ventilation immediately after birth.10,12 All devices uses a small, low dead space (~1ml) flow sensor, which an accuracy of ±8% (manufacturer’s data). The flow sensor is general placed between a ventilation device (for example, face mask). The inspiratory and expiratory VT are automatically calculated by integrating the flow signal. A separate monitoring line measures and displays the airway pressures. The monitor can be set to continuously display pressure, flow and tidal volume waves. It also measures and displays numerical values for PIP and PEEP, VT, respiratory rate, minute ventilation and the leak between mask and face as a percentage ((inspiratory VT – expiratory VT)/inspiratory VT)*100.

Optimal mask ventilation
Figure 1A illustrates gas flow, airway pressure, ECO2 and VT waveforms during mask PPV in a newborn with well aerated lungs and without any mask leak or airway obstruction. At the start of the inflation PIP increases from baseline PEEP to the set PIP. This PIP increase correlates with increase in gas flow towards the infant and increase in inspiratory VT. The ECO2 waveform remains at zero. During expiration, PIP returns to baseline PEEP, gas flow moves away from the infant (negative gas flow).12 The expiratory VT waveform returns to zero and ECO2 waveform is displayed.12

Pitfalls during mask ventilation
Mask leak and airway obstruction are common enemies of mask ventilation in the delivery room.10,12,13 The delivery of adequate PPV in the DR is dependent on adequate face mask technique. Several factors can reduce the effectiveness of PPV. These include poor face mask application resulting in leak or airway obstruction, spontaneous movements of the baby, movements by or distraction of the resuscitator, and procedures such as changing the wet towels or fitting a hat.10,13

Mask leak
Mannequin and DR studies reported a wide variation in mask leak between operators,3 which can be reduced by optimising mask hold.3 This is further supported by a recent randomised controlled trial demonstrated that an RFM can be used to reduce mask leak, which was not always facilitated by the operator.4 These studies demonstrated that an observation of gas flow and VT waves could be used to optimise mask position to minimise mask leak during respiratory support. Also, no study has yet reported if ECO2 or CO2 detectors can be used to assess mask leak during PPV. Figure 1B demonstrates that during PPV a large mask leak is present and no ECO2. The inspiratory gas flow has a greater area underneath the flow curve compared to expiratory flow curve. The VT curve displays larger inspiratory VT compared to expiratory VT and leak is displayed as a straight line in the VT curve. ECO2 is zero suggesting no ventilation.12

Airway obstruction
Significant airway obstruction occurs in about half of the very preterm infants who received PPV in the DR.13,14 In a recent report, Finer et al. described airway obstruction during mask PPV in the DR using a colourimetric CO2 detector.13 They found airway obstruction in 75% of infants receiving PPV.13 An ECO2 detector is a very useful device to assess effective ventilation, however it cannot differentiate between inadequate VT delivery, airway obstruction or mask leak.12 In contrast, an RFM, which displays flow and VT signal allows distinguishing between mask leak and airway obstruction.14 A recent observational study in the DR showed that severe airway obstruction, defined as a reduction in VT of >75% occurs in 25% of infants receiving mask ventilation.14 Figure 1C shows mask PPV with severe airway obstruction, with significant reduction in both the inflation and expiratory gas flow (compared to Figure 1A), no VT and zero ECO2. The PIP and PEEP are maintained during PPV.

Adjusting tidal volume during mask ventilation
The purpose of PPV is to achieve lung aeration with an appropriate VT to facilitate gas exchange. Using a fixed PIP during PPV the delivered VT depends on the weight of the infant, lung and chest wall compliance and airway resistance.10 The appropriate VT during various phases of resuscitation is unknown. A VT that is too high can damage the lungs by over inflation and a VT that is too small will result in inadequate gas exchange and atelectasis.2 Studies in the DR suggest that VT during mask PPV should be within the range (4–6ml/kg).4,15 Using an RFM during PPV enables the resuscitation team to monitor the delivered VT and adjust PIP to deliver the required VT. The optimum PIP will vary between infants and in the same infant over time as the lung aerates and compliance and resistance change.10 Causes for little or no VT displayed during PPV includes mask leak (Figure 1B), airway obstruction (Figure 1C), a very low PIP (Figure 1D) or non-compliant lungs (Figure 1D). If a very low VT during PPV is displayed the resuscitator can either reposition the facemask to alleviate mask leak or airway obstruction, or an increase in PIP until an appropriate VT is displayed. Once an adequate VT is delivered the PIP can be then adjusted to the required VT depending on ECO2.

Assessing lung aeration during PPV
At birth, lung liquid has to be cleared from the airways to allow air entry and establishment of a functional residual capacity. However, when infants fail to breathe adequately immediately after birth, it is important to apply PPV to facilitate gas exchange without causing lung injury. Hooper et al. recently reported that changes in ECO2 during mask PPV provides could guide PPV immediately after birth (Figure 1D). Inability to detect ECO2, in the absence of mask leak or airway obstruction, indicates that air has not reached distal gas exchange structures to allow gas exchange (Figure 1D). Increasing ECO2 levels in subsequent inflations indicates aeration of distal gas exchange regions. This increase in ECO2 is closely associated with increasing end inflation lung volumes,11 which has been reported during first minutes after birth in spontaneously breathing infants and infants requiring mask PPV.11,12,16 During real-life resuscitations with good mask ventilation, zero ECO2 indicates that distal gas exchange has not occurred. Figure 1D displays adequate PPV with VT of 8ml/kg but no ECO2 indicating no gas exchange. After intubation and increase PIP a rapid increase in ECO2 is observed indicating lung aeration.
 

Figure 1.

Possible problems from using this technology
Inexperience and lack of knowledge about the displayed waveforms may lead to misinterpretation of the signals.10,12 Therefore anyone using this technology must be trained to interpret ECO2, pressure, flow and VT signals. Furthermore, the monitor only displays waves and data to aid the resuscitator and does not provide interpretation of the signals or a diagnosis.10,12 There are an increasing number of monitoring devices introduced into the DR, however, it is important that human factors are considered when introducing new technologies, to avoid overwhelming the clinical team with the additional data.10,12 The attention of an inexperienced user may be diverted from the baby to the monitor screen. For people unfamiliar with the device they may find that placement of a flow sensor between the mask and resuscitation device makes holding the device a little awkward. In addition, these new technologies have to be validated before they can become standard of care.10,12
      
A further issue is the cost of these devices. RFMs can cost up to €10,000, requires a new flow sensor for each patient (this could cost up to €50 per flow sensor/patient), and also requires maintenance. A cheap alternative is an ECO2 detector, which can be purchased for €1 per piece, however this device has several limitations. The future approach might be to use equipment already routinely used in the NICU (for example, all mechanical ventilators can measure gas flow or VT and end-tidal CO2 is routinely used to guide mechanical ventilation). With this approach a system of flow sensor and end tidal CO2 could be transitioned into the DR and connected to a face mask to provide adequate mask ventilation.

Gaps in knowledge
There is increasing evidence that monitoring gas flow, VT, and ECO2 has an important role to guide respiratory support in the DR. However, further studies are needed to investigate whether gas flow and VT during resuscitations can: 1) reduce mask leak or airway obstruction during PPV, 2) target VT during PPV, 3) to confirm correct endotracheal tube placement, 4) reduce the need for endotracheal intubation and improve its success, and 5) improve survival and long-term outcomes. Further studies are also needed to investigate whether ECO2 during resuscitations can: 1) be used to assess lung aeration, 2) improve lung recruitment immediately after birth, 3) reduce the need for endotracheal intubation and improve its success, and 4) improve survival and long-term outcomes. Currently several randomised controlled trials are underway examine different aspects to assess if either VT or ECO2 improves improve survival and short- and long-term outcomes.

Conclusions
There is increasing evidence that VT and ECO2 waveforms can aid during PPV. There is evidence that an RFM can help to reduce mask leak and reduce large VT delivery during mask PPV in the DR. In addition, there is evidence that trends in ECO2 can be used to guide PPV.

References

  1. Kattwinkel J et al. Part 15: neonatal resuscitation: 2010 American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care. Circulation 2010;122:S909–19.
  2. Schmölzer GM et al. Reducing lung injury during neonatal resuscitation of preterm infants. J Pediatr 2008;153:741–5.
  3. Schmölzer GM et al. Assessment of tidal volume and gas leak during mask ventilation of preterm infants in the delivery room. Arch Dis Child Fetal Neonatal 2010;95:F393–7.
  4. Schmölzer GM et al. Respiratory function monitor guidance of mask ventilation in the delivery room: a feasibility study. J Pediatr 2012;160:377–381.e2.
  5. Poulton DA et al. Assessment of chest rise during mask ventilation of preterm infants in the delivery room. Resuscitation 2011;82:175–9.
  6. Dawson JA et al et al. Changes in heart rate in the first minutes after birth. Arch Dis Child Fetal Neonatal 2010;95:F177–81.
  7. Dawson JA et al. Defining the reference range for oxygen saturation for infants after birth. Pediatrics 2010;125:e1340–7.
  8. Schmölzer GM, Roehr C. Use of Respiratory Function Monitors during Simulated Neonatal Resuscitation. Klin Padiatr 2011;223:261–6.
  9. Tingay DG. Monitoring of end tidal carbon dioxide and transcutaneous carbon dioxide during neonatal transport. Arch Dis Child Fetal Neonatal 2005;90:F523–6.
  10. Schmölzer GM et al. Respiratory monitoring of neonatal resuscitation. Arch Dis Child Fetal Neonatal 2010;95:F295–303.
  11. Hooper SB et al. Expired CO2 levels indicate degree of lung aeration at birth. PLoS ONE 2013;8:e70895.
  12. van Os S et al. Exhaled carbon dioxide can be used to guide respiratory support in the delivery room. Acta Paediatrica 2014;103:796–806.
  13. Finer N et al. Airway Obstruction During Mask Ventilation of Very Low Birth Weight Infants During Neonatal Resuscitation. Pediatrics 2009;123:865–9.
  14. Schmölzer GM et al. Airway obstruction and gas leak during mask ventilation of preterm infants in the delivery room. Arch Dis Child Fetal Neonatal 2011;96:F254–7.
  15. Mian QN et al. Tidal volumes in spontaneously breathing preterm infants supported with continuous positive airway pressure. J Pediatr 2014;165:702–706.e1.
  16. Kang LJ et al. Monitoring Lung Aeration during Respiratory Support in Preterm Infants at Birth. PLoS ONE 2014;9:e102729.