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

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 (V_T) 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 V_T.^1,2 However, the delivered V_T is rarely measured and therefore PIP is not adjusted to optimise V_T delivery.³ The appropriate V_T to be delivered during various phases of resuscitation is unknown. A V_T that is too high can damage the lungs by over inflation and a V_T that is too small will result in inadequate gas exchange and atelectasis.² Several delivery room (DR) studies reported a large variation of V_T 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 V_T 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 V_T 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, V_T, respiratory rate, minute ventilation and the leak between mask and face as a percentage ((inspiratory V_T – expiratory V_T)/inspiratory V_T)*100.

Optimal mask ventilation
Figure 1A illustrates gas flow, airway pressure, ECO₂ and V_T 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 V_T. 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 V_T 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 V_T 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 V_T curve displays larger inspiratory V_T compared to expiratory V_T and leak is displayed as a straight line in the V_T 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 V_T delivery, airway obstruction or mask leak.¹² In contrast, an RFM, which displays flow and V_T 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 V_T 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 V_T 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 V_T to facilitate gas exchange. Using a fixed PIP during PPV the delivered V_T depends on the weight of the infant, lung and chest wall compliance and airway resistance.¹⁰ The appropriate V_T during various phases of resuscitation is unknown. A V_T that is too high can damage the lungs by over inflation and a V_T that is too small will result in inadequate gas exchange and atelectasis.² Studies in the DR suggest that V_T 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 V_T and adjust PIP to deliver the required V_T. 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 V_T 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 V_T 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 V_T is displayed. Once an adequate V_T is delivered the PIP can be then adjusted to the required V_T 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 V_T 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 V_T 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 V_T 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, V_T, and ECO₂ has an important role to guide respiratory support in the DR. However, further studies are needed to investigate whether gas flow and V_T during resuscitations can: 1) reduce mask leak or airway obstruction during PPV, 2) target V_T 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 V_T or ECO₂ improves improve survival and short- and long-term outcomes.

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

References

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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.
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