A non-invasive respiratory volume monitor

Myron Yaster MD

Original article

Atkinson DB, Sens BA, Bernier RS, Gomez-Morad AD, Imsirovic J, Nasr VG. The Evaluation of a Noninvasive Respiratory Volume Monitor in Mechanically Ventilated Neonates and Infants. Anesth Analg. 2022 Jan 1;134(1):141-148. PMID: 33929346

Consider a couple of cases:

Case 1:  A baby born at 34 weeks gestation presents for an inguinal hernia repair.  He is now 54 weeks post-conceptual age.  Do you admit overnight to monitor for post-anesthetic apnea?  What if the baby was 48 or 60 post conceptual age?  How is the child monitored if admitted to the hospital or if sent home?

Case 2: A 4 year old with sleep study confirmed sleep disordered breathing presents for a T&A.  Do you admit the patient postoperatively overnight for respiratory monitoring?  What if there was no sleep study?  Where and how do you monitor for respiratory depression? And for how long?

Case 3: A 7 year old is being treated with IVPCA (morphine).  Where do you monitor this patient in the hospital and if so how?

All 3 of these everyday cases present with a unifying problem.  How or if you should monitor for respiratory depression in at-risk patients?  First a couple of teaching points. The most common technique to evaluate respiration is for a person (usually a nurse or a nursing assistant) to visually or acoustically (stethoscopy) observe the character (e.g. depth) and rate of respirations.  Obviously, this approach is labor intensive, episodic, and occurs only haphazardly and randomly. Electronic monitoring devices may be used to detect respiratory rate and are preprogrammed to alarm if the rates fall below a set value. These electronic vigilance/surveillance aids measure respiration and can detect its absence over some predefined time limit ("apnea"). Unfortunately, most are inaccurate and will not detect respiratory failure from upper airway obstruction because chest wall motion may still be detected even the setting of low tidal volume. Alternative monitors that detect the downstream consequences of apnea or inadequate ventilation such as hypoxia (pulse oximetry) or bradycardia (pulse oximetry or electrocardiography) don’t really work that well either.   

Pulse oximetry is a late, and I do mean late sign of respiratory failure. particularly if the patient is receiving supplemental oxygen.  Unfortunately, at the earliest suggestion of falling SaO2, supplemental oxygen is almost universally given which makes the monitor an almost useless early warning system.  Capnography, the “gold standard”, works wonderfully in the OR or procedure suites but not so well in spontaneously breathing, moving patients, particularly if the monitoring canulae are removed from the face by the patient (eating, talking, finding it annoying)!  Additionally, even when properly positioned, nasal canulae will entrain room air which dilutes end-tidal CO2 and gives falsely low measures.  Additionally, condensation and secretions commonly obstruct the sampling cannula.(1)

What to do?  What to do?  A relatively new device on the market, the respiratory volume monitor (RVM) (ExSpiron, Respiratory Motion Inc, Watertown, MA) uses thoracic impedance technology to noninvasively and continuously measure tidal volume (TV), respiratory rate (RR), and minute ventilation (MV) may be the answer.  Unlike the monitors you are familiar with that count respirations from the ECG leads (plethysmography), this monitor uses a special lead that measures change in electrical impedance associated with thoracic expansion and contraction during breathing and thus provides measurements of both RR and TV. I have been watching the development and hoping for the release and validation of this product for several years.  It has already been validated in adults (2) and children(3), with the latter paper from the same institution as today’s PAAD evaluating the RVM in infants and children 1-17 years of age.   In today’s paper the RVM was tested for accuracy in 40 intubated and mechanically ventilated neonates and infants with a mean age of 93 days.  Although the device works best if there is an initial calibration with a pneumotachometer it also works reasonably well without one (average relative error in TV of <11.0 and < 16.9% with and without calibration respectively). Only 11% of the monitored breaths in this mechanically ventilated population were spontaneously as opposed to controlled. But the spontaneous breaths were equally well detected.

“Further study is needed to demonstrate that the RVM’s performance is accurate and reliable in non-intubated patients and neonates” especially in normal clinical conditions of patient motion and care delivery seen in post-surgical patients”.

As I suspect you can tell, I am hoping that this device will prove to be accurate and useful in non-intubated, spontaneously ventilated pediatric patients, including neonates and infants, and it may become a valuable part of our armamentarium.  (I have no financial interest in the company or the product). At the Society for Pediatric Anesthesia’s Quality and Safety Committee I’ve been advocating for prospective studies using this type of monitor in all of the types of cases I listed at the start of the PAAD.  Let me know if you are interested and hopefully we can finally answer the questions I posed at the start of today’s PAAD with data and evidence. Myron Yaster MD

References

1.           Miller KM, Kim AY, Yaster M, Kudchadkar SR, White E, Fackler J, Monitto CL. Long-term tolerability of capnography and respiratory inductance plethysmography for respiratory monitoring in pediatric patients treated with patient-controlled analgesia. Paediatr Anaesth 2015;25:1054-9.

2.           Voscopoulos C, Brayanov J, Ladd D, Lalli M, Panasyuk A, Freeman J. Special article: evaluation of a novel noninvasive respiration monitor providing continuous measurement of minute ventilation in ambulatory subjects in a variety of clinical scenarios. Anesth Analg 2013;117:91-100.

3.           Gomez-Morad AD, Cravero JP, Harvey BC, Bernier R, Halpin E, Walsh B, Nasr VG. The Evaluation of a Noninvasive Respiratory Volume Monitor in Pediatric Patients Undergoing General Anesthesia. Anesth Analg 2017;125:1913-9.