What is a “valid” breath? - Methodological IssuesMichael B. Jaffe PhD

Philips-Respironics, Wallingford, CT

One dictionary defines a breath as “the air taken in and expelled by the expansion and contraction of the thorax” while the Webster’s Revised Unabridged Dictionary‘s definition associates a breath with gas exchange and defines it as:

The air inhaled and exhaled in respiration; air which, in the process of respiration, has parted with oxygen and has received carbonic acid, aqueous vapor, warmth, etc.

As in different dictionaries, there is in the medical community generally a lack of agreement about what constitutes a valid breath. A clear definition is particularly important to developers of computer based algorithms which estimate clinically important measures such as a respiratory rate, tidal volume and end-tidal gas measurements that are considered critical in the management of patients in clinical environments ranging from pre-hospital to the OR and ICU.

1. Govindarajan N, Prakash O. Breath detection algorithm in digital computers. Int J Clin Monit Comput. 1990 Jan;7(1):59-64.

2. Folke M, Cernerud L, Ekström M, Hök B. Critical review of non-invasive respiratory monitoring in medical care. Med Biol Eng Comput. 2003 Jul;41(4):377-83

3. Karlen, W., Turner, M., Cooke, E., Dumont, G. Ansermino, J. M. CapnoBase: Signal database and tools to collect, share and annotate respiratory signals. 2010 Society for Technology in Anesthesia Annual Meeting.

4. Orr JA, Brewer LM, Jaffe MB. Evaluation of Adequacy of Tidal Volumes Using a Volumetric Capnography Reference Data Set. AARC 2010

5. Idris AH, Banner MJ, Wenzel V, Fuerst RS, V, Becker LB, Melker RJ. Ventilation caused by external chest compression is unable to sustain effective gas exchange during CPR: a comparison with mechanical ventilation. Resuscitation 1994;28:143-150.

The criteria for what constitutes a valid breath needs to clearly defined, context specific and clinically relevant. Algorithms need to better disclose their breath detection criteria and to be judged against relevant bench and clinical standards.

Conclusions

Table 3 – Clinical Environments, Context and Breath Criteria

Table 1– Respiratory monitoring methods and variables that can be estimated (Adapted from Folke et al., 2003 )

DiscussionDiscussionIntroductionIntroduction The output of computer based algorithms is dependent upon

the proper detection and clear definition of respiratory events such as the start of breath (SOB), end of breath (EOB), and the transition between inspiratory and expiratory phases. These boundaries can be inferred in a number of ways – using pressure, flow, a constituent component of the breath (e.g. gas such CO2), combinations of these measurements (1) or through more indirect measurements such as chest wall movement, or acoustic measurements. (Table 1) (2)

The definitions of SOB, EOB and what constitutes sufficient volume to be considered a breath are dependent upon clinical environment, context and technology. The criteria for what constitutes a patient effort or breath may vary between the pre-hospital and hospital environments (Figure 1) and context (Table 2). Also what constitutes a useful gold standard needs further clarification. In support of this waveform databases with annotations and/or reference waveforms are being developed. (3) It is suggested that the criteria for breath detection and measurement should be optimized for the environment of use, clinical expectations and therapeutic procedure (e.g. procedural sedation, CPR, general anesthesia, and invasive and non-invasive ventilation).(Table 3)

The relevant clinical and physiological questions asked in determining what is a breath also vary in a similar manner (e.g. is the breath “effective”, does it clear the deadspace, and does the breath represent a patient effort?). Folke (1) notes as a caveat that devices providing only respiratory rate and lacking information about actual gas exchange may have limited clinical value.

The issue of breath size and rate is readily apparent with some algorithms where the reported breath rate can vary widely in presence of artifacts and small patient efforts. A recent study (4) using a large OR and ICU dataset found that the fraction of breaths for which the tidal volume was too small to clear the serial dead volume can be significant (Table 4), and that algorithms which do not indicate the presence of very small breaths may fail to indicate hypoventilation. Similarly, algorithms must be careful to distinguish between chest compressions which fail to clear the deadspace and mechanically delivered breaths which likely will (5).

ReferencesReferences

Table 4 - Frequency of Inadequate Breaths* (4)

Patient Type Percent Breaths Too Small

N (Breaths) N (patients)

ICU, Adult 3.57 % 229,187 28OR, Adult 0.6 % 32,331 38

Adult, Non-intubated 17.3 % 28,078 55

Pediatric, OR 0.44 % 50,398 13

Volume (ml)

Gas movement > 0

Patient effort 10-20

Breath attempt (fails to clear deadspace but sufficient to trigger)

< 150

Breath (clears deadspace and provides trigger) 150-750

Table 2 – Possible volume criteria for breaths (adult)

Respiratory rate

estimation

Tidal volume

estimation

Estimation of CO2

elimination

Estimation of O2

saturation

Notes

Airway sensing Flow/pressure sensing ● ● ●* With CO2

Temperature sensing ●

Humidity sensing ●

Acoustic ●

Gas sensing (e.g.CO2) ● ●* With flow

Movement, volume and tissue composition detection

Transthoracic impedance/ Inductance/ Fiber-optic plethysmography

● ●

Strain-gauge transducers ● ●

Mutual inductance ● ●

Magnetometer ● ●

Capacitance displacement ● ●

Microwave radiation ●

Sensors in mattress ●

Photoplethysmography ● ● * Using POX

Muscle activity ●

Clinical Environment

Context Representative Clinical Problems and Breath Criteria

OR General anesthesia

Small patient efforts (e.g. inadequate anesthesia) may be obscured with sidestream gas monitoring

OR Procedural sedation

Small patient efforts may be labeled as breaths indicating normal breath rate with hypoventilation

Pre-hospital CPR Small fluctuations in volume and gas may or may not indicate gas exchange

Pre-hospital Respiratory distress

Nasal cannula placement problematic

ICU Invasive ventilation

Patient asynchrony may complicate definition of SOB/EOB

ICU Non-invasive ventilation

Patient efforts/breath may be obscured in the presence of mask leak

*inadequate breaths defined as fraction of breaths for which the tidal volume was too small to clear the serial deadspace volume of the patient.

Figure 1- Different Criteria for different clinical environments . With the capnogram as an example – CO2 waveforms in (a) mechanically ventilated ICU patient with significant rebreathing (with flow shown); (b) patient receiving procedural sedation with small breath efforts; and (c) patient during CPR with compression oscillations which fail to clear the deadspace (5)

a.

b.

c.

12/15/10 MBJ final