Effect of spontaneous movement on respiration in preterm infants

Abstract

New findings: What is the central question of this study? The respiratory centres in the brainstem that control respiration receive inputs from various sources, including proprioceptors in muscles and joints and suprapontine centres, which all affect limb movements. What is the effect of spontaneous movement on respiration in preterm infants? What is the main finding and its importance? Apnoeic events tend to be preceded by movements. These activity bursts can cause respiratory instability that leads to an apnoeic event. These findings show promise that infant movements might serve as potential predictors of life-threatening apnoeic episodes, but more research is required.

Abstract: A common condition in preterm infants (<37 weeks' gestational age) is apnoea resulting from immaturity and instability of the respiratory system. As apnoeas are implicated in several acute and long-term complications, prediction of apnoeas may preempt their onset and subsequent complications. This study tests the hypothesis that infant movements are a predictive marker for apnoeic episodes and examines the relation between movement and respiration. Movement was detected using a wavelet algorithm applied to the photoplethysmographic signal. Respiratory activity was measured in nine infants using respiratory inductance plethysmography; in an additional eight infants, respiration and partial pressure of airway carbon dioxide ( $P_{C{O}_2}$ ) were measured by a nasal cannula with side-stream capnometry. In the first cohort, the distribution of movements before and after the onset of 370 apnoeic events was compared. Results showed that apnoeic events were associated with longer movement duration occurring before apnoea onsets compared to after. In the second cohort, respiration was analysed in relation to movement, comparing standard deviation of inter-breath intervals (IBI) before and after apnoeas. Poincaré maps of the respiratory activity quantified variability of airway $P_{C{O}_2}$ in phase space. Movement significantly increased the variability of IBI and $P_{C{O}_2}$ . Moreover, destabilization of respiration was dependent on the duration of movement. These findings support that bodily movements of the infants precede respiratory instability. Further research is warranted to explore the predictive value of movement for life-threatening events, useful for clinical management and risk stratification.

Keywords: Poincaré map; apnoea; breathing; movement; preterm infants; respiration; respiratory stability.

Figure 1:

Time series of respiratory activity (from pneumogram), heart rate, SpO₂ and movement, before (PRE-APNEA-ONSET) and after (POST-APNEA-ONSET) the onset of apnea, defined as time=0 (Comparison 1). Bradycardia threshold is indicated by the green dashed line. Movement was estimated using a wavelet-based algorithm on the photoplethysmogram. Total movement durations in the PRE-APNEA-ONSET and POST-APNEA-ONSET conditions were computed from the binary movement time series.

Figure 2:

Schematic representation of all segments compared

Figure 3:

A. Time series of movement, respiratory activity (PCO₂) and its first derivative (dPCO₂), from Comparison 2: before (PRE-MOVEMENT-ONSET: left) and after (POST-MOVEMENT-ONSET) the onset of movement. B. Phase portrait of the same time series of PCO₂ with a Poincaré section in the expiratory phase (green line). Markers ‘o’ and ‘x’ represent the two points used to define the Poincaré section. The red points indicate the intersections of the trajectories with the Poincaré section. These locations are projected onto the time series in panel A (red points). C. Zoomed versions of the orbits in the rectangles in panel B. For this subject (#14), the reference point (p*) on the Poincaré section is denoted by the black point. The distance of each intersection p_n from the reference point p* are denoted by d_n.

Figure 4:

Example phase space with three Poincaré sections: expiratory (green), end-expiratory (pink) and inspiratory (orange).

Figure 5:

Changes in mean and standard deviation of inter-breath intervals (IBIs) and movement duration from PRE-APNEA-ONSET to POST-APNEA-ONSET segments (Comparison 1) in Cohort 1 (subject # 1–9). Movement duration in PRE-APNEA-ONSET to POST-APNEA-ONSET segments were also calculated for a subset of apneas that were classified as pathological. Each colored marker represents a single subject.

Figure 6:

Differences in mean and standard deviation of inter-breath intervals in Comparison 2: PRE-MOVEMENT-ONSET vs POST-MOVEMENT-ONSET segments in Cohort 2 (subject # 10–17). Each colored marker represents a single subject.

Figure 7:

Differences in respiratory stability in Comparison 2: PRE-MOVEMENT-ONSET vs POST-MOVEMENT-ONSET segments in Cohort 2 (subject # 10–17). Each colored marker represents a single subject. A. Mean (Mean-d) and standard deviation (SD-d) of the distance from reference point using Poincaré sections taken at the expiratory Poincaré section B. Mean and standard deviation of end-tidal CO₂ obtained at the end-expiration Poincaré section. Note: Subject denoted by red had lower P_etCO₂ values due to the use of a non-standard nasal canula for sampling of CO₂. The subject was not excluded from analysis; excluding the subject would have yielded stronger significance.

Figure 8:

Differences in mean and standard deviation of inter-breath intervals in Comparison 3: PRE-MOVEMENT-OFFSET vs POST-MOVEMENT-OFFSET segments in Cohort 2 (subject # 10–17). Each colored marker represents a single subject.

Figure 9:

Differences in respiratory stability for Comparison 3 (PRE-MOVEMENT-OFFSET and POST-MOVEMENT-OFFSET segments) in Cohort 2 (subject # 10–17). Each colored marker represents a single subject. A. Mean (Mean-d) and standard deviation (SD-d) of the distance from reference point at the expiratory Poincaré sections. B. Mean and standard deviation of end-tidal CO₂ obtained at the end-expiratory Poincaré section.

Figure 10:

Variability measures for Comparison 4 (PRE-MOVEMENT-ONSET, DURING-MOVEMENT and POST-MOVEMENT-OFFSET segments) in Cohort 2 (subject #10–17). Each colored marker represents a single subject. A. Standard deviation (SD-d) of the distance from reference point at the expiratory Poincaré sections. B. Standard deviation of end-tidal CO₂ obtained at the end-expiratory Poincaré section.

Figure 11:

Time interval between onset of movements and onset of apneas.