Howto: Ventilation properties of baby mattresses
This measurement setup provides data on the ventilation propertiues of baby mattresses. The test parameters are chosen according to the physiological conditions of a newborn child (~6 months).
Test bench
The ventilation tests are performed on a force-displacement bench, to which a mannequin baby head is attached. The head is equipped with a differential pressure sensor (Freescale semiconductor MPXV5004DP) and a ventilation tube. The tube is connected to an air pump through an airflow sensor (Honeywell AWM5102VN). The products under testing are placed on a metal grill, providing free air circulation underneath the test sample.
The software allows for different testing conditions: cyclic testing until specified maximal indentation [%] or force [N], maintaining specified airflow [slpm] or pressure [cmH2O].
In terms of suffocation risk, inspiration is more critical than expiration. Therefore, the test conditions are performed under constant inspiration flow, while the mannequin head is pushed into the surface. The measurement values of the pressure at the mouth opening at different
indentation forces provides insight into the breathability differences between the different test samples.
The test are performed in dry or in wet conditions:
- Dry: samples are conditioned at room temperature, no addition of extra liquids
- Wet: 15ml of tap water is distributed equally over a circular area (7.5cm diameter), after which a 5 minute delay was applied to allow the water to spread homogeneously
1 measurement consists of 5 indentation cycles. During each cycle the mannequin head is pushed into the mattress at a constant speed until the indentation force reaches 25N (approximately 2,5 kg), and subsequently retracted until its original position (5 mm above surface). The displacement, force, airflow and pressure are recorded at 100Hz. Each combination is measured 5 times.
Test parameters
To simulate under the physiological conditions that occur for 6 month old infants, the airflow is calculated from the tidal volume and the respiratory frequency at this age. According to Wilkinson [6] the respiratory frequency is dependent on the infant’s age and also on sleep state (Hoppenbrouwers [4]), but not on the sleep position (Elder [2]).
| state | 1wk | 1mo | 2mo | 3mo | 4mo | 6mo |
| AS (active sleep) | 50.5 | 41.7 | 36.3 | 30.8 | 29.8 | 28.1 |
| QS (quiet sleep) | 38.2 | 34.3 | 30.3 | 25.1 | 25.5 | 24.0 |
| IN (intermediate state) | 46.6 | 41.5 | 37.3 | 27.1 | 28.3 | 27.8 |
| AW (awake) | 50.0 | 48.1 | 47.6 | 40.6 | 40.1 | 42.9 |
Although sleep state has a significant effect on the respiratory rate, the effects on tidal volume are inconclusive (Hathorn [3] found an increase of tidal volume in prone position for most subjects, and a decrease for some subjects). According to literature the tidal volume seems independent of
age and lies between 6 and 8 mL/kg (Connors [1], Letzin [5]).
Assuming the airflow is following a sinusoidal pattern, the maximal airflow during a normal inspiratory cycle is calculated as :
Airflow=2.pi.f.Vt
Taking the respiratory rate during active sleep, and the upper boundary of the tidal volume, the maximal inspiratory airflow results in 188,33 ml/s = 11,3 l/min
Analysis
- Comparison of the average pressure curves (pressure as a function of applied force)
- Comparison of the average pressure at 2,0kg (lower pressure means less airway blockage)
- Calculation of the relative breathing resistance: ratio of the pressure at 2,0kg to the pressure obtained for a reference situation
References
- Connors CA and Rosenthal-Dichter C, “Components of breathing: pediatric ventilatory challenges”, Crit.Care Nurse, 17(1)60-70, 1997.
- Elder D.E. et al, “Respiratory variability in preterm and term infants: Effect of sleep state, position and age”, Resp.Phys&Neurobiol., 175, p234-238, 2011
- Hathorn MKS, “The rate and depth of breathing in new-born infants in different sleep states”, J.Physiol, 243, p101-113, 1974.
- Hoppenbrouwers T et al, “Polygraphic studies of normal infants during the First Six Months of Life. II. Respiratory Rate and Variability as a function of state”, Ped.Res (12)120-125, 1978.
- Latzin P et al “Lung Volume, breathing pattern and ventilation inhomogeneity in preterm and term infants”, PONE 4(2), 2009
- Wilkinson MH et al “Postnatal development of periodic breathing cycle duration in term and preterm infants”, Ped.Res 62(3)331-336, 2007
