Table 1.

Methods to assess respiratory muscle function and the work of breathing.

	Methodology	Strengths and clinical relevance	Prediction of extubation, highest reported area under the curve	Limitations
Electromyography (EMG)	Detection of the electrical signal of the diaphragm on the surface or with indwelling oesophageal and gastric catheters	Sensitive detection of the start of a breathing effort and provides insight in the amount of breathing effort, applicable in NAVA ventilation	0.7760	Specialised equipment is required
Maximal respiratory pressures (PImax, PEmax)	Measurement of the maximal pressures during inspiration and expiration generated during crying against an occluded airway	Maximal pressures are a surrogate for muscle strength and increase with maturation	0.9035	Assessment is effort-dependent. Large scatter of normal values
Phrenic nerve stimulation	Electric or magnetic stimulation of the phrenic nerve and measurement of the diaphragm EMG and transdiaphragmatic pressure	Non-volitional method	Has not been assessed in neonates	Specialised equipment is required
Tension Time Index (TTI) of the diaphragm	Product of the ratio of the mean transdiaphragmatic pressure to the maximum inspiratory transdiaphragmatic pressure times the ratio of the inspiratory time to the total breathing cycle time	Composite index of respiratory muscle efficiency: less efficient function when inspiration involves a high proportion of the maximal inspiratory pressure and happens during a large part of the respiratory cycle	1.0040	Specialised equipment and post measurement analysis are required
Thoraco-abdominal asynchrony (TAA)	Lack of synchrony between the chest and abdomen during respiration and calculation of the corresponding phase angle	Non-invasive method. Continuous positive airway pressure decreases TAA. Asynchrony decreases post feeding	Not applicable	Only useful in non-ventilated infants
Relaxation rate of the respiratory muscles	A longer time to relax after contraction signals respiratory muscle fatigue	Ventilator pressure waveforms can be used as a surrogate for calculating the rate of relaxation. Less efficient respiratory muscle function in the presence of systemic infection and in infants of a lower gestational age	0.93753	Difficult calculations. Currently not available in real time
Diaphragmatic ultrasound	The thickness of the diaphragm and the range of diaphragmatic displacement can be non-invasively measured	Inexpensive, non-ionising and accessible method. Diaphragmatic velocity decreased during fatigue. Diaphragmatic thickness is higher in term compared to preterm infants	0.9863	More meaningful in spontaneous breathing infants