Sniff nasal inspiratory pressure: simple or too simple?

R espiratory muscle weakness, be it of acute or chronic onset, is a potentially threatening condition. Weakness of inspiratory muscles generates an imbalance between muscle load and capacity that, when severe enough, leads to hypercapnic respiratory failure. Conversely, weakness of expiratory muscles impairs cough and airway clearance and favours lung atelectasis and infection. Dysfunction of both respiratory muscle groups commonly precipitates acute respiratory failure in neuromuscular disorders. In recent years, the importance of respiratory muscle assessment has been recognised and a variety of tests has been proposed [1, 2]. The strength of inspiratory muscles can be assessed either by volitional or by nonvolitional tests. The volitional tests are simple, portable and inexpensive. Their main limitation lies in their dependence on maximal voluntary neuromuscular activation, which, in practice, is difficult to ascertain. In contrast, the cortical motor command is bypassed by nonvoli-tional tests such as phrenic nerve magnetic stimulation [3]. Phrenic nerve stimulation offers the most reliable measure of diaphragm contractility, but is not widely available because it requires expensive equipment. It must be added that phrenic nerve stimulation may overestimate the diaphragm strength that is actually available to the patient in case of upper motor neuron lesions [4]. Thus, notwithstanding their limitations, volitional tests remain on the first line and must be best exploited. Maximum inspiratory pressure (PI,max) is the classic volitional test of inspiratory muscle strength. It is measured as the highest mouth pressure sustained for 1 s during a maximum inspiratory effort against a quasi occlusion. Although simple in principle, the PI,max manoeuvre is difficult for many and requires a hermetic seal around the mouthpiece. As a consequence, low values may be due to true muscle weakness, a submaximal effort, or air leaks in the case of facial muscle weakness. The sniff is an alternative manoeuvre that is more natural and easier for most subjects. During a maximal sniff, there is strong activation of the diaphragm and of the scalene muscles [5, 6]. Thus, the sniff has proved valuable to assess diaphragm strength using transdiaphragmatic pressure (sniff Pdi), or global inspiratory muscle strength using oesophageal pressure (sniff Poes) [7, 8]. More recently, the method of sniff nasal inspiratory pressure (SNIP) was proposed as a non-invasive test of inspiratory muscle strength [9]. This very simple procedure consists of measuring peak nasal pressure in one occluded nostril during a maximal sniff performed from relaxed end-expiration through the contralateral patent nostril. …