In the RRM, the lower graph (Fig. 27.1) displays the turbulence behavior of nasal airflow in relation to airflow velocity. The level on the x-axis corresponds to pure laminar flow, and the upper blue-gray bars correspond to marked turbulence.

At very low flow velocities, flow is laminar in any flow channel (Mlynski and Loew 1992) and thus in the nose as well. In inspiration and expiration, as the velocity of flow increases, laminar flow portions transition into turbulent flow portions. This “transitional zone” is important for the respiratory function of the nose. It creates the optimal situation for conditioning the air. It provides for adequate mucosal contact by the streaming air without leading to drying out or cooling of the mucosa. At very high airflow velocities, nasal airflow becomes purely turbulent (Mlynski and Loew 1992; Churchill et al. 2004; Sawyer et al. 2007; Chen et al. 2009, 2010; Leong et al. 2010). Pure turbulence hardly ever occurs in a normal nose. When high airflow velocities are required to provide adequate oxygen supplies during heavy physical activity, oral bypass breathing is unconsciously switched on so that the nasal airstream decreases (see above: zone of “physiologically required nasal airflow”).

Nasal airflow should become more turbulent when the mucosa is in a decongested state (corresponding to the working phase of the nasal cycle) as a condition for sufficient mucosal contact than in the congested state (corresponding to a resting phase in the nasal cycle). In the resting phase (decongested mucosa), the flow character should not become purely turbulent up to an airflow velocity of 250 ml/s (Fig. 27.1, bilaterally; Fig. 27.4, right side of the nose).

In pathological turbulence behavior, there is a rapid transition to pure turbulence so that pure turbulence will already develop in the nose at a flow rate <250 ml/s (Fig. 27.4, left side of the nose).

Excessive turbulence in the nose can be the cause of elevated airway resistance, and it can also cause a sense of stuffiness and sicca symptoms at low levels of airway resistance.

Besides using the graphical representations to establish a “diagnosis at a glance,” the numerical values make it possible to precisely estimate the extent and the causes of obstruction. Different individuals with the same degree of nasal obstruction may have quite varied levels of symptoms depending on their age, gender, body mass index, level of physical fitness, and associated illnesses. Therefore, the reference values presented here should not be applied rigidly. They are simply benchmarks for the purpose of orientation. The values were determined through studies on both rhinologically healthy patients and patients with nasal obstruction (Marschall 1997; Enßen 2005; Fiebig 2007; Gogniashvili et al. 2011).

Nasal air resistance (R) is numerically reported at a flow velocity of 250 ml/s before and after decongestion. As presented in Sect. 27.2.1.1, the maximum flow velocity still within the zone of physiologically required flow is around 500 ml/s. If we assume that with moderate physical activity, both sides of the nose will enter the working phase of the nasal cycle (which is known as the “in-concert” cycle), at this level of activity, each side of the nose should have a flow rate of about 250 ml/s. No mouth-bypass breathing should occur with moderate physical activity (Olson and Strohl 1987). This is the reason that for a total flow rate = 500 ml/s, there must be no nasal obstruction present. Thus, for the practical evaluation of nasal obstruction, the resistance measured at 250 ml/s is an important value (Gehring et al 2000; Sawyer et al. 2007).

The extent of nasal obstruction can be estimated according to the reference values in Table 27.1.

Hydraulic diameter (d _h ) is a measure of the width of the nasal cavum. The nasal cavity is a space with irregular cross sections. Therefore, its width cannot be defined simply by its diameter as with a round tube. In technological science, the usual practice when dealing with an irregular cross section is to use the “hydraulic diameter.” This is the diameter of a tube of the same length with a round cross section that has the same resistance to flow as the irregularly shaped flow channel. The figures shown in Table 27.2 can be used as reference values for estimating the width of the interior of the nose.

Hydraulic diameter is shown before and after decongestion. Using these values, one can identify a mucosal swelling and a skeletal narrowing as the cause of nasal obstruction. The increase in magnitude of the hydraulic diameter by decongestion is an index of mucosal swelling. If the hydraulic diameter remains low after decongestion, this is evidence of skeletal stenosis. A very large hydraulic diameter indicates a nose that is too wide.

Numerical values for NVC are calculated before and after decongestion. To objectify the extent of NVC, a calculation is made of the percentage increase in resistance (∆R) caused by suction at a flow velocity of 500 ml/s (Fig. 27.5, left). If this flow velocity cannot be achieved, the calculation is performed at maximum inspiration (Fig. 27.5, right).

The numerical value for the increase in resistance from NVC (ΔR) allows us to estimate the magnitude of the suction phenomena as well as to differentiate between physiological and pathological NVS, in accordance with Table 27.3.

In addition, the nasal airflow velocity at which suction/collapse begins (F _NVC) is quantified. The nasal valves should not be significantly collapsed/sucked in (ΔR < 25 %) at airflow velocities up to the maximum physiologically required nasal airflow of 500 ml/s.

The friction coefficient λ is reported during inspiration and expiration, both before and after decongestion. This index characterizes the configuration of the inner nasal wall in relation to its degree of “streamlining,” that is, its impact on the development of turbulent flow, which is important for the warming and humidifying function of the nose (Keck and Lindemann 2010). A low λ-value suggests an internal configuration that causes few turbulent regions of flow. High λ-values indicate greater turbulence formation.

Using the reference values in Table 27.4, the interior of the nose can be evaluated with respect to its tendency to trigger turbulence.

In the nose, the λ-value changes over the nasal cycle: it increases after decongestion in the working phase. This promotes the mucosal contact with the flowing air that is required for thermal and humidity exchange. In the resting phase, the friction coefficient becomes lower, and the airflow becomes more laminar (Lang et al. 2003).

Values over 0.030 indicate severe turbulence, which can cause elevated airway resistance, sicca symptoms, and/or a feeling of stuffiness.

Figure 27.6 presents rhinoresistometry graphs and numerical values from a patient with right mucosal congestion, right skeletal narrowing, and left pathological turbulence behavior.

Figure 27.7 shows RRM findings from a patient with “empty nose syndrome” (Beule 2010; Scheithauer 2010), with post-decongestion values on both sides that are characteristic of an excessively wide nose (hydraulic diameter after decongestion >6.5 mm) with marked turbulence (λ > 0.03), in the graph prior to and after decongestion, there is pure turbulence at a flow <250 ml/s.

ARM is presented in detail in Chap.​ 26 Here, we will describe in addition how acoustic rhinometry can be used to obtain evidence about the causes of pathological turbulence in the nose (Fig. 27.8).

The occurrence of turbulence in a diffuser rises directly in proportion to increases in the cross-sectional area and to decreases in the size of the diffuser entry opening (see Sect. 23.​4).

The entry opening in the nasal diffuser corresponds to the inner nasal valves and thus to MCA1 as calculated by acoustic rhinometry. It is only when there is a ballooning phenomenon present that the diffuser starts out already at the external nasal ostium.

Increases in cross-sectional area along the nasal diffuser are evaluated by measuring the diffuser opening angle φ. This indicator is used in fluid physics in order to characterize cross-sectional expansion in circular diffusers (Fig. 27.9).

In accordance with this principle, the opening angle φ is calculated for the nasal diffuser (anterior cavum) using the cross-sectional area measured by ARM (Fig. 27.10). The angle φ indicates the occurrence of turbulence in the nose in the inspiratory flow direction.

The diffuser angle in relation to triggering nasal turbulence can be assessed using the reference values in Table 27.5.

In the presence of a pathological friction coefficient λ > 0.03 as measured by RRM, the diffuser opening angle provides information about one possible cause of the high level of turbulence. Figure 27.10 shows the results of ARM with a large diffuser opening angle on the right, especially after decongestion.

It has to be considered that even if the opening angle in the nasal diffuser plays the principal part in the occurrence of turbulence, structures other than a narrow diffuser entry opening (e.g., a deformed nasal vestibule, bands, or spurs) may also increase the level of turbulence in the nose.

LRM was developed because RMM, RRM, and ARM only allow for estimating nasal obstruction at a certain time point of the measurement (Lang et al. 2003). Some patients complain about symptoms that occur at other times of the day. LRM makes it possible to measure nasal flow separately for each side of the nose, along with heart rate to serve as an index for physical activity over a 24-h time period under the patient’s everyday conditions of life. Nasal flow is measured using standard commercial nasal oxygen cannulas and the heart rate using standard EVG electrodes. Recording is performed by means of a battery-powered portable device (Fig. 27.11).

Figure 27.12 illustrates the graphical curves resulting from an LRM examination. The upper graph presents the maximal nasal flow values during inspiration separately for each side of the nose in relation to time. The lower graph shows the heart rate for estimating physical activity, respiratory rate, as well as nasal minute ventilation in relation to time.