Intra-abdominal pressure measurement: validation of intragastric pressure as a measure of intra-abdominal pressure

BACKGROUND

The diagnosis of abdominal compartment syndrome depends upon the demonstration of an elevated intra-abdominal pressure (IAP). Direct measures of IAP are impractical in the critical care unit; intravesical pressure (IVP) and intragastric pressure (IGP) should represent acceptable surrogate measures. IVP is the preferred measure of IAP in critical care. We considered that IGP represents a practical alternative. The objective of this preliminary study was to observe the relationship between IGP and IAP.

METHODS

After Institutional Ethics Board approval, 29 patients having elective laparoscopic surgery were recruited. IAP was measured directly via the abdominal trochar. This was compared with IGP measured via a commercial balloon catheter placed into the stomach.

RESULTS

Measured IGP was always more positive than IAP; both showed linear correlation (r2>0.9). When IGP was calibrated against IAP, an estimated difference between the IGP and IAP of+/-2.5 mm Hg for 95% of the measurements was seen.

CONCLUSIONS

The study demonstrates the strength of the relationship between IGP and IAP in normal individuals. Application of IGP measurement in the critical care patient is necessary to demonstrate its suitability for continuous IAP assessment.

Assessment of diaphragm function after stellate ganglion block using magnetic stimulation

Stellate ganglion block is a procedure frequently used for the management of patients with chronic sympathetically mediated pain affecting the arm, neck or head. We studied the effect of stellate ganglion block on ipsilateral phrenic nerve function, and hence diaphragmatic strength, in 11 adult patients with chronic sympathetically mediated pain. Pre- and post-block forced vital capacity (FVC) measurements were recorded using a pneumotachograph and a Magstim nerve stimulator was used to generate pre- and post-block twitch mouth pressures (P(TWM)). This device can be used to stimulate the phrenic nerves and hence the diaphragm. The resulting change in airway pressure was measured at the mouth and has previously been shown to reflect diaphragm strength. There was no statistically significant difference in FVC or P(TWM) pre- or post stellate ganglion block. In conclusion, a stellate ganglion block has no adverse effect on ipsilateral phrenic nerve function or diaphragm strength in healthy adult patients.

Tracheal tube pressure change during magnetic stimulation of the phrenic nerves as an indicator of diaphragm strength on the intensive care unit

Diaphragm strength can be assessed from twitch gastric (TwPgas), twitch oesophageal (TwPoes), and twitch transdiaphragmatic pressure (TwPdi) in response to phrenic nerve stimulation. This requires the passage of balloon catheters, which may be difficult. Changes in pressure measured at the mouth during phrenic nerve stimulation avoid the need for balloon catheters. We hypothesized that pressures measured at the tracheal tube during phrenic stimulation, could also reflect oesophageal pressure change as a result of isolated diaphragmatic contraction and, therefore, reflect diaphragm strength. We aimed to establish the relationship between twitch tracheal tube pressure (TwPet), TwPoes, and TwPdi in patients in the supine and sitting positions. The phrenic nerves were stimulated magnetically bilaterally, in 14 ICU patients while supine and on another occasion while sitting up at 45 degrees. In the sitting position mean TwPoes was 9.1 cm H2O and TwPet 11.3 cm H2O (mean(SD) difference -2.2 (SD 1.5)). In the supine position mean TwPoes was 8.1 cm H2O and TwPet 9.9 cm H2O (mean difference -1.8 (2.2)). The difference between TwPoes and TwPet was less at low twitch amplitude; less than +/- 1 cm H2O below a mean twitch height of 8 cm H2O supine and 10 cm H2O sitting. Sitting TwPet was related to TwPoes r2=0.93 and TwPdi r2=0.65 (P<0.01). Supine TwPet was related to TwPoes r2=0.84 and TwPdi r2=0.83 (P<0.01). The mean within occasion coefficient of variation while sitting was TwPet=13.3%, TwPoes=13.9%, TwPdi=11.2%, and supine TwPet=11.6%, TwPoes=14.6%, TwPdi=11.8%. We conclude that TwPet reflects TwPoes during diaphragmatic stimulation and is worthy of further study to establish its place as a guide to the presence of respiratory muscle strength and fatigue.

Respiratory muscle strength in Cushing's syndrome

The effect of Cushing's syndrome on respiratory muscle strength is unknown. Therefore, we studied 10 consecutive patients with severe Cushing's syndrome. The respiratory muscles were assessed using maximal inspiratory and expiratory mouth pressures (MIP, MEP), maximal sniff transdiaphragmatic pressures (max sniff Pdi), and maximal sniff esophageal pressures (max sniff Pes). Maximal quadricep strength was also assessed. The patients demonstrated an overall mean MIP 92 cm H(2)O, SD 19 (mean 105% of predicted; SD, 23%), mean MEP 134 cm H(2)O, SD 35 (mean 99% of predicted; SD, 25%), mean max sniff Pdi 107 cm H(2)O, SD 12 (mean 78% of predicted; SD, 10%) and mean max sniff Pes of 92 cm H(2)O, SD 11 (mean 92% of predicted; SD, 11%). Quadriceps muscle strength was reduced in all 10 patients: mean 26 kg, SD 9 (mean 49% of predicted strength, SD 21%). Respiratory muscle weakness was not found, despite the presence of severe quadriceps impairment. We conclude that major weakness of the respiratory muscles is not usual in Cushing's syndrome.

Measurement of sniff nasal and diaphragm twitch mouth pressure in patients

BACKGROUND

Inspiratory muscle weakness is a recognised cause of unexplained dyspnoea. It may be suggested by the finding of a low static inspiratory mouth pressure (MIP), but MIP is a difficult test to perform, with a wide normal range; a low MIP may also occur if the patient has not properly performed the manoeuvre. Further investigation conventionally requires balloon catheters to obtain oesophageal (Poes) and transdiaphragmatic pressure (Pdi) during sniffs or phrenic nerve stimulation. Two non-invasive tests of inspiratory muscle strength have recently been described--nasal pressure during a maximal sniff (Sn Pnas) and mouth pressure during magnetic stimulation of the phrenic nerves (Tw Pmo). The use of these two tests in combination might identify patients without inspiratory muscle weakness who are unable to produce a satisfactory MIP< therefore avoiding the need for investigation with balloon catheters.

METHODS

Thirty consecutive patients with clinically suspected inspiratory muscle weakness and a low MIP underwent both conventional (Sn Poes and Tw Pdi) and non-invasive testing (Sn Pnas and Tw Pmo). Weakness was considered to be excluded by a Sn Poes of > or = 80 cm H20 or a Tw Pdi of > or = 20 cm H20. The limit values used to test the hypothesis were Sn Pnas > or = 70 cm H20 or Tw Pmo > or = 12 cm H20.

RESULTS

Inspiratory muscle weakness was excluded in 17 of the 30 patients. Fifteen of these would have been identified using Sn Pnas and Tw Pmo, with better results when the two tests were combined. The cut off values selected for Sn Pnas and Tw Pmo were shown by ROC plots to indicate normal strength conservatively, avoiding failure to detect mild degrees of weakness. No patient with global weakness was considered normal by Sn Pnas or Tw Pmo.

CONCLUSIONS

In most patients with normal inspiratory strength and a low MIP, Tw Pmo and Sn Pnas used in combination can reliably exclude global inspiratory muscle weakness, reducing the number of patients who need testing with balloon catheters.

Paired phrenic nerve stimuli for the detection of diaphragm fatigue in humans

The transdiaphragmatic pressure (Pdi) elicited by paired bilateral phrenic nerve stimulation may be viewed as the sum of the Pdi values produced by the first (t1) and second (t2) stimulus. The Pdi at t2 (P[di,t2]) is a function of the interstimulus interval. A reduction in the ratio obtained by dividing Pdi,t2 at 10 Hz (P[di,t2,10]) by Pdi at 100 Hz (P[di,t2,100]) (t2(10:100)) has been proposed as a test of low frequency diaphragm fatigue. The aim of the present study was to establish whether this change could also be detected using paired cervical magnetic nerve stimulation (pCMS), and whether t2(10:100) was influenced by lung volume. We studied healthy subjects at functional residual capacity (FRC), at 0.5 and 1.0 L below FRC, and at 0.5, 1.0 and 1.5 L above FRC. The subjects were then subjected to a fatiguing protocol (2 min of maximal isocapnic ventilation (MIV)). Studies were repeated at FRC 20 and 60 min after MIV and between these times at 1.0 L below and 1.5 L above FRC. In the unfatigued state, t2(10:100) had a negative relationship with increasing lung volume (r2=0.98, p=0.002). After MIV there was a fall in the Pdi elicited by a single stimulus (mean fall at 20 min 17.9% and at 60 min 14.6%, p<0.03 for both). t2(10:100) fell by a mean 28.1% after 20 min and mean 22.9% at 60 min (p<0.03 for both). This change was mainly mediated by a fall in the P[di,t2,10]. The t2(10:100) was not able to distinguish between fatigue and acute hyperinflation. We conclude that paired cervical magnetic nerve stimulation may be used to detect low frequency diaphragm fatigue but that it remains important to control for lung volume.

Cervical magnetic stimulation of the phrenic nerves in bilateral diaphragm paralysis

Cervical magnetic stimulation (CMS) produces a greater twitch transdiaphragmatic pressure (TwPdi) than electrical stimulation. This may be because CMS produces rib cage muscle activation, thus producing an inspiratory action independent of the diaphragm. Alternatively, CMS could merely stiffen the rib cage, allowing the diaphragm to act efficiently, by contracting against a stable rib cage. To examine these two hypotheses we studied five patients with isolated bilateral diaphragm paralysis using CMS and bilateral electrical phrenic stimulation (BES). TwPdi, twitch esophageal pressure (TwPes), and twitch gastric pressure (TwPgas) were measured. We also assessed maximal sniff esophageal and transdiaphragmatic pressures (SnPes) (SnPdi), maximal inspiratory and expiratory mouth pressures (MIP) (MEP), and fall in VC on moving from an upright to a supine position. Respiratory muscle strength tests were consistent with bilateral diaphragm paralysis, and the MEPs confirmed normal expiratory muscle function. The patients were able to generate a mean SnPes of -30 cm H2O, mainly because of inspiratory activity of rib cage and neck muscles. However, TwPdi and TwPes during both CMS and BES were close to zero. We conclude that in our patients with diaphragm paralysis caused by neuralgic amyotrophy, CMS stiffens the rib cage but does not have an inspiratory action independent of the diaphragm.

Diaphragm performance during maximal voluntary ventilation in chronic obstructive pulmonary disease

In normal subjects 2 min of maximal voluntary hyperventilation results in failure of tension generation and low-frequency fatigue of the diaphragm. Patients with severe chronic obstructive pulmonary disease (COPD) do not develop diaphragm fatigue during exhaustive treadmill exercise despite excessive inspiratory muscle loading and we hypothesized that they might be relatively resistant to the development of diaphragm fatigue during maximal ventilation. In six patients with severe COPD (mean FEV1 0.671) we therefore loaded the diaphragm using 2 min of maximal isocapnic ventilation (MIV). Initial mean ventilation was 28.6 L/min and diaphragm pressure-time product (PTPdi) 602 cm H2O x s/min; these values were sustained throughout MIV without significant decline. Mean twitch transdiaphragmatic pressure (Tw Pdi) was 19.7 cm H2O 25 min after a control run and 20.5 cm H2O at the same time after MIV [corrected]. Compared with normal subjects previously studied in our laboratory (Hamnegard, C.-H., et al. Eur. Respir. J. 1996;9:241-247) the reduction in PTPdi was disproportionately greater than the reduction in Tw Pdi. We conclude that, unlike normal subjects, 2 min of MIV causes neither failure of diaphragm performance nor low-frequency diaphragm fatigue in patients with severe COPD. It is likely that the diaphragm makes a relatively limited contribution to the generation of maximal levels of ventilation in severe COPD.

Diaphragm strength in chronic obstructive pulmonary disease

The diaphragm is normally the main inspiratory muscle and diaphragm strength in chronic obstructive pulmonary disease (COPD) is therefore of interest. We assessed diaphragm strength in 20 patients with severe stable COPD (mean FEV1 0.61, mean thoracic gas volume [Vtg] 5.31) and seven normal control subjects, measuring both maximal sniff transdiaphragmatic pressure (sniff Pdi(max)) and twitch transdiaphragmatic pressure (Tw Pdi) elicited by cervical magnetic stimulation (CMS) of the phrenic nerve roots at FRC. Acute-on-chronic hyperinflation was examined in four patients. Mean Tw Pdi in patients and control subjects was 18.5 cm H2O and 25.4 cm H2O, respectively (p < 0.01), and mean sniff Pdi was 81.9 cm H2O and 118 cm H2O, respectively (p < 0.001). Reduction in mean intrathoracic pressures was more marked; twitch esophageal pressure (Tw Pes) was -7.3 cm H2O and -16.3 cm H2O, respectively (p < 0.001) and sniff Pes was -67 cm H2O and -97.8 cm H2O (p < 0.001). During acute-on-chronic hyperinflation there was a linear negative correlation of Tw Pdi with increasing lung volume of 3.5 cm H2O/L. The ability of the diaphragm to generate transdiaphragmatic, and particularly a negative intrathoracic, pressure is reduced in COPD and these changes are exaggerated with acute-on-chronic hyperinflation.

Inspiratory pressure support reduces slowing of inspiratory muscle relaxation rate during exhaustive treadmill walking in severe COPD

When patients with COPD walk to a state of intolerable dyspnea, there is excessive inspiratory muscle loading, as evidenced by slowing of the maximum relaxation rate of the inspiratory muscles, measured from esophageal pressure during a sniff (Sn Pes MRR). In this setting, inspiratory pressure support (IPS) delivered via an orofacial mask increases walking distance and reduces dyspnea, but the mechanism by which this benefit is achieved remains unclear. In this study we compared Sn Pes MRR after equidistant treadmill walking in six men with severe COPD (mean FEV1: 0.6 L, 22% predicted). After the free walk there was a mean slowing of Sn Pes MRR of 41% (p < 0.03). After the IPS-assisted walks, the slowing of Sn Pes MRR was 20% of baseline; this was significantly less than after the free walk (p < 0.05). Four subjects performed shorter walks; after free walks of one third and two thirds of maximum distance, the mean slowing of Sn Pes MRR was 23% and 28%, respectively. We conclude that when patients with COPD walk to exhaustion, IPS reduces slowing of inspiratory muscle MRR, and that this represents a considerable unloading of the inspiratory muscles. The magnitude of the reduction is approximately the same as reducing the distance walked by two thirds.

Bilateral magnetic stimulation of the phrenic nerves from an anterolateral approach

We investigated whether bilateral magnetic stimulation of the phrenic nerves from an anterolateral approach (BAMPS) could combine the reproducibility and ease of use of cervical magnetic stimulation (CMS) with the specificity of bilateral electrical stimulation (BES) and whether it could be used in supine subjects. We placed two double 43-mm coils over the phrenic nerves in the neck. BAMPS produced supramaximal phrenic stimulation by electromyogram (EMG) assessment in six of seven subjects. There was no significant difference in the twitch gastric pressure/twitch esophageal pressure ratio (twitch Pgas/Pes) between BAMPS (1.2) and BES (1.3). Both differed from CMS (0.9, p < 0.001). The effect of a change in posture on twitch transdiaphragmatic pressure (TwPdi) and Pgas/Pes ratio was the same for BAMPS and BES. In normal subjects and patients BAMPS correlated significantly with BES (r = 0.97), maximal sniffs (r = 0.85), and CMS (r = 0.92). The mean difference between BAMPS and BES was 0.3 cm H2O (SD = 2.3). Two-minute maximal isocapnic ventilation produced a 19% fall in TwPdi elicited by BAMPS. BAMPS is easy, well tolerated and can be used in the supine subject. TwPdi and partitioning of Pes and Pgas were very close for BAMPS and BES, suggesting similar specificity for the diaphragm.

Abdominal muscle fatigue after maximal ventilation in humans

Abdominal muscles are the principal muscles of active expiration. To investigate the possibility of abdominal muscle low-frequency fatigue after maximal ventilation in humans, we stimulated the nerve roots supplying the abdominal muscles. We used a magnetic stimulator (Magstim 200) powering a 90-mm circular coil and studied six normal subjects. To assess the optimum level of stimulation and posture, we stimulated at each intervertebral level between T7 and L1 in the prone, supine, and seated positions. At T10, we used increasing power outputs to assess the pressure-power relationship. Care was taken to avoid muscle potentiation. Twitch gastric pressure (Pga) was recorded with a balloon-tipped catheter. Mean (+/-SD) baseline twitch Pga measured with the subjects in the prone position at T10 was 23.5 +/- 5.4 cmH2O. Within-occasion mean twitch Pga coefficient of variation was 4.6 +/- 1.1%. Twitch Pga was measured with the subjects in the prone position with stimulation over T10 before and after 2 min of maximal isocapnic ventilation (MIV). Twenty minutes after MIV, mean twitch Pga fell by 17 +/- 9.1% (P = 0.03) and remained low 90 min after MIV. We conclude that after maximal ventilation in humans there is a reduction of twitch Pga and, therefore, of low-frequency fatigue in abdominal muscles.

Effect of maximum ventilation on abdominal muscle relaxation rate

BACKGROUND

When the demand placed on the respiratory system is increased, the abdominal muscles become vigorously active to achieve expiration and facilitate subsequent inspiration. Abdominal muscle function could limit ventilatory capacity and a method to detect abdominal muscle fatigue would be of value. The maximum relaxation rate (MRR) of skeletal muscle has been used as an early index of the onset of the fatiguing process and precedes failure of force generation. The aim of this study was to measure MRR of abdominal muscles and to investigate whether it slows after maximum isocapnic ventilation (MIV).

METHODS

Five normal subjects were studied. Each performed short sharp expiratory efforts against a 3 mm orifice before and immediately after a two minute MIV. Gastric pressure (PGA) was recorded and MRR (% pressure fall/10 ms) for each PGA trace was determined.

RESULTS

Before MIV the mean (SD) maximum PGA MRR for the five subjects was 7.1 (0.8)% peak pressure fall/10 ms. Following MIV mean PGA MRR was decreased by 30% (range 25-35%), returning to control values within 5-10 minutes.

CONCLUSIONS

The MRR of the abdominal muscles, measured from PGA, is numerically similar to that described for the diaphragm and other skeletal muscles. After two minutes of maximal isocapnic ventilation abdominal muscle MRR slows, indicating that these muscles are sufficiently heavily loaded to initiate the fatiguing process.

Quadriceps strength and fatigue assessed by magnetic stimulation of the femoral nerve in man

There is no nonvolitional method of assessing quadriceps strength which both supramaximally activates the muscle and is acceptable to subjects. In 10 normal subjects and 10 patients with suspected muscle weakness we used magnetic stimulation of the femoral nerve to elicit an isometric twitch and measured twitch tension (TwQ), surface electromyogram in addition to the maximum voluntary contraction force (MVC). Supramaximality was achieved in all subjects at a mean of 83% of maximum stimulator output. When supramaximal, TwQ was reproducible (mean coefficient of variation 3.6%, range 0.7-10.9) and correlated well with MVC (r2 = 0.83, P<0.001). In 7 normal subjects we measured TwQ before and after a fatiguing protocol; after 20 min TwQ was a mean of 55% (range 29-77%) of baseline and remained substantially reduced at 90 min. Magnetic femoral nerve stimulation is a painless, supramaximal method of assessing quadriceps strength and fatigue which is likely to be of value in clinical and physiological studies.

Exhaustive exercise slows inspiratory muscle relaxation rate in chronic obstructive pulmonary disease

The excessive load placed on inspiratory muscles when patients with COPD exercise could lead to fatigue and contribute to exercise limitation. Slowing of maximal relaxation rate (MRR) of skeletal muscle is an early index of the fatiguing process. We investigated whether inspiratory muscle MRR slows when patients with COPD walk to exhaustion. We studied nine well-trained and motivated patients with stable severe COPD (mean FEV1: 0.7 L, 28% predicted). Each subject performed sniff maneuvers before and after walking on a treadmill until they were forced to stop because of dyspnea. Esophageal (Pes), gastric, and transdiaphragmatic pressures were measured using balloon-tipped catheters. MRR was calculated as the percent Pes drop/10 ms. In the first minute after exercise there was a mean decrease of Pes MRR of 42% (range, 21 to 65%) (p < 0.01), which returned to baseline within 3 to 5 min. The fall in MRR indicates that the inspiratory muscles of patients with COPD walking to exhaustion are sufficiently heavily loaded to initiate the fatiguing process.

Unilateral magnetic stimulation of the phrenic nerve

BACKGROUND

Electrical stimulation of the phrenic nerve is a useful non-volitional method of assessing diaphragm contractility. During the assessment of hemidiaphragm contractility with electrical stimulation, low twitch transdiaphragmatic pressures may result from difficulty in locating and stimulating the phrenic nerve. Cervical magnetic stimulation overcomes some of these problems, but this technique may not be absolutely specific and does not allow the contractility of one hemidiaphragm to be assessed. This study assesses both the best means of producing supramaximal unilateral magnetic phrenic stimulation and its reproducibility. This technique is then applied to patients.

METHODS

The ability of four different magnetic coils to produce unilateral phrenic stimulation in five normal subjects was assessed from twitch transdiaphragmatic pressure (TwPDI) measurements and diaphragmatic electromyogram (EMG) recordings. The results from magnetic stimulation were compared with those from electrical stimulation. To determine whether the magnetic field affects the contralateral phrenic nerve as well as the intended phrenic nerve, EMG recordings from each hemidiaphragm were compared during stimulation on the same side and the opposite side relative to the recording electrodes. The EMG recordings were made from skin surface electrodes in five normal subjects and from needle electrodes placed in the diaphragm during cardiac surgery in six patients. Similarly, the direction of hemidiaphragm movement was evaluated by ultrasonography. To determine the usefulness of the technique in patients the 43 mm mean diameter double coil was used in 54 patients referred for assessment of possible respiratory muscle weakness. These results were compared with unilateral electrical phrenic stimulation, maximum sniff PDI, and TwPDI during cervical magnetic stimulation.

RESULTS

In the five normal subjects supramaximal stimulation was established for eight out of 10 phrenic nerves with the 43 mm double coil. Supramaximal unilateral magnetic stimulation produced a higher TwPDI than electrical stimulation (mean (SD) 13.4 (2.5) cm H2O with 35 mm coil; 14.1 (3.8) cm H2O with 43 mm coil; 10.0 (1.7) cm H2O with electrical stimulation). Spread of the magnetic field to the opposite phrenic nerve produced a small amplitude contralateral diaphragm EMG measured from skin surface electrodes which reached a mean of 15% of the maximum EMG amplitude produced by ipsilateral stimulation. Similarly, in six patients with EMG activity recorded directly from needle electrodes, the contralateral spread of the magnetic field produced EMG activity up to a mean of 3% and a maximum of 6% of that seen with ipsilateral stimulation. Unilateral magnetic stimulation of the phrenic nerve was rapidly achieved and well tolerated. In the 54 patients unilateral magnetic TwPDI was more closely related than unilateral electrical TwPDI to transdiaphragmatic pressure produced during maximum sniffs and cervical magnetic stimulation. Unilateral magnetic stimulation eliminated the problem of producing a falsely low TwPDI because of technical difficulties in locating and adequately stimulating the nerve. Eight patients with unilateral phrenic nerve paresis, as indicated by a unilaterally elevated hemidiaphragm on a chest radiograph and maximum sniff PDI consistent with hemidiaphragm weakness, were all accurately identified by unilateral magnetic stimulation.

CONCLUSIONS

Unilateral magnetic phrenic nerve stimulation is easy to apply and is a reproducible technique in the assessment of hemidiaphragm contractility. It is well tolerated and allows hemidiaphragm contractility to be rapidly and reliably assessed because precise positioning of the coils is not necessary. This may be particularly useful in patients. In addition, the anterolateral positioning of the coil allows the use of the magnet in the supine patient such as in the operating theatre or intensive care unit.

Exhaustive treadmill exercise does not reduce twitch transdiaphragmatic pressure in patients with COPD

Reduced diaphragm contractility has been described in normal subjects after whole body endurance exercise, and it indicates low frequency fatigue (LFF); it is unknown whether LFF is of clinical importance. We therefore studied the effect of treadmill exercise to exhaustion on diaphragm contractility in six patients with severe chronic obstructive pulmonary disease (COPD) (mean FEV1, 0.71, 27% predicted). The subjects first performed a short (control), treadmill walk and then, after resting, a second walk to a state of severe dyspnea. Cervical magnetic stimulation of the phrenic nerve roots was performed at the start of the study and 20 and 30 min after each walk. The twitch transdiaphragmatic pressure (Tw Pdi) was reproducible (mean coefficient of variation, 5.3%; range, 2 to 12.5%). Mean Twi Pdis were 18.4 cm H2O at baseline and 19.6 cm H2O and 19.2 cm H2O 20 and 30 min after the control walk. At the same times after the exhaustive walk, mean Tw Pdis were 19.6 and 20.4 cm H20. Tw Pdi was not reduced by exhaustive treadmill walking (p > 0.9), and a power calculation showed that the study had a 95% chance of detecting a 10% fall at the 5% significance level. We conclude that Tw Pdi is not reduced when patients with severe COPD walk to a state of extreme breathlessness and that therefore low frequency fatigue of the diaphragm does not occur.

The effect of lung volume on transdiaphragmatic pressure

Diaphragm strength can be assessed by measurement of transdiaphragmatic pressure (Pdi) in response to stimulation of the phrenic nerves. The length-tension relationship of the diaphragm can be studied by measuring twitch Pdi over the range of lung volume. Previous studies of the relationship between lung volume and diaphragm strength have used the technique of electrical stimulation of the phrenic nerves. In these studies, the phenomenon of twitch potentiation has not been taken into account. It has previously been shown that prior contraction of the diaphragm can greatly enhance the twitch response, thus affecting the measurements. The aim of this study was to investigate the relationship between unpotentiated twitch Pdi and lung volume for volumes ranging from residual volume (RV) to total lung capacity (TLC) in normal subjects. Great care was taken to avoid muscle potentiation. For this purpose, we stimulated the phrenic nerves with a magnetic stimulator. In addition, we used positive pressure to inflate the lungs to high lung volumes. The impact of twitch potentiation on the length-tension relationship was investigated by subjects making maximum inspiratory efforts prior to phrenic nerve stimulation. The unpotentiated twitch Pdi decreased in a linear fashion with increasing lung volume over the full range of vital capacity by 0.54 kPa.L-1. Potentiation increased twitch Pdi by 40% at FRC and the effect was similar, in absolute terms, at all lung volumes. In relative terms, the effect of potentiation became greater as lung volume increased, and more than doubled twitch Pdi at TLC. With increasing lung volume, there is a linear fall in unpotentiated twitch Pdi with a slope that is less steep, over the same range of absolute lung volume, than previously reported. When assessing diaphragm strength by the twitch technique, it is essential to control for lung volume and equally important to control for twitch potentiation.

Inspiratory muscle relaxation rate assessed from sniff nasal pressure

BACKGROUND

Slowing of the maximum relaxation rate (MRR) of inspiratory muscles measured from oesophageal pressure (POES) during sniffs has been used as an index of the onset and recovery of respiratory muscle fatigue. The purpose of this study was to measure MRR at the nose (PNASAL MRR), to investigate its relationship with POES MRR, and to establish whether PNASAL MRR slows with respiratory loading.

METHODS

Five normal subjects were studied. Each performed sniffs before and after two minutes of maximal isocapnic ventilation (MIV). In a separate session the subjects performed submaximal sniffs. POES and PNASAL were recorded during sniffs and the MRR (% pressure fall/10 ms) for each sniff was determined.

RESULTS

Before MIV mean POES MRR was 8.9 and PNASAL MRR was 9.3. The mean (SD) difference between PNASAL MRR and POES MRR during a maximal sniff was 0.48 (0.34) (n = 64) and during submaximal sniffs was 0.28 (0.46) (n = 526). The subjects showed a mean decrease in sniff POES MRR of 27.4% (range 22.5-36%) after MIV and a similar reduction in sniff PNASAL MRR of 28.5% (range 24.1-41.3%). Both returned to control values within 5-10 minutes.

CONCLUSIONS

PNASAL MRR reflects POES MRR over a wide range of sniff pressures, PNASAL MRR of maximal sniffs reflects POES MRR in normal subjects at rest and following MIV, so measurement of PNASAL MRR may be a useful non-invasive method for measuring inspiratory muscle MRR, thereby providing an index of respiratory muscle fatigue.