Cough Peak Flow Assessment: Is There More to the Story?

Assessment of cough peak flow (CPF) is frequently utilized to determine the need for cough augmentation. CPF < 270 L/min is the established standard for implementing assisted coughing techniques in patients with neuromuscular disease (NMD). ¹ Whereas the indications during invasive mechanical ventilation are not as well defined, CPF < 60 L/min along with signs of secretion retention have been used to initiate therapy in this patient population. ^2,3

CPF has also been utilized to assess effectiveness of cough augmentation. Measurements can be obtained through a variety of methods including a spirometer, peak flow meter, mechanical insufflation-exsufflation (MI-E) devices, and ventilators. In the early 1990s, Bach ⁴ compared CPF obtained with a peak flow meter during assisted and unassisted coughing in adults with NMD and found that assisted techniques including MI-E significantly increased CPF. An evaluation comparing CPF measured with a spirometer and with a peak flow meter did not identify significant differences between the devices; however, the peak flow meter may underestimate CPF. ⁵

Cough strength during mechanical ventilation has been associated with extubation outcomes, as a weak cough effort has been linked to increased rates of re-intubation. ^6,7 Low-level evidence has suggested use of MI-E during invasive ventilation may reduce extubation failure. ⁸ Gobert ⁹ and others measured CPF using an ICU ventilator to construct a composite score for predicting extubation failure. A different study that evaluated CPF obtained from the ventilator and from a spirometer following successful completion of a spontaneous breathing trial demonstrated that CPF had high sensitivity and specificity to predict re-intubation, and both measures had similar predictive accuracy. ⁷

This issue of the Journal includes 2 papers that examined CPF obtained from different devices, contributing to the growing body of literature on this topic. ^10,11 Terzi and colleagues ¹⁰ compared CPF from 4 different MI-E devices in an in vitro model with stable and collapsed airway conditions. Significant differences among MI-E devices were observed for CPF measured at the tracheal level with a pneumotachograph, and values were lower with the collapsed condition. In contrast, CPF measured with the MI-E device was higher in the collapsed condition.

Laryngeal collapse during MI-E therapy has been associated with treatment failure in many cases. ^12,13 Evaluation of CPF during MI-E administered with a face mask in adults with NMD demonstrated that CPF alone was unable to detect upper-airway collapse. ¹⁴ High insufflation pressures have been attributed to upper-airway collapse in patients with amyotrophic lateral sclerosis and in a child with spinal muscular atrophy. Transnasal fiberoptic laryngoscopy identified laryngeal closure during insufflation that resolved with decreased insufflation pressure. ^13,15 More recently, waveform analysis was successfully used to recognize obstruction during insufflation and optimize therapy settings. ¹⁶

The study by Terzi ¹⁰ highlights the limitations in using CPF alone to evaluate cough effectiveness during MI-E. Furthermore, different MI-E devices produced varying results. The authors also evaluated effective cough volume (ECV), defined as volume exhaled > 180 L/min. ECV was found to significantly differ for all devices with both the collapsed and stable airway and was lower in the collapsed condition. ¹⁰ The authors suggest that measuring ECV might identify postextubation upper-airway collapse in the presence of increased CPF. However, Andersen and others ¹² were unable to identify an association between visualized laryngeal response and unique air flow patterns. Additional study is warranted for the role and clinical application of measuring ECV during MI-E therapy.

Use of a peak flow meter with a mask to measure CPF is a common practice for assessing cough effectiveness in individuals with NMD. ¹⁷ The device also can be adapted to an artificial airway to evaluate cough strength. ¹⁸ Compared to spirometry, CPF may be underestimated when measured with a peak flow meter. ⁵ Both methods require disconnection from the ventilator when obtaining CPF during mechanical ventilation.

Fossat and colleagues ¹¹ evaluated CPF in mechanically ventilated patients and compared values obtained from an ICU ventilator to an electronic peak flow meter connected to the endotracheal tube. Previous studies that examined CPF measured by a ventilator did not compare values to those obtained from a peak flow meter. ^7,9 In the present study, there was significant correlation between values obtained from the ventilator and the peak flow meter. ¹¹ Although 2 different ventilator models were used, no statistically significant differences were identified in CPF values. However, use of different ventilators introduced the possibility of variability in measurements. Using the ventilator to measure CPF not only eliminates the need for disconnection but also the requirement for additional equipment such as a spirometer or peak flow meter.

Unsuccessful extubation is often attributed to airway issues such as cough impairment and excessive secretions or weaning failure due to hypoventilation. ² Glottic closure is affected by the endotracheal tube, directly impacting the ability to produce an effective cough. ¹⁹ Poor cough strength has been associated with re-intubation. ^6,7 Cough augmentation during invasive mechanical ventilation is increasing, and some evidence has suggested that MI-E therapy may help reduce extubation failure. ⁸ The routine use of CPF assessment during mechanical ventilation has potential to assist with both need for cough augmentation and predict extubation failure. However, CPF typically requires patient participation to cough on command; therefore, clinical application would be limited.

Both of the papers ^10,11 assessing CPF under different circumstances provide important contributions to the literature and implications for future study. Answering research questions often leads to more questions. The use of MI-E during mechanical ventilation is increasing. ²⁰ More research is needed to establish the role of CPF assessment in ventilated patients and evidence-based criteria for initiating cough augmentation. This could help create standards of care for cough augmentation in mechanically ventilated patients and introduce an additional tool for ventilator liberation protocols for extubation readiness.

CPF measured by different MI-E devices was found to be significantly different in both collapsed and stable airway conditions. ¹⁰ Comparison of CPF values with different MI-E devices in ventilated patients represents another potential investigation as intubated patients would not typically experience airway collapse that may occur with noninvasive application of MI-E. Comparison of values from MI-E devices with a ventilator is also warranted. Additionally, an understanding of how different ventilators and MI-E devices measure CPF is an important consideration.

Given the limitations of CPF alone to assess cough efficacy during MI-E, further study regarding ECV is necessary to understand the potential role for this measure and the relationship to CPF. Transnasal fiberoptic laryngoscopy and waveform analysis during MI-E therapy should be explored as potential standard assessments to optimize therapy settings and to correct instances of upper-airway collapse. The more we learn about CPF in various applications the more we realize there is so much more to be learned.