Open Journal Systems

ORIGINAL CONTRIBUTIONS

Received: March 14^th 2015

Accepted: May 15^th 2015

Threshold value of f/Vt index for predicting successful weaning from mechanical ventilation in active smokers

Rommel Jesús Rivas-Salazar,^a José Ángel Baltazar-Torres,^a Perla Cristina Arvizu-Tachiquín,^a Israel Moreno-López,^a Sergio Zamora-Varela,^a Abraham Antonio Cano-Oviedo,^a Alejandro Esquivel-Chávez,^a Luis Alejandro Sánchez-Hurtado^a

^aUnidad de Cuidados Intensivos, UMAE Hospital de Especialidades “Dr. Antonio Fraga Mouret”, Centro Médico Nacional La Raza, Instituto Mexicano del Seguro Social, Ciudad de México, México

Communication with: Rommel Jesús Rivas-Salazar

Telephone: 5724 5900, extensión 23137

Email: rommeljesus@hotmail.com

Background: Mechanical ventilation (MV) is used in 60-70 % of patients admitted to the intensive care unit (ICU). An f/Vt threshold value of 105 is a predictor of successful weaning from mechanical ventilation (WMV) in patients with asthma, cardiac surgery, pneumonia, sepsis, neurocritical, etc. However, there are no reports about the usefulness of the f/Vt value to predict successful WMV in patients with active smoking (AS). The purpose of this paper is to identify a threshold value for f/Vt to predict successful WMV in patients with AS.

Methods: Prospectively, 85 patients with AS and MV >24 hours admitted to de ICU were included. Demographic and clinical data were registered. The f/Vt value was measured with a Wright’s spirometer, and the sensibility, specificity, positive predictive value (PPV) and negative predictive value (NPV) were calculated to predict a successful WMV. A p value <0.05 was considered statistically significant.

Results: The average of f/Vt was 68.69. Successful WMV was of 75.3 %. An f/Vt threshold value of 79.5 had sensibility of 76 %, specificity of 61 %, PPV of 85 %, and NPV of 46 % to predict successful WMV in this group of patients.

Conclusions: An f/Vt threshold value of 79.5 is useful to predict successful WMV in patients with AS.

Keywords: Artificial respiration; Respiratory insufficiency; Smoking; Mechanical ventilators

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Mechanical ventilation (MV) is a respiratory function support system used in a significant proportion (60-70%) of patients admitted to the intensive care unit (ICU).¹ There are undoubted benefits that MV offers the management of patients with respiratory failure, but it is also true that it has multiple harmful effects.² In addition, it is estimated that patients with prolonged MV consume up to 37% of ICU resources, increasing costs of care.³ Removing MV prematurely or unnecessarily prolonging it have deleterious consequences for the patient: unnecessary inconvenience, increased incidence of ventilator-associated pneumonia, airway injury, oxygen toxicity, gastrointestinal bleeding, deep vein thrombosis, increased stay in the ICU, mortality, and costs of care.^4-6 On the other hand, premature MV withdrawal and subsequent reintubation increase the risk of lung infection, tracheostomy, increased MV time, ICU stay, and mortality. Weaning from mechanical ventilation (WMV) is mainly based on the value of the f/Vt index, which has not been evaluated in patients with active smoking without prior respiratory function tests. In Mexico, the prevalence of active smoking in adults 18 to 65 years is 30.2%, and it is more common in men (45.3%) than women (18.4%).⁷ Active smoking is defined as the habitual consumption of tobacco, and it is the most important determinant of deterioration in pulmonary ventilatory function (accelerated decline in lung function, residual volume, forced expiratory volume in the first second, vital capacity, and functional residual capacity).⁸ Forced expiratory volume in the first second decreases annually -8.4 ml per pack smoked a day.⁹ It also produces disorders in gas exchange manifested by arterial hypoxemia and hypercapnia.¹⁰ Active smoking can be considered a predictor of WMV failure due to changes in the physiology of the patient’s respiratory system,^8,9,11-13 and it is an independent risk factor for difficulty and failure in WMV.¹⁴

The f/Vt index, also called the rapid shallow breathing index, proposed by Yang and Tobin,¹⁶ is a parameter that evaluates the patient's ventilatory function. It divides the respiratory rate by the current volume in liters.^15,16 Under normal conditions, the breathing pattern is characterized by low breathing rate and high tidal volume, which results in a low f/Vt value. If the patient’s ventilatory function deteriorates, the ventilatory pattern changes and is now characterized by rapid, shallow breaths, i.e., high respiratory rate and low tidal volume, resulting in increased value of the f/Vt rate. Therefore, the lower the f/Vt rate value, the lower the deterioration of the patient’s ventilatory function.¹³ A f/Vt threshold of 105 is a predictor of successful WMV in patients with asthma, cardiac surgery, pneumonia, sepsis, neurocritical conditions, etc. In patients with active smoking in whom the threshold value is 105, the f/Vt rate may not be useful. There are no reports of the usefulness of the f/Vt rate to predict successful WMV in patients with active smoking.  

The aim of this article is to identify a threshold f/Vt rate value to predict successful WMV in patients with active smoking.

Methods

A prospective, longitudinal, observational, and descriptive study was performed in patients admitted to the ICU of a tertiary care hospital between August 1 and December 31, 2014.

The sample size included 85 patients, incorporated by consecutive sampling with the following selection criteria: men and women over 15 years with active smoking, smoking index > 1, who had MV for more than 24 hours, and who signed the written informed consent to participate in the study. We excluded those with noninvasive MV, tracheotomy, neuromuscular diseases, or unplanned extubation.

The interventions recorded demographic and clinical variables: gender, age, ICU admission diagnosis, and cause for intubation and mechanical ventilation, as well as the severity of the disease, which was assessed by APACHE II.¹⁴ Interventions on patient management, including scheduling the mechanical ventilator and the decision to begin withdrawing it, was left to the discretion of the treating physician. Once it was decided to start WMV, SVT was done using a T-piece for 30 to 120 minutes (Annex 1). At the end of the test tidal volume was measured using a Wright spirometer and respiratory rate was recorded. The f/Vt rate was calculated with these parameters by dividing the respiratory rate by the tidal volume. The decision to remove the ventilator and extubate the patient was left to the discretion of the treating physician. After removing MV, patients were monitored for 48 hours. WMV was considered successful if the patient continued breathing spontaneously during this time, and WMV failed if the patient was reintubated or died within the first 48 hours after being removed from the ventilator.

Statistical analysis

Continuous variables are expressed as mean ± standard deviation for parametric data and as medians with interquartile range (IQR) for nonparametric data. The normal distribution of numerical data was assessed using the Kolmogorov-Smirnov test. Student’s t-test was used for comparison of parametric data, and the Mann-Whitney U test to compare nonparametric data. Categorical variables are expressed as percentages and Chi-square test was used to compare these data. An ROC curve was made to identify the f/Vt rate value with the best sensitivity and specificity to predict successful WMV. Once the threshold value was identified, sensitivity (S), specificity (SP), positive predictive value (PPV), negative predictive value (NPV), false positives (FP), and false negatives (FN) were determined to predict successful WMV. In all cases a p-value < 0.05 was considered statistically significant. This project met international ethical standards and was approved by the Local Research Commission of the Instituto Mexicano del Seguro Social (IMSS). 

Results

In total, 85 patients were admitted to the study. The median age was 60 years (IQR 45.5-68.5), and 64.7% were men. Smoking history had a median of 4 (RIC 2-8.5) and the average APACHE II score was 10.42 ± 3.78 (Table I).

The most frequent causes of ICU admission are shown in Table II. Highlights include postoperative for high-risk surgery (27.1%), hemorrhagic shock (16.5%), and pneumonia (12.9%).

The most frequent reasons for tracheal intubation and initiation of mechanical ventilation were hemorrhagic shock (18.8%), acute respiratory distress syndrome (15.3%), and postoperative for high-risk surgery (10.6%) (Table III).

Table IV shows the parameters of WMV. The median duration of MV was 4 days (IQR 2-7). Patients most often required 2 (IQR 1-2) SVTs before WMV was achieved, which was successful in 75.3% of patients. The average f/Vt rate was 68.69 ± 19.95. 

The f/Vt rate showed good discriminative ability to identify the successful removal of MV. The area under the ROC curve was 0.78 (95% CI 0.68-0.88), with p < 0.05. Using this tool, the f/Vt value identified with the best sensitivity and specificity was 79.5.  

Fifty-seven patients (67%) had a f/Vt rate ≤ 79.5, while in 33% (28/85) the rate was > 79.5. Patients with f/Vt ≤ 79.5 had a successful WMV in 86% of cases (49/57), while for patients with f/Vt> 79.5, only 46.4% (13/28) were removed successfully. The difference was statistically significant, with p < 0.05 (Figure 1).

The threshold f/Vt rate value of 79.5 had S 76% and 61% SP for predicting the successful removal of MV. The PPV was 85% and NPV of 46%. FP were presented in 38% of cases and FN in 23%.

Discussion

WMV is an aspect of care of the seriously ill that clinicians should keep in mind from the moment they start MV. Active smoking can be considered a predictor of failure in WMV due to alterations caused in the respiratory system of the patient with MV.⁹ It is very important to identify the right time for WMV, as both premature withdrawal and unnecessary prolonging of MV have deleterious consequences. Identifying the right time for WMV is based on clinical and gasometrical criteria,⁶ and it is recommended to use a WMV protocol including SVT.^18,19 The decision to withdraw MV is based on predictors of WMV success, especially f/VT rate.^16,20,21 The f/Vt rate was proposed by Yang and Tobin¹⁶ as a parameter for predicting success in WMV, and is currently the most widely used worldwide. This index consists of measuring the patient’s spontaneous tidal volume and respiratory rate during the SVT. The index results from dividing the respiratory rate by the tidal volume.^15,16 These authors proposed a threshold level of 105 to decide whether a patient is removed from MV or not. If the f/Vt rate value is less than 105, they recommend removing MV, but if it is greater, they recommend continuing MV and making a new SVT 24 hours later.¹⁶ This threshold f/Vt rate value has shown useful in predicting successful WMV in a variety of patients,²² and it is recommended by the Task Force for Weaning²³ for use in the general population.  

The threshold value of the f/Vt rate in our population with active smoking was much lower than that described in the original study by Yang and Tobin¹⁶ and recommendations proposed in most WMV protocols.^20,21,23,25

However, it has not been evaluated in patients with active smoking with smoking index > 1. We believe that in these patients the threshold of 105 is very high, because the alterations in ventilatory mechanics conditioned by tobacco use, characterized by rapid, shallow breaths, cause a higher f/Vt rate.^8,9,11,13

The main finding of this study is that, by analyzing the ROC curve, we identified a threshold f/Vt value 79.5 for patients with active smoking. This value is similar to that reported in reviews of more than 29 studies, where it was found that a f/Vt rate value between 77.4 and 89.1 could predict successful WMV with greater reliability, both in patients with lung disease, and in patients without lung disease.²³

Using this threshold, 86% of our patients were successfully removed from MV, which is higher than previously reported in patients with chronic obstructive pulmonary disease.²⁶ These authors found a failure rate of 61% using a f/Vt value < 105 to decide on WMV, while we had a 14% failure rate using a threshold value of ≤ 79.5. Even in our patients with f/VT rate > 79.5 that were removed from MV, the failure rate was 46.4%, which is also lower than reported by Zhonghua and colleagues.²⁶

In our patients admitted to the ICU with MV, the f/Vt rate showed good discriminative ability to predict success in WMV, with area under the ROC curve of 0.78. This value is lower than found by Yang et al.,¹⁶ who reported an area under the curve of 0.89; however, it must be considered that their population is not specifically patients with active smoking. When compared with patients with lung disease, the area under the ROC curve has better performance, as the area under the ROC curve reported by Boutou et al. in this group of patients was 0.39.²⁷ The area under the ROC curve of the f/Vt rate shows better discriminative behavior for predicting successful WMV in this population of patients with mechanical ventilation and lung disease.

The performance of the threshold value of 79.5 to predict success in WMV showed S of 76%, SP of 61%, PPV of 85%, and NPV of 46%. Compared with those reported by Yang and Tobin,¹⁶ our values ​​are lower than those found in patients with < 8 days of MV (S 100%, SP 64%, PPV 78%, and NPV of 95%), and also that found in patients with > 8 days of MV (S 88%, SP 67%, PPV 64%, and NPV 89%). However, these authors did not report whether their study population included patients with active smoking, which could explain the difference in results. On the contrary, when compared with patients with underlying impaired lung function, our values ​​obtained are higher than reported by Boutou et al. (S 38%, SP 63%, PPV 66%, and NPV 35%).²⁷

To date there are no similar studies to ours that specifically propose the use of a threshold value of the f/Vt rate to predict successful WMV in patients with active smoking, so we propose using 79.5 as this threshold value.

However, our study has some weaknesses to be taken into account. First, it was conducted in a single hospital, which may limit extrapolation of the results. Second, the sample size is small, which can limit the correct identification of this threshold value. Still, 86% of our patients with f/Vt rate ≤ 79.5 were removed from MV successfully, higher than figures previously reported in other patient groups.

Conclusions

The f/Vt rate has good discriminative ability to predict successful WMV in patients with active smoking (area under the ROC curve of 0.78 [95% CI 0.68 to 0.88], p < 0.05).

The threshold value of the f/Vt rate with the best S and SP to predict successful WMV in patients with active smoking is 79.5.

The threshold f/Vt rate value of 79.5 has adequate S, SP, PPV, and NPV to predict successful WMV in patients with active smoking.

We propose using a threshold value of 79.5 in the f/Vt rate as a predictor of successful WMV in patients with active smoking.

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