Predictors of post-stroke respiratory infection and the level of cough reflex that is enough to protect against respiratory infection remains a challenge. So far, clinical predictors and tests have offered prognostic values . We included acute stroke patients whether they had dysphagia or not, and the citric acid used in titrating concentrations from 0.8 to 2.6, and the follow-up was only for 1 week. The goal of this work is to administer the swallowing ability test and cough reflex challenge to all stroke patients to predict post-stroke aspiration pneumonia.
In this observational prospective study, the mean age of stroke patient was around 60 years old, males more than females, most of our stroke patients were unemployed and 60.4% had previous positive medical problems such as DM, our results were nearly the same as reported by Thrift and colleagues 2017 .
In the current study, stroke patients who were older ages and unemployed had significantly higher rates of developing respiratory infections than those who did not. High NIHSS score exposes stroke patients to a higher risk of post-stroke pneumonia. Sellars and colleagues  found that NIHSS score more than 6 is predictive of post-stroke pneumonia. Also, these findings were consistent with the study carried out by Westendrop and colleagues .
No statistically significant difference was found between males and females as regards developing chest infection, so was smoking. Also, stroke location either anterior or posterior circulation, type, past medical history, and medications did not show a significant difference between patients with and without post-stroke chest infection.
All our stroke patients had a significantly higher cough threshold level (1.08 mol/L) than the normal value (0.8 mol/L); this is consistent with the results of  who studied the cough reflex in post-stroke patients and found that its mean values were higher than the values of the healthy volunteers, in common we both found that stroke patients needed higher citric acid concentrations than healthy individuals.
It is worth the attention that impairment of swallowing is associated with an increased risk of aspiration. In our study, the difference in the total MANN score between the patients that developed respiratory infection was significantly higher than that those did not have any respiratory complications, this is consistent with the results of Vilardell and colleagues  who stated that incidence of post-stroke pneumonia has been found to be significantly increased with both impaired swallowing and cough responses.
Moreover, regarding the level of cough reflex and incidence of respiratory infections, chest infection occurs in 80% of our stroke patients with absent cough, in comparison to 50% in stroke patients with a weak cough. Our results were consistent with the results from Field and colleagues  who found higher aspiration pneumonia rates in patients with absent cough (25%) than those with weak cough (7.7%) and the least found in patients with strong cough (0%). These findings interpreted in the context that the absence of a protective cough reflex exposes stroke patients to one of the most post-stroke morbidities which is pneumonia. Although our results were inconsistent with those found by Miles and colleagues .
In the present study, we found significant negative correlation between the NIHSS score and cough reflex levels, as patients with no cough had the highest mean NIHSS then weak then normal cough group who had the lowest NIHSS mean score; this result is consistent with Vilardell and colleagues  that stated that incidence of post-stroke pneumonia has been found to be significantly increased with both impaired swallowing and cough responses.
Stroke patients with absent cough reflex had the highest score of A2DS2 followed by those with weak cough, and the least score was found in patients with normal cough threshold; the difference was significant between the three groups as the higher the patient score A2DS2, the higher the possibility to develop a chest infection. These results were supported by a previous study by Zapata-Arriaza and colleagues  that stated that the A2DS2 score is a valid tool to predict aspiration pneumonia. In our study, it correlated significantly with the cough threshold, PCF, and no. of coughs. Unfortunately, we did not find a study that made such a correlation before, but the cough threshold and peak flow were correlated with the risk of pneumonia in many studies.
We found that dysphagia correlated significantly with weak voluntary cough ability in acute stroke patients as the MASA scores correlated significantly with all the items of the cough challenge test. These results matched with another important study by Ward and colleagues  that assessed VC and RC by measurements of airflow and gastric pressure.
The present study revealed that inflammatory markers ESR and CRP showed non-significant between the three groups on admission, but after 1 week the CRP showed significant difference unlike the ESR that the difference remained nonsignificant, this goes in accordance with Zapata-Arriaza and colleagues  who stated that CRP more sensitive and responds more quickly to changes in the clinical situation. To the best of our knowledge, this study is the first to highlights this correlation.
In the current study, cut-off values showed that the citric acid threshold was > 1 mol/L. Nakajoh and colleagues  documented swallowing latency of response longer than 5 s and a cough threshold higher than a concentration of 1.35 mol/L. We, thus, recommend that every institution must have its own reference cut-off value.
We detected the cut-off value for the peak cough flow in post-stroke respiratory infection to be <= 100 L/min. Bianchi and colleagues, 2012 reported a cough peak flow of lower than 242 L/min predicting the development of pneumonia  while Sohn and colleagues, 2018 found that cut-off flow of 59 L/min diagnosed dysphagia attributable to cerebrovascular disease and the incidence of aspiration pneumonia in 6 months and the citric acid concentration used was 0.28 mol/L , this is different than our methodology. Regarding the cough latency and number of coughs after stimulation, their cut-off values were non-significant.
We should address that our study is limited by a small number of participants and a short follow-up duration.